L-Citrulline

amino acidL-citrulline

What is it

L-citrulline is a non-essential amino acid named after watermelon (Citrullus), the fruit from which it was first isolated. The body uses it as part of the urea cycle and as a precursor that converts to L-arginine in the kidneys, ultimately raising nitric oxide production.

How it works

L-citrulline is one of those rare cases where a precursor outperforms the substance it makes. When you take oral L-arginine, most of it is destroyed by intestinal and hepatic arginase before reaching systemic circulation. L-citrulline bypasses both barriers: it is absorbed efficiently, escapes first-pass metabolism, and is converted to L-arginine in the kidneys via the enzymes argininosuccinate synthase and argininosuccinate lyase. The net effect is that an oral dose of citrulline raises blood arginine levels more than the same oral dose of arginine itself. The arginine then feeds endothelial nitric oxide synthase, which converts it to NO. NO relaxes vascular smooth muscle, dilates blood vessels, increases tissue perfusion, and lowers blood pressure. This is the foundation for L-citrulline's three best-supported use cases: exercise performance, erectile function, and blood pressure. Citrulline malate, the most common sports supplement form, combines L-citrulline with malic acid, a Krebs cycle intermediate. The malate may contribute small additional effects on energy metabolism, but the L-citrulline does most of the work.

Evidence for 5 uses

AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.

Exercise performance (strength endurance)

Grade B

Good evidence

Multiple randomized trials and meta-analyses show 6 to 8 g of citrulline malate pre-workout improves repetition volume in resistance training, reduces muscle soreness 24 to 72 hours post-exercise, and modestly improves anaerobic performance. Effects on aerobic endurance are smaller but still positive in some trials.

Erectile dysfunction (mild)

Grade C

Moderate evidence

A 2011 single-blind randomized trial in 24 men with mild ED showed 1.5 g/day of L-citrulline for 1 month improved erection hardness from grade 3 to grade 4 in 50 percent of men versus 8.3 percent on placebo. Effects are modest and less reliable than PDE5 inhibitors but appropriate for mild cases.

Blood pressure

Grade C

Moderate evidence

Several small trials in adults with normal or pre-hypertensive blood pressure have shown 3 to 6 g/day L-citrulline modestly lowers systolic and diastolic readings. Effects are similar in magnitude to L-arginine.

Heart failure exercise tolerance

Grade C

Moderate evidence

Trials of L-citrulline in patients with heart failure with preserved ejection fraction have shown improvements in exercise capacity and reduced systemic vascular resistance. Evidence is preliminary but mechanistically plausible.

Sickle cell disease

Grade D

Mixed evidence

Small trials in sickle cell disease have suggested benefits on NO-related vascular function, but evidence is preliminary. Should be considered investigational and pursued under hematology guidance.

2 commercial forms

Citrulline malate (2:1 ratio)

L-citrulline bound to malic acid; the form used in most positive sports research.

The default for pre-workout. A standard scoop is 6 to 8 g, delivering 4 to 5 g L-citrulline plus 2 to 3 g malic acid. Slightly sour taste.

Pure L-citrulline

Higher per-gram delivery of L-citrulline than malate, but without the modest contribution from malic acid.

Better choice if you want pure L-citrulline effects without the malic acid. 3 to 5 g per dose is comparable to 6 to 8 g of malate.

Dosage

There is no RDA. Typical doses are 6 to 8 grams of citrulline malate (about 4 to 5 grams of L-citrulline) taken 30 to 60 minutes pre-workout. For pure L-citrulline, 3 to 6 grams per dose is common. Blood pressure trials have used 3 to 6 grams per day in divided doses for weeks. Erectile dysfunction studies have used 1.5 to 3 grams per day.

When and how to take it

For exercise performance, take 6 to 8 grams of citrulline malate (or 3 to 5 g of pure L-citrulline) 30 to 60 minutes before training on an empty stomach. Effects on blood flow last about 2 to 4 hours. For blood pressure or erectile support, split daily totals (1.5 to 3 g twice daily) and take with or between meals. Unlike some pre-workout ingredients, citrulline does not stack uncomfortably with caffeine. Avoid taking late in the day if you find it disrupts sleep, though this is uncommon.

Food sources

Food	Amount	%DV
Watermelon (1 cup)	~250 mg	—
Watermelon rind (concentrated)	much higher than flesh	—
Cucumber (1 cup)	small amounts	—
Cantaloupe (1 cup)	small amounts	—
Pumpkin (1 cup)	modest amounts	—

Safety

L-citrulline has an excellent tolerability profile, generally better than L-arginine because it doesn't trigger the same level of GI upset at effective doses. Mild stomach discomfort or headache can occur, especially with citrulline malate due to malic acid's mild acidity. No formal Tolerable Upper Intake Level has been established. Doses up to 15 grams per day have been used safely in short-term research. Like arginine, L-citrulline can lower blood pressure and may compound the effect of antihypertensives or other vasodilators. People with low blood pressure or who take PDE5 inhibitors and nitrates should be cautious. There is no comparable post-MI safety signal to that observed with high-dose L-arginine, but cardiology consultation is still wise after a heart attack.

Who should be cautious

Use cautiously with low blood pressure, on antihypertensive medication, or with PDE5 inhibitors or nitrates. Avoid combining with nitrates. People with severe kidney disease should consult a clinician since renal conversion to arginine is the key step. Pregnant and breastfeeding women have limited safety data. Coordinate with cardiology after a heart attack despite the absence of clear adverse signals.

Interactions

L-citrulline compounds the blood-pressure-lowering effects of antihypertensive medications and PDE5 inhibitors (sildenafil, tadalafil, vardenafil). Combining with nitrates can cause dangerous hypotension. May enhance the effects of other vasodilators. No known significant interactions with most cardiovascular, metabolic, or psychiatric drugs at typical doses.

Frequently asked questions

Is L-citrulline better than L-arginine?⌄

For most use cases, yes, in terms of raising plasma arginine and producing nitric oxide effects. Oral arginine is heavily metabolized in the gut and liver before reaching circulation; citrulline bypasses both barriers and is converted to arginine in the kidneys, ending up at higher blood levels than the same dose of arginine.

What's the difference between L-citrulline and citrulline malate?⌄

Citrulline malate combines L-citrulline with malic acid in roughly a 2:1 ratio. The L-citrulline does most of the work; the malic acid contributes mildly to energy metabolism. Most sports research uses citrulline malate at 6 to 8 g doses.

When should I take citrulline?⌄

Pre-workout, 30 to 60 minutes before training. Effects on blood flow last 2 to 4 hours. For blood pressure or ED support, split into morning and evening doses.

Can I stack citrulline with caffeine or pre-workout?⌄

Yes, no known negative interaction. Citrulline is a common ingredient in pre-workout blends and pairs well with caffeine for combined alertness and pump effects.

Does citrulline really come from watermelon?⌄

Yes, that's where it was first isolated. A cup of watermelon contains about 250 mg, well below typical supplement doses. The watermelon rind is much more concentrated, which is why some old-school athletes drink watermelon rind juice.

References

Wikidata: L-Citrulline — Wikidata link

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Disclaimer: These statements have not been evaluated by the FDA. This page is educational, not a substitute for personalized medical advice. Evidence grades are AI-assisted assessments — talk to your doctor before starting any new supplement, especially if you're pregnant, breastfeeding, on medications, or managing a chronic condition.