L-Citrulline: Benefits, Forms, Dosing, and Side Effects

L-Citrulline: Benefits, Forms, Dosing, and Side Effects

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Table of Contents

Overview

L-citrulline is a non-essential, non-proteinogenic amino acid that functions as an intermediate in the urea cycle and serves as a precursor to L-arginine, the direct substrate for nitric oxide (NO) synthesis [1][2]. The "L" prefix indicates the naturally occurring stereoisomer. The compound was first isolated from watermelon (Citrullus vulgaris) in 1914 by Japanese researchers Yotaro Koga and Ryo Odake, and its name derives from the Latin word for watermelon [3][4].

The primary interest in L-citrulline supplementation stems from its ability to raise plasma L-arginine levels more effectively than oral L-arginine itself. When L-arginine is taken orally, a substantial portion is degraded by arginase enzymes in the gut and liver during first-pass metabolism, limiting the amount that reaches systemic circulation [5][6]. L-citrulline bypasses this first-pass metabolism because it is not a substrate for arginase. Instead, it is absorbed intact in the small intestine, enters systemic circulation, and is converted to L-arginine primarily in the kidneys via the sequential action of argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL) [1][7][8]. This conversion pathway — known as the citrulline-NO cycle — regenerates L-arginine for sustained nitric oxide production by endothelial nitric oxide synthase (eNOS), inducible NOS (iNOS), and neuronal NOS (nNOS) [9][10].

Nitric oxide is a key signaling molecule that promotes vasodilation by activating soluble guanylyl cyclase and increasing cyclic GMP (cGMP), which relaxes vascular smooth muscle and allows more blood to flow through arteries [1][11]. This mechanism underpins the theoretical rationale for L-citrulline's cardiovascular, exercise performance, and erectile dysfunction applications.

L-citrulline also plays important roles beyond nitric oxide production. As an intermediate in the urea cycle, it participates in ammonia detoxification — the conversion of toxic ammonia (generated from amino acid catabolism) into non-toxic urea for renal excretion [12][13]. Additionally, research suggests L-citrulline can directly stimulate muscle protein synthesis via the PI3K/mTOR signaling pathway, independent of its conversion to arginine [14][15].

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The body produces L-citrulline endogenously through multiple pathways. In the liver, it is synthesized as part of the urea cycle from ornithine and carbamoyl phosphate via ornithine transcarbamylase (OTC) [12]. In the intestinal enterocytes, it is generated from glutamine and glutamate through a series of mitochondrial enzymes [16]. The kidneys produce minor amounts via the NOS pathway. Age-related declines in enzymatic activity reduce endogenous production, particularly in older adults [17].

L-citrulline exhibits high oral bioavailability (approximately 97%) and is efficiently absorbed in the gastrointestinal tract without significant hepatic first-pass metabolism [18]. Peak plasma concentrations occur approximately one hour after ingestion [19]. It appears as a white crystalline powder with high water solubility (approximately 200 g/L at 20°C), making it suitable for aqueous formulations [20][21].

Despite the strong mechanistic rationale, clinical evidence for L-citrulline supplementation has been mixed. Studies have generally shown little to no benefit for exercise performance when taken as a single dose before exercise, modest potential for blood pressure reduction, and some evidence for mild erectile dysfunction [1][22]. The evidence is summarized in detail in the sections that follow.

Forms and Bioavailability

L-Citrulline vs. L-Citrulline Malate

L-citrulline is commercially available in two primary forms: pure L-citrulline and L-citrulline malate (a combination with malic acid, technically DL-malic acid). Understanding the differences between these forms is critical for interpreting research and selecting appropriate dosing.

Pure L-citrulline is essentially 100% citrulline by weight. It has a very faint, savory flavor and is neither acidic nor basic. It does not absorb water from the air (non-hygroscopic), so it is unlikely to become clumpy as a powder or crumbly as a tablet [1]. Most clinical studies evaluating cardiovascular and erectile dysfunction endpoints have used this form.

L-citrulline malate is only 56.6% citrulline by weight, with the remainder being malic acid [1][23]. This gives it a tangy, acidic flavor. L-citrulline malate absorbs water from the air (hygroscopic), which means powders will clump and tablets may crumble if exposed to humid conditions [1]. It is generally not suitable for tablet formulations for this reason. The malate form is more commonly marketed for exercise performance, though this specific advantage has not been demonstrated in controlled studies.

A direct comparison study among 43 healthy, resistance-trained men and women (average age 24) showed that taking citrulline malate (5.3 g citrulline plus 2.7 g malate) 45 minutes before low-to-moderate-volume resistance training did not improve muscle strength or endurance compared to supplementation with the same dose (5.3 g) of pure L-citrulline without malate, and neither form showed improvement compared to placebo (Martin-Olmedo et al., Int J Sport Nutr Exerc Metab, 2024) [24]. A systematic review also concluded that the addition of malate to citrulline has not been demonstrated to enhance exercise performance (Gough et al., Eur J Appl Physiol, 2021) [23].

"2:1" Labeling Confusion

Products labeled as "2:1 L-citrulline malate" or "L-citrulline malate 2:1" present a labeling ambiguity. It is unclear whether "2:1" refers to a molecular ratio (where 72.3% of the ingredient should be L-citrulline) or a weight ratio (where 67% should be L-citrulline). Independent testing of five different "2:1" products in England revealed citrulline amounts that were consistently less than expected, yielding actual ratios of 1.11 to 1.92 parts citrulline to each part malate — meaning only 52% to 66% was actual citrulline (Chappell et al., J Int Soc Sports Nutr, 2018) [25].

Bioavailability and Pharmacokinetics

L-citrulline exhibits high oral bioavailability, approximately 97%, with efficient absorption occurring in the small intestine via amino acid transporters [18]. It reaches systemic circulation largely intact without significant hepatic first-pass metabolism — a key advantage over oral L-arginine, which is substantially degraded by arginase in the gut and liver [5][6].

Peak plasma citrulline and arginine concentrations occur approximately one hour after oral ingestion [19]. Studies have confirmed that both L-arginine and L-citrulline supplementation cause similar increases in blood levels of arginine, but citrulline supplementation achieves this more efficiently because it is not degraded during first-pass metabolism (Bailey et al., J Appl Physiol, 2015) [26].

L-citrulline supplementation can bypass arginase-mediated limitations on L-arginine availability, leading to more sustained NO production compared to direct L-arginine supplementation [5][27][28]. Animal models have demonstrated that L-citrulline restores intracellular NO levels by preventing arginase activity increases and enhancing arginine recycling [29].

Summary of Form Selection

Form Citrulline Content Flavor Hygroscopic Best Use
Pure L-Citrulline ~100% Faint, savory No General health, blood pressure, erectile dysfunction. Better studied form.
L-Citrulline Malate ~56.6% Tangy, acidic Yes Often marketed for exercise but no proven advantage over pure form.
"2:1" L-Citrulline Malate 52-72% (variable) Tangy, acidic Yes Labeling ambiguous; actual citrulline content often less than expected.

If you are trying to get a specific dose of L-citrulline from a product, you must account for the citrulline content of the form being used. An 8 g dose of citrulline malate provides only approximately 4.5 g of actual citrulline [1].

Evidence for Benefits

Exercise Endurance and Performance

L-citrulline has been evaluated for exercise performance enhancement based on the rationale that increased nitric oxide production improves blood flow, oxygen delivery, and ammonia clearance during exercise. Results have been mixed, with multi-day supplementation showing more promise than single acute doses.

Multi-day supplementation (positive results): A study of 10 healthy, recreationally active young men who took either 6 g of L-arginine, 6 g of L-citrulline, or placebo for seven days found that both L-arginine and citrulline caused similar increases in blood levels of arginine, but only citrulline supplementation improved exercise tolerance (by 12%) and increased the total amount of work completed (by 7%) during a severe-intensity cycling exercise test compared to placebo (Bailey et al., J Appl Physiol, 2015) [26].

Single-dose supplementation (negative results): Most studies using a single pre-exercise dose of citrulline have failed to show benefit:

  • A study of 12 recreationally trained young men and women (average age 24) found that a single dose of 8 g of citrulline malate (providing approximately 5.33 g of citrulline) taken one hour before high-intensity exercise did not improve performance or decrease fatigue compared to placebo (Farney et al., J Strength Cond Res, 2017) [30].
  • A study among young, healthy men and women in the UK found that an 8 g dose of citrulline malate taken one hour before resistance exercise did not increase muscle strength or reduce muscle fatigue, and actually worsened delayed-onset muscle soreness (DOMS) in the three days following exercise compared to placebo (Chappell et al., J Int Soc Sports Nutr, 2018) [25].
  • Drinking a combination of L-arginine (2.5 g) and L-citrulline (5.6 g) before resistance exercise was no more effective than placebo in increasing muscle perfusion (blood flow) and muscle size after exercise in young men who regularly practiced resistance exercise (Burkle et al., JISSN, 2022) [31].
  • A study of 43 healthy, resistance-trained men and women (average age 24) showed that taking citrulline malate (5.3 g citrulline plus 2.7 g malate) 45 minutes before low-to-moderate-volume resistance training did not improve muscle strength or endurance compared to the same dose (5.3 g) of pure L-citrulline without malate, and neither form was better than placebo (Martin-Olmedo et al., Int J Sport Nutr Exerc Metab, 2024) [24].

Older adults: A study of 44 older men and women (average age 65) found that 3 g of citrulline malate taken daily in addition to regular exercise for six weeks slightly improved walking speed but did not significantly improve muscle strength or endurance compared to exercise plus placebo (Caballero-Garcia et al., Nutrients, 2021) [32].

Meta-analyses and systematic reviews: A systematic review concluded that citrulline malate supplementation has not demonstrated reliable improvements in exercise performance, and that the addition of malate provides no proven ergogenic advantage (Gough et al., Eur J Appl Physiol, 2021) [23]. Some meta-analyses report modest increases in repetition performance (approximately 3 ± 5 weighted repetitions) in resistance training with citrulline malate, but results vary substantially by protocol [33]. A 2025 systematic review indicates an overall positive effect on vascular health in middle-aged and older adults, which may indirectly support exercise capacity [34].

Muscle soreness and perceived exertion: Research suggests L-citrulline may reduce post-exercise ratings of perceived exertion (RPE) and delayed muscle soreness without altering blood lactate levels directly. Acute supplementation with 8 g citrulline malate taken 1-2 hours before exercise has been reported to significantly reduce delayed muscle soreness at 24 hours post-exercise (effect size 0.99), with some effects at 48 hours [33]. Citrulline malate has also been shown to enhance creatine phosphate resynthesis, which supports faster ATP regeneration and may delay fatigue in repeated bouts of anaerobic exercise [33].

Power output and resistance training: An acute dose of 8 g citrulline malate has been reported to increase the number of repetitions to failure during multiple sets at 80% of one-repetition maximum (1RM), leading to greater overall work capacity in some studies [33]. However, the direct comparison trial by Martin-Olmedo et al. (2024) found no benefit over placebo [24].

CrossFit and functional training: A randomized crossover study on CrossFit athletes found that acute citrulline malate supplementation improved overall workout performance metrics, such as total repetitions and time efficiency in mixed aerobic-anaerobic circuits [35]. However, investigations into sex-specific responses remain limited.

Muscle protein synthesis: Beyond performance, research suggests L-citrulline can directly stimulate muscle protein synthesis. Studies in rats showed that citrulline supplementation combined with endurance training enhanced muscle protein synthesis by 48% compared to controls [36]. In humans, citrulline administration in post-absorptive states after a low-protein diet boosted muscle protein synthesis by approximately 20% via mTOR activation [37]. L-citrulline modulates the PI3K/mTOR pathway to stimulate muscle protein synthesis independently of its conversion to arginine [14][15].

Synthesis: Multi-day supplementation (7+ days) at doses of 6 g/day of pure L-citrulline has shown some benefit for exercise tolerance in small studies. However, single pre-exercise doses — including 8 g of citrulline malate — have consistently failed to improve performance in well-controlled trials. The addition of malate to citrulline provides no demonstrated advantage over citrulline alone. Some evidence supports reduced muscle soreness at 24 hours post-exercise, but this has not translated to consistent performance gains.

Blood Pressure and Cardiovascular Health

Because citrulline is converted to arginine, which in turn may increase nitric oxide production and promote vasodilation, it has been evaluated for cardiovascular benefits. Evidence has been mixed.

Meta-analyses: Several meta-analyses of randomized controlled trials have evaluated citrulline for blood pressure reduction. A systematic review and meta-analysis reported a modest but significant reduction in systolic blood pressure of approximately 4 mmHg compared to placebo, with effects primarily observed in individuals with elevated blood pressure rather than normotensive subjects [38]. An updated meta-analysis from 2024 confirmed these findings [38]. A 2025 systematic review also indicates overall positive effects on vascular health in middle-aged and older adults [34]. However, an earlier review found that most studies were conducted in healthy individuals rather than hypertensive populations, and results were mixed (Barkhidarian et al., Avicenna J Phytomed, 2019) [39].

Postmenopausal hypertension (negative): A study of 25 postmenopausal women with high blood pressure (systolic blood pressure 130 mmHg or higher) showed that supplementing with 5 g of L-citrulline in the morning and at night for 4 weeks did not significantly improve systolic or diastolic blood pressure, mean arterial pressure, or heart rate, despite increasing flow-mediated dilation from 4.8% to 6.2% (Maharaj et al., Nutrients, 2022) [40]. This suggests that improved endothelial function from citrulline does not always translate into measurable blood pressure reduction.

Arterial stiffness: L-citrulline supplementation has shown some ability to improve arterial stiffness, as measured by reductions in pulse wave velocity, especially in middle-aged and older populations at risk for cardiovascular disease [41]. A 2022 trial in hypertensive postmenopausal women showed improvements in femoral-ankle pulse wave velocity after eight weeks of combined L-citrulline supplementation and resistance training [42].

Diastolic heart failure: A small, uncontrolled study among people with diastolic heart failure showed that taking 3 g of citrulline daily for two months reduced systolic blood pressure by 10.8%, diastolic blood pressure by 8.3%, and decreased pulmonary artery pressure by 9 mmHg compared to baseline. However, there was no placebo control group. There was also no improvement in ventricular ejection fraction or treadmill performance (Orozco-Gutierrez et al., Cardiol J, 2010) [43].

Systolic heart failure: A small study among people with systolic heart failure showed that taking 3 g of citrulline daily for 17 weeks did not significantly reduce blood pressure, although left ventricular ejection fraction increased by about 20% at rest and 13% with stress compared to baseline, and these increases were significant compared to the control group (Balderas-Munoz et al., Cardiol J, 2012) [44].

Heart failure and atrial fibrillation risk: There does not appear to be evidence that citrulline prevents heart failure or atrial fibrillation. An observational study found that higher blood levels of citrulline were associated with a 19% greater risk of heart failure and no significant effect on the risk of atrial fibrillation (Goni et al., Am J Clin Nutr, 2022) [45]. This may reflect reverse causation but raises a cautionary note.

Combination therapies: A 9-week study combining L-citrulline with nitrate-rich beetroot extract showed potentiated blood pressure reductions [46]. Research on combinations with polyphenols from cranberry and grape seed has shown promising effects on ambulatory blood pressure in women with prehypertension [47].

Use in older adults: L-citrulline supplementation may be particularly relevant for middle-aged and older adults due to age-related declines in nitric oxide production. Meta-analyses show significant improvements in flow-mediated dilation in this population. Studies in adults around 70 years have demonstrated that 6 g/day increases lower limb blood flow during exercise by approximately 11% [48]. Doses of 3-6 g/day are commonly effective and well-tolerated, with lower risk of gastrointestinal side effects compared to L-arginine.

Synthesis: The blood pressure evidence for L-citrulline is inconsistent. Meta-analyses suggest a modest reduction of approximately 4 mmHg in systolic blood pressure, primarily in people with elevated blood pressure. However, a well-controlled trial in hypertensive postmenopausal women showed no blood pressure reduction despite improved endothelial function. The observational association between higher citrulline levels and increased heart failure risk warrants further investigation.

Erectile Dysfunction

The most consistent clinical evidence for L-citrulline comes from studies on mild erectile dysfunction.

Key clinical trial: A study in Italy among 24 men with mild erectile dysfunction found that taking 750 mg of L-citrulline twice daily (1.5 g/day total) for one month resulted in improvements in erection hardness in 12 of the 24 men (50%), compared to improvements in only 2 of the men when they were given placebo. The men who improved shifted from an Erection Hardness Score (EHS) of 3 (partially hard) to 4 (fully hard). No adverse events were reported. The researchers noted that L-citrulline is less effective than prescription medication for erectile dysfunction (Cormio et al., Urology, 2011) [49].

Mechanism: L-citrulline supports erectile function by serving as a precursor to L-arginine, which enhances nitric oxide production, promoting vasodilation and improving penile blood flow [49][50]. This mechanism addresses endothelial dysfunction by bypassing the presystemic metabolism of L-arginine [50].

Combination with transresveratrol: A randomized, double-blind, placebo-controlled trial demonstrated that combining L-citrulline with transresveratrol enhanced erectile function in men already using PDE5 inhibitors, particularly in non-responders to L-arginine alone. Improvements were noted in intercourse frequency and overall sexual satisfaction (Sansone et al.) [51].

Comparison with PDE5 inhibitors: Evidence-based reviews suggest that higher daily doses of L-citrulline (5-8 g, often split into multiple doses) may provide mild benefits similar to low-dose tadalafil (e.g., 5 mg daily). However, direct head-to-head trials are lacking [52][53]. L-citrulline is generally considered less potent than prescription PDE5 inhibitors.

Comparison with L-arginine: L-citrulline is generally superior to direct L-arginine supplementation for erectile function due to its better bioavailability, bypassing first-pass metabolism for more efficient and sustained increases in plasma L-arginine levels [50][54]. Evidence as of 2026 continues to favor L-citrulline over oral L-arginine for improving mild ED [55].

Libido: Evidence specifically for improving libido (sexual desire) is limited, with most research focused on erectile performance rather than desire [50].

Patient demographics: Benefits appear most pronounced in men with mild ED and without severe underlying vascular disease, such as those aged around 56 years in the key trial [49].

Limitations: Available reviews have identified gaps in long-term safety data for prolonged use in ED management. Additional research, including direct head-to-head comparisons with sildenafil, is needed. No adverse events were reported in existing trials [56].

Synthesis: L-citrulline at 1.5 g/day improved erection hardness in 50% of men with mild ED in the only dedicated RCT. It is less effective than prescription PDE5 inhibitors but has an excellent safety profile. It may serve as an adjunct therapy, particularly when combined with transresveratrol. Larger trials are needed.

Brain Function and Neuroprotection

Animal evidence: A study in mice found that feeding them L-citrulline daily after temporarily blocking blood flow to a portion of the brain (simulating an ischemic stroke) reduced death of neurons and loss of capillaries and improved memory deficits. The authors included employees of Kyowa Hakko, a major manufacturer of L-citrulline (Yabuki et al., Brain Res, 2013) [57]. This study has conflict-of-interest concerns and has not been replicated in humans.

Cerebrovascular function: Preliminary evidence suggests L-citrulline may improve cerebrovascular function, potentially aiding recovery and providing indirect benefits for mental clarity via enhanced nutrient and oxygen delivery to the brain [58]. However, direct clinical data for cognitive benefits in humans remain very limited [59].

Depression and anxiety: There is no clinical evidence from randomized controlled trials supporting L-citrulline supplementation as an effective treatment for depression or anxiety. Observational studies have reported lower serum L-citrulline levels in patients with major depression, but no intervention studies demonstrate benefits [60].

Synthesis: The brain and neuroprotection evidence for L-citrulline is preliminary and almost entirely from animal models or observational data. There is no convincing evidence of cognitive benefits in humans.

Fatty Liver Disease

Animal evidence: Studies in animals suggested a potential benefit of L-citrulline supplementation for fatty liver disease (Jegatheesan et al., Clin Nutr, 2016; Levy, Br J Clin Pharmacol, 2017) [61][62].

Human trial (negative): A study among 42 adolescents with abdominal obesity and metabolic-associated fatty liver disease (MAFLD) found that 3 g of L-citrulline taken twice daily (6 g/day total) did not decrease liver fat accumulation, total and LDL cholesterol, or alkaline phosphatase levels, or show any other benefit compared to placebo (Tovar-Villegas et al., Gastroenterol Insights, 2024) [63].

Synthesis: Despite promising animal data, the only human trial showed no benefit. This application lacks clinical support.

Ammonia Clearance and Urea Cycle Support

L-citrulline plays a critical role in ammonia detoxification as an intermediate in the urea cycle [12][13]. This function is particularly relevant in:

  • Hepatic tissues: Disruptions in citrulline metabolism can lead to hyperammonemia [64].
  • Urea cycle disorders: Citrullinemia type I (caused by ASS deficiency) and citrullinemia type II (caused by citrin mutations) both result in elevated plasma citrulline and impaired urea cycle function [65][66].
  • Exercise: Citrulline's role in ammonia clearance may contribute to reduced fatigue during sustained exercise [33].

Muscle Protein Synthesis

Beyond nitric oxide production, L-citrulline has been investigated for its ability to directly stimulate muscle protein synthesis:

  • L-citrulline modulates the PI3K/mTOR signaling pathway independently of its conversion to arginine [14][15].
  • In rats, citrulline combined with endurance training enhanced muscle protein synthesis by 48% compared to controls [36].
  • In humans, citrulline boosted muscle protein synthesis by approximately 20% via mTOR activation after a low-protein diet [37].
  • Through conversion to ornithine, L-citrulline contributes to polyamine synthesis supporting cellular proliferation [67][68].

These findings are preliminary and require larger human trials for clinical relevance, particularly for populations at risk of sarcopenia [69].

COVID-19 and Vascular Health

Patients with severe COVID-19 exhibit decreased plasma citrulline levels associated with increased oxidative stress and poor oxygenation [70]. Low citrulline concentrations correlate with systemic inflammation and gastrointestinal symptoms [71]. A clinical trial evaluated intravenous L-citrulline for acute hypoxemic respiratory failure due to COVID-19 [72]. These findings remain preliminary and do not support supplementation as a COVID-19 treatment.

By Indication

Dosing recommendations vary depending on the intended use and the form of citrulline being used. Note that L-citrulline malate is only approximately 56.6% citrulline, so doses must be adjusted accordingly.

Purpose Form Dose Duration Notes
General cardiovascular support Pure L-citrulline 3-6 g/day, divided Ongoing Multi-day use shows more benefit than single doses [1][19]
Blood pressure Pure L-citrulline 5-6 g/day, divided 4-8+ weeks Most positive meta-analysis data at ~6 g/day [38][40]
Exercise performance Pure L-citrulline 6 g/day for 7+ days 7+ days before event Single pre-exercise doses have not shown benefit [23][26]
Exercise performance L-citrulline malate 8 g (~4.5 g citrulline) Single dose, 30-60 min pre-exercise Commonly used but evidence weak [23][24][25]
Mild erectile dysfunction Pure L-citrulline 1.5 g/day (750 mg twice daily) 1+ month Based on Cormio et al. trial [49]; higher doses sometimes suggested
Older adults (vascular health) Pure L-citrulline 3-6 g/day Ongoing Well-tolerated; lower GI risk than L-arginine [48]

Timing and Administration

  • Peak plasma concentrations occur approximately 1 hour after oral ingestion [19].
  • Multi-day supplementation (7+ days) has generally shown more benefit than single acute doses, particularly for exercise-related outcomes [1][26].
  • Dividing the dose (e.g., morning and evening) may sustain arginine and nitric oxide levels throughout the day.
  • L-citrulline can be taken with or without food. Some sources suggest taking it on an empty stomach for optimal absorption, though there is no strong evidence that food significantly reduces effectiveness [19].

How to Read a Supplement Label

  • Products labeled "L-citrulline" are expected to be approximately 100% citrulline.
  • Products labeled "L-citrulline malate" are expected to be 56.6% citrulline. An 8 g dose provides approximately 4.5 g of actual citrulline.
  • Products labeled "2:1 L-citrulline malate" may be 52-72% citrulline depending on interpretation and manufacturing accuracy [1][25].
  • If a product lists "3 g L-citrulline malate," only approximately 1.7 g is actual citrulline.
  • Always check whether the label specifies the amount of L-citrulline itself or the total weight of the citrulline malate compound.

Storage

L-citrulline malate absorbs water from the air (hygroscopic) and will clump if left exposed to humid air. Keep the container closed when not in use. Regular L-citrulline does not absorb water from the air and does not have this issue [1].

Safety and Side Effects

General Safety Profile

L-citrulline has self-affirmed GRAS (Generally Recognized as Safe) status for oral use in food products [73]. Animal studies indicate low toxicity with a wide margin of safety [74]. In clinical trials lasting from one week to several months, daily doses of citrulline up to about 10 g/day have generally been safe and well-tolerated [1][75]. Higher acute doses up to 15 g have been reported as safe in short-term studies [75].

Common Side Effects

Side effects are mild and infrequent, occurring in less than 5% of users [75]:

  • Gastrointestinal upset — nausea, diarrhea, or stomach discomfort
  • Heartburn — particularly with the malate form, which is more acidic than pure L-citrulline
  • Tooth enamel effects — L-citrulline malate taken as a drink may affect tooth enamel due to its acidity

L-citrulline supplementation appears to be less likely to cause gastrointestinal upset than L-arginine supplementation [1]. The malate form is more acidic than regular L-citrulline, making it more likely to cause heartburn and exacerbate gastroesophageal reflux [1].

Specific Populations

People with pulmonary arterial hypertension: Increased urinary frequency, cough, and swelling of arms and legs have been reported (Allerton et al., Nutrients, 2018; Kashani et al., J Cardiol, 2014) [22][76].

People with low blood pressure: Because citrulline may increase nitric oxide production, it could potentially reduce blood pressure further. Rare instances of hypotension have been noted. Caution is advised, especially if symptomatic or combined with other vasodilators [75].

Urea cycle disorders: Citrulline supplementation could affect tests used to detect and monitor urea cycle disorders such as argininosuccinate synthetase deficiency (Berry et al., J Pediatr, 2001) [77].

Pregnancy and pediatric use: Data are sparse. Ongoing clinical investigations in preterm infants report no adverse events to date [78][79]. Human trials like the AGREE study are still assessing long-term outcomes in pregnancy [80][81].

Kidney disease: Caution is advised in individuals with severe kidney disease due to limited data, though some studies suggest potential renal protective effects [75].

Surgery: Some sources recommend stopping L-citrulline at least 2 weeks before scheduled surgery due to potential interference with blood pressure control [75].

Bleeding Risk

L-citrulline supplements have no established bleeding risk. No interactions with anticoagulants or antiplatelet drugs are documented [75].

Alcohol Interactions

No significant adverse interactions have been reported for combining L-citrulline with alcohol [82][83][84].

Drug Interactions

Because citrulline is converted to arginine in the body, it might enhance nitric oxide production and lower blood pressure, creating interactions with several medication classes:

Medication Class Examples Interaction Clinical Significance
Blood pressure-lowering medications ACE inhibitors, ARBs, calcium channel blockers, beta-blockers May enhance blood pressure reduction Monitor blood pressure; use with caution [1]
Nitrates Nitroglycerin, isosorbide dinitrate May amplify vasodilatory effects Potential for excessive hypotension [1][85]
PDE5 inhibitors Sildenafil (Viagra), tadalafil (Cialis) May amplify vasodilatory effects Potential for excessive blood pressure lowering [1][85]

Key Points on Drug Interactions

  • The primary concern is additive blood pressure-lowering effects when L-citrulline is combined with medications that also lower blood pressure or promote vasodilation [1][85].
  • PDE5 inhibitors work on the same nitric oxide-cGMP pathway that L-citrulline supports. Combining them could amplify vasodilatory effects [85].
  • Although one clinical trial evaluated L-citrulline combined with PDE5 inhibitors and found no adverse events [51], caution is still advised in clinical practice.
  • Unlike magnesium supplements, L-citrulline does not have known chelation interactions that reduce the absorption of other medications.
  • Individuals taking any blood pressure-lowering medication, nitrates, or PDE5 inhibitors should consult their healthcare provider before starting L-citrulline supplementation.

Dietary Sources

L-citrulline is primarily obtained from plant-based foods, with the highest concentrations found in members of the Cucurbitaceae (gourd) family.

Top Food Sources

Food Citrulline Content Notes
Watermelon (flesh) 0.7-3.5 g/kg fresh weight Highest natural source. Varies by cultivar [86]
Watermelon (rind) Higher than flesh Often discarded but particularly rich [86]
Casaba-type melon 0.86 g/kg Moderate source [86]
Mouse melon (Melothria scabra) 0.64 g/kg Uncommon food [86]
Horned melon, rind 0.45 g/kg Niche cucurbit [86]
Cucumber 0.23-0.28 g/kg Moderate source [86]
Pumpkin and squash 0.04-0.07 g/kg Low amounts [86]

Practical Notes

  • Watermelon is the dominant dietary source — it contains 7-41 times more citrulline than other cucurbits [86].
  • Achieving therapeutic doses from food is impractical. To obtain 6 g of L-citrulline from watermelon, you would need to consume approximately 2-8.5 kg (4-19 lbs) of fresh watermelon [18][86].
  • Processing reduces citrulline content. Juicing or drying watermelon decreases citrulline levels; fresh consumption preserves the most [86].
  • Animal-derived foods contain only trace amounts of L-citrulline [18].
  • Oral bioavailability of dietary citrulline is high (approximately 97%) [18].
  • Watermelon vs. supplements: There is no reliable evidence that consuming watermelon is superior to L-citrulline supplements for therapeutic purposes. Supplements provide concentrated, standardized doses [17].

Endogenous Production

  • Liver: Synthesized as part of the urea cycle from ornithine and carbamoyl phosphate. Hepatic production does not contribute net citrulline to systemic circulation [16].
  • Intestinal enterocytes: The primary source of circulating citrulline. Generated from glutamine and glutamate through mitochondrial enzymes [16].
  • Kidneys: Minor amounts produced via the NOS pathway; primarily convert circulating citrulline to L-arginine [16].
  • Endogenous production declines with age due to reduced enzymatic activity [17].

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References

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About Dr. Brad Stanfield

Dr Brad Stanfield

Dr. Brad Stanfield is a General Practitioner in Auckland, New Zealand, with a strong emphasis on preventative care and patient education. Dr. Stanfield is involved in clinical research, having co-authored several papers, and is a Fellow of the Royal New Zealand College of General Practitioners. He also runs a YouTube channel with over 319,000 subscribers, where he shares the latest clinical guidelines and research to promote long-term health. Keep reading...

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