L-citrulline is an amino acid made in the body and found in some foods, most famously watermelon. It is not used to build proteins; instead, the kidneys convert it into L-arginine, which blood vessels and nerves use to produce nitric oxide. Nitric oxide relaxes the smooth muscle of the penis so it can fill with blood during an erection.
The appeal is absorption. Oral L-arginine is heavily broken down before it reaches the circulation, whereas L-citrulline passes through largely intact. In a randomised crossover study of 20 healthy volunteers, L-citrulline raised blood arginine more effectively than L-arginine did, and boosted markers of nitric oxide signalling in a dose-dependent way; the highest dose tested was 3 g twice a day.1
Its best-known study in sexual health is an Italian trial in 24 men with mild erectile dysfunction, described below.2
What have studies found on L-citrulline and erections?
In men with mild erectile dysfunction, one month of L-citrulline improved erection hardness compared with a month on placebo. Larger trials would help confirm the findings.
In the Italian study, 24 men with mild ED took a placebo for one month, then 1.5 g of L-citrulline a day for another month. Erection hardness improved from 'mild ED' to 'normal' in 12 of 24 men (50%) while on citrulline, compared with 2 of 24 (8.3%) on placebo. The number of times the men had sex each month also rose, every man whose hardness improved said he was very satisfied, and no side effects were reported.2
The study was single-blind, meaning the researchers knew which phase was which, and the placebo month always came first. The authors described citrulline as less potent than PDE5 inhibitor medicines, at least in the short term, but safe and well accepted, and called for further research.2
L-arginine, which citrulline is converted into, has a larger body of trials: a meta-analysis of 10 randomised trials in men with mild to moderate ED found improved erectile function.3
Why is L-citrulline in semen volume supplements?
It is included for its role in blood flow and nitric oxide. Studies have measured erection hardness and blood arginine rather than the amount of semen.
L-citrulline is a common ingredient in semen volume supplements, often alongside L-arginine and minerals such as zinc. Semen volume itself comes mostly from the seminal vesicles and prostate, and has not been an outcome in citrulline trials so far. When a product combines several ingredients, the research on each one describes that ingredient at the amount studied.
Is L-citrulline safe?
It appeared well tolerated in the short studies available. Because it raises nitric oxide, it may add to the effects of medicines that lower blood pressure.
Talk to a doctor before using L-citrulline if you take nitrates, blood pressure medicines or PDE5 inhibitors, or if you have heart disease. Erection problems can share causes with heart and blood vessel disease, so it is worth getting checked as well. For another approach with research behind it, see whether cardio improves erections.
- Studied for: mild ED and nitric oxide levels
- Dose in the ED study: 1.5 g a day for one month
- Check with a doctor first if: you take nitrates, blood pressure medicines or PDE5 inhibitors
Common questions
Is L-citrulline better than L-arginine?
It is better absorbed, and raised blood arginine more effectively than L-arginine itself in a study of healthy volunteers.1 The two have not yet been compared head to head for erections; L-arginine has more trials, and citrulline's ED study found improved erection hardness.23
Is watermelon a good source of L-citrulline?
Watermelon is the best-known food source of citrulline. The erection study used a concentrated supplement dose of 1.5 g a day,2 and eating watermelon has not been studied for erections in the same way.
How much L-citrulline did the ED study use?
The study in men with mild ED used 1.5 g a day for one month.2 That is a research dose, not a recommendation, so check with a doctor before trying it.
Read more on this
Supplement ingredients for men’s sexual health: what the research shows
Does cardio really improve erections? What the research says
Sources
- Schwedhelm E, et al. Pharmacokinetic and pharmacodynamic properties of oral L-citrulline and L-arginine: impact on nitric oxide metabolism. Br J Clin Pharmacol. 2008;65(1):51-9. PubMed
- Cormio L, et al. Oral L-citrulline supplementation improves erection hardness in men with mild erectile dysfunction. Urology. 2011;77(1):119-22. PubMed
- Rhim HC, et al. The potential role of arginine supplements on erectile dysfunction: a systemic review and meta-analysis. J Sex Med. 2019;16(2):223-234. PubMed
Not medical advice. Written by Eric Stephens from the research cited above; not reviewed by a doctor. If something worries you, see a GP.