L-citrulline is an amino acid your body uses to produce nitric oxide, a molecule that relaxes and widens blood vessels. Unlike many amino acids, it isn’t used to build proteins. Instead, it works behind the scenes to support circulation, exercise performance, and blood pressure regulation. Your body makes some on its own, but supplemental doses can raise levels well beyond what you’d produce naturally.
How L-Citrulline Works in Your Body
L-citrulline follows a somewhat roundabout path to do its job. When you take it orally, it passes through the gut and into the bloodstream without being broken down by the liver, a quality that sets it apart from many other amino acids. Your kidneys then convert it into another amino acid called L-arginine, which serves as the raw material for nitric oxide production.
Nitric oxide signals your blood vessels to relax and expand. That increased blood flow is the mechanism behind most of L-citrulline’s benefits, from lower blood pressure to better exercise performance to improved erections. Once nitric oxide is produced from arginine, the leftover byproduct is actually citrulline again, creating a recycling loop that keeps the process going.
Why Not Just Take L-Arginine?
This is one of the most practical things to understand about L-citrulline. Taking arginine directly seems like it would be more efficient, since arginine is what your body actually needs to make nitric oxide. But roughly 70% of supplemental arginine gets broken down during digestion before it ever reaches your bloodstream. Only about 31% of an oral arginine dose makes it into general circulation.
L-citrulline, by contrast, bypasses that breakdown almost entirely. In animal research published in The Journal of Nutrition, essentially 100% of supplemental citrulline appeared in plasma. At matched doses, citrulline raised blood arginine levels 35% higher than arginine supplements themselves did. Even more telling, citrulline supplementation increased the actual rate of arginine flowing through the body, while arginine supplements did not. So L-citrulline is, somewhat paradoxically, the better way to raise your arginine levels.
Blood Pressure Effects
The blood vessel relaxation caused by nitric oxide translates into measurable drops in blood pressure. A meta-analysis of 15 randomized controlled trials in middle-aged and older adults found that L-citrulline supplementation reduced systolic blood pressure (the top number) by about 4 mmHg and diastolic pressure (the bottom number) by about 2.5 mmHg. Those numbers are modest but meaningful, roughly comparable to what some people achieve with lifestyle changes like reducing sodium.
When citrulline was combined with arginine supplementation, the effects were considerably larger: systolic pressure dropped by about 10.4 mmHg and diastolic by nearly 5 mmHg. These reductions approach what you’d see from some blood pressure medications, though the evidence base is still relatively small (415 total participants across all included trials).
Exercise Performance
L-citrulline is a common ingredient in pre-workout supplements, and the research here is genuinely mixed. Some studies show clear improvements in total work completed and exercise tolerance, while others find no significant effect on time to exhaustion, power output, or speed.
The studies that show benefits tend to involve multi-day supplementation (6 to 16 days of loading) or larger single doses taken 60 to 90 minutes before exercise. For example, 6 grams of citrulline taken daily for a week plus a dose before testing improved average power and time to complete a cycling test. But a single 12-gram dose of citrulline malate taken 60 minutes before exercise produced no improvement in time to exhaustion, power, or distance traveled in another trial. Timing and duration of use seem to matter, and one-off doses before a workout are less reliably effective than consistent daily use.
Erectile Function
Because erections depend heavily on blood flow, the nitric oxide connection makes L-citrulline relevant here too. In a study of 24 men with mild erectile dysfunction, half the participants improved from a hardness score of 3 (mild dysfunction) to 4 (normal function) while taking L-citrulline, compared to just 8.3% while taking a placebo. Every man who saw that improvement reported being “very satisfied.” The effect is less potent than prescription medications for erectile dysfunction, but the study suggests citrulline offers a real benefit for mild cases.
Citrulline Malate vs. Pure L-Citrulline
You’ll find two forms on supplement shelves: pure L-citrulline and citrulline malate, which combines citrulline with malic acid (a compound involved in energy production inside your cells). Most resistance exercise studies use 8 grams of citrulline malate in a 2:1 ratio, which delivers about 5.3 grams of actual citrulline. The remaining weight is malate.
The hypothesis behind adding malate is that it could improve the rate at which your muscles regenerate energy during exercise, creating a synergistic effect. In practice, both forms raise arginine and nitric oxide levels effectively. If you’re comparing labels, just be aware that an “8 grams of citrulline malate” product contains less citrulline per scoop than “8 grams of L-citrulline.”
Dosage
The minimum effective dose for raising circulating arginine levels is about 3 grams per day of L-citrulline. The maximum clinically effective dose appears to be around 10 grams per day, which has been shown to increase both arginine levels and nitric oxide production in healthy adults. Tolerability studies have pushed as high as 15 grams per day without safety concerns in healthy volunteers.
For general cardiovascular support, 3 to 6 grams daily is the most common range used in research. For exercise, studies typically use 6 to 8 grams of either pure L-citrulline or citrulline malate, taken consistently rather than just on training days.
Food Sources
L-citrulline gets its name from Citrullus lanatus, the Latin name for watermelon, which is its richest natural food source. The rind contains nearly twice the citrulline of the red flesh. But the amounts are small in practical terms: a 100-calorie serving of watermelon flesh provides only about 10 mg of L-citrulline, and even the rind delivers just 19.3 mg per equivalent serving. Supplemental doses start at 3,000 mg, so you’d need to eat an impractical amount of watermelon to match even a modest supplement dose.
Safety Profile
L-citrulline is well tolerated in the dosage ranges studied. Up to 15 grams per day has been tested in healthy adults without notable adverse effects. Because it increases nitric oxide and lowers blood pressure, it has the potential to amplify the effects of medications that also lower blood pressure or increase blood flow, including nitrate drugs and medications for erectile dysfunction. If you’re taking any of those, the interaction is worth discussing with a pharmacist or physician before adding citrulline.