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L-citrulline is an amino acid your body makes on its own and that watermelon contains in unusually high amounts. Supplement interest comes from one useful quirk: when you swallow it, most of it slips past the gut and liver and is turned into L-arginine in the kidneys, and arginine is the raw material the body uses to make nitric oxide. That is why L-citrulline turns up in pre-workout powders and “circulation” formulas. The human research is real but modest: small blood pressure changes in some analyses and none in others, a small effect on lifting and sprint performance, and one tiny study in men with mild erection problems. Here is what it is, what the studies found and who should be careful.
Peptide Labs sells dietary supplements, but none of our current products list L-citrulline or citrulline malate on the label, and this article does not recommend any product. It is general information, not medical advice.
Key Takeaways
- L-citrulline is a non-protein amino acid made in the body and found in high amounts in watermelon.
- Taken by mouth, it largely escapes the gut and liver and is converted to L-arginine in the kidneys.
- In a 2008 crossover study, L-citrulline raised blood arginine more effectively than L-arginine itself.
- Blood pressure meta-analyses disagree, and exercise benefits in meta-analyses are small.
- Short studies report few side effects, but long-term safety and drug interactions are not well studied.
What is L-citrulline?
A 2018 review in Nutrients describes L-citrulline as a neutral, non-essential amino acid. Unlike most amino acids, it is not used to build proteins. Instead, it is a working part of the urea cycle, the chemical loop the liver and kidneys use to handle nitrogen. The review notes that citrulline is rarely found in food but is highly concentrated in watermelon. Your own gut also releases citrulline into the blood, where the kidneys pick it up.
How L-citrulline becomes L-arginine
Here is the part that matters for supplements. According to a 2013 review in Nutrition, citrulline is not used up by the intestine and is not taken up by the liver, so it reaches the bloodstream largely intact. Cells in the kidneys then convert it into L-arginine, and blood vessel and immune cells can do the same. L-arginine taken by mouth has the opposite problem: an enzyme called arginase breaks down much of it in the gut before it reaches the blood.
A 2008 crossover study from the University Medical Centre Hamburg-Eppendorf tested this directly. Twenty healthy volunteers took placebo, citrulline or arginine in six different regimens, each for a week. Citrulline raised blood arginine levels more effectively than arginine did, and it increased urine markers of nitric oxide activity. None of the regimens improved a standard ultrasound measure of blood vessel function compared with baseline, a reminder that a change in a blood marker is not the same as a health benefit.
L-citrulline vs L-arginine
The two amino acids are closely linked, but they behave differently as supplements. Our companion guide to L-arginine covers that amino acid in more detail.
| Feature | L-citrulline | L-arginine |
|---|---|---|
| Used to build proteins | No | Yes |
| Main food sources | Watermelon; little elsewhere | Fish, red meat, poultry, soy, whole grains, beans, dairy |
| What happens after swallowing | Mostly escapes the gut and liver; kidneys convert it to arginine | Much is broken down in the gut before reaching the blood |
| Effect on blood arginine (2008 study) | Raised arginine more | Raised arginine less |
| Stomach upset | Reported in some studies | Nausea, diarrhea and bloating are listed side effects |
| Major safety signal | None identified in short studies | More deaths after heart attack in one trial |
L-citrulline benefits: what the research shows
Claims for L-citrulline benefits fall into a few areas. The table below summarizes how strong each one is; the sections after it give the details.
| Area studied | What the evidence shows | Strength |
|---|---|---|
| Blood pressure | One meta-analysis found no effect; a later one in older adults found small drops | Mixed, small trials |
| Strength and sprint performance | 12 studies pooled: a small benefit over placebo | Small effect |
| Citrulline malate and repetitions | 8 studies pooled: about 6% more repetitions on average | Small effect |
| Erection hardness | One single-blind study in 24 men | Very limited |
| Muscle protein in older adults | Lab and early clinical research | Early |
Blood pressure research
Results here point in different directions. A 2018 meta-analysis in Current Hypertension Reports pooled five interventions and found no significant change in systolic or diastolic blood pressure. A larger 2025 meta-analysis in Clinical Nutrition ESPEN included 15 randomized trials with 415 middle-aged and older adults and found that L-citrulline or watermelon lowered systolic blood pressure by about 4 mm Hg and diastolic by about 2.5 mm Hg on average, with a larger drop in trials that combined citrulline with arginine. The trials were small and varied in design.
These are research findings, not a reason to change any prescribed plan. High blood pressure needs a diagnosis and a plan from a clinician, and supplements are not a replacement for prescribed medicines. Dietary nitrate from vegetables reaches nitric oxide by a different route; our beetroot powder guide explains that pathway and its own evidence.
Exercise performance and citrulline malate
Citrulline malate is L-citrulline bound to malic acid, a compound found in fruit. It is the form most often used in pre-workout products. In a 2010 randomized, double-blind crossover study in The Journal of Strength and Conditioning Research, 41 men did more bench press repetitions and reported less soreness after citrulline malate than after placebo. About 15% reported stomach discomfort.
Pooled studies show a smaller picture. A 2019 meta-analysis in Sports Medicine combined 12 studies with 198 healthy participants and found a small benefit for strength and power, but the confidence interval of every individual study crossed the line of no effect. A 2021 meta-analysis of citrulline malate found about three extra repetitions, or roughly 6%, before muscle failure, with a trend for leg exercises and no significant effect for upper-body exercises. If performance is your goal, our guide to creatine benefits covers a supplement with a much larger body of research.
Erectile function research
Because nitric oxide helps blood vessels relax, researchers have looked at citrulline for erectile function. A 2011 study in Urology from the University of Foggia gave 24 men with mild erectile dysfunction a placebo for one month and L-citrulline for the next. Erection hardness scores improved in 12 of 24 men on citrulline and 2 of 24 on placebo, with no side effects reported. The authors said citrulline was less effective than prescription PDE5 inhibitor drugs and needed more research. The study was tiny, single-blind and always gave placebo first.
Erection problems can be linked to blood vessel, hormone or other health issues that deserve a proper checkup, so talk to a clinician rather than starting with a supplement. Never combine citrulline with erectile dysfunction medicines without asking your prescriber.
L-citrulline side effects and safety
L-citrulline side effects in the published studies were uncommon. The 2018 Nutrients review noted a lack of adverse effects in the trials it covered, and stomach discomfort was the only side effect in the citrulline malate bench press study. Most studies lasted days to weeks, so long-term safety has not been well studied. Keep these points in mind:
- Blood pressure, heart and erectile dysfunction medicines: formal interaction studies for citrulline are limited. Because the body converts it to arginine, the cautions Mayo Clinic lists for L-arginine, including nitrates, sildenafil and blood pressure drugs, are worth raising with your prescriber.
- After a heart attack: a trial of L-arginine after heart attack was stopped early because of more deaths in the arginine group. No similar citrulline trial exists, so talk to your cardiologist first.
- Kidney or liver disease: these organs handle citrulline and nitrogen, so check with your clinician.
- Pregnancy and breastfeeding: safety has not been established; ask your obstetric clinician.
Supplement quality varies, so look for products with an independent testing seal. Our guide to third-party tested supplements explains what those seals check.
Reading L-citrulline labels
If you compare L-citrulline products, a few label details matter more than front-of-pack claims:
- Citrulline vs citrulline malate: a citrulline malate ratio on the label tells you how much of the powder is malic acid rather than citrulline, so equal scoops are not equal amounts.
- Proprietary blends: pre-workout blends often list citrulline inside a blend without a separate amount, which makes comparison impossible.
- Caffeine and other stimulants: many pre-workouts pair citrulline with caffeine. See how much caffeine is too much before stacking products.
- Watermelon products: watermelon juice contains citrulline, but amounts vary by variety and are rarely stated on food labels.
Frequently Asked Questions
What does L-citrulline do?
L-citrulline is part of the urea cycle, and the kidneys convert it into L-arginine, which the body uses to make nitric oxide. Supplements are studied for blood flow, blood pressure and exercise, with modest results.
Is L-citrulline better than L-arginine?
In a 2008 study, L-citrulline raised blood arginine more effectively than L-arginine because it escapes breakdown in the gut. That does not prove better health outcomes.
What are the side effects of L-citrulline?
Short studies report few side effects; stomach discomfort is the one most often noted. Long-term safety and drug interactions are not well studied.
What is citrulline malate?
Citrulline malate is L-citrulline combined with malic acid. It is common in pre-workout products, and pooled studies show a small effect on repetitions.
Does watermelon contain L-citrulline?
Yes. Watermelon is unusually rich in it, while most other foods contain very little. Amounts vary by variety.
Can I take L-citrulline with blood pressure medicine?
Ask your prescriber first. Citrulline raises arginine, and Mayo Clinic warns that L-arginine with blood pressure drugs may lower blood pressure too much.
Questions about a label? Contact us or read our Buyer’s Guide. Follow the label and ask a healthcare provider before starting any supplement, especially if you take medicines or are pregnant. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
Last reviewed October 2026.
Related guides: L-Arginine: Benefits, Side Effects and What the Research Shows, Beetroot Powder Benefits: Nitric Oxide, Blood Pressure and Exercise Evidence, Creatine Benefits: What Research Shows, Side Effects and Myths.