L-citrulline is a non-essential amino acid that your body uses to produce another amino acid, L-arginine, which in turn fuels the production of nitric oxide, a molecule that relaxes and widens blood vessels. Your body makes some L-citrulline on its own, but many people take it as a supplement to support blood flow, exercise performance, and cardiovascular health. It gets its name from Citrullus, the Latin word for watermelon, which is one of the richest natural sources.
How L-Citrulline Works in Your Body
L-citrulline follows a specific path through your organs before it does its main job. Your intestines produce citrulline and release it into the bloodstream, where it travels to the kidneys. Inside the kidney’s filtering tubes, two enzymes convert citrulline into arginine. This kidney-based conversion is the primary way your body manufactures new arginine to maintain healthy levels throughout the body.
Arginine then serves as the raw material for nitric oxide production. Nitric oxide signals the smooth muscle lining your blood vessels to relax, which lowers resistance to blood flow. This is why L-citrulline supplementation is linked to benefits involving circulation: better blood pressure, improved exercise performance, and stronger erections. Taking citrulline actually raises blood arginine levels more effectively than taking arginine supplements directly, because arginine taken by mouth gets partially broken down in the liver before it reaches the bloodstream. Citrulline bypasses this breakdown entirely.
Blood Pressure Effects
A meta-analysis of clinical trials found that L-citrulline supplementation lowered systolic blood pressure (the top number) by an average of about 4 mmHg. The effect on diastolic blood pressure (the bottom number) was smaller and only reached statistical significance at doses of 6 grams per day or more, where it dropped by roughly 2.75 mmHg. These reductions are modest compared to prescription medications, but they’re meaningful for people with mildly elevated blood pressure or those looking for complementary approaches alongside lifestyle changes.
Exercise Performance and Recovery
Most exercise research uses citrulline malate (a combination of L-citrulline and malic acid) at doses of 6 to 8 grams, taken 40 to 60 minutes before training. A systematic review and meta-analysis of strength training studies found that this protocol increased repetitions to failure by about 6.4% compared to placebo. The effect was slightly more pronounced for lower body exercises (around 8.1%) than upper body work (5.7%), though neither subgroup reached strong statistical significance on its own. In practical terms, you might squeeze out one to three extra reps on a hard set.
The recovery side of the equation is where citrulline shows more consistent results. A meta-analysis of seven studies found that citrulline supplementation significantly reduced muscle soreness 24 hours after exercise, with a large effect size. It also lowered ratings of perceived exertion, meaning people felt like the same workout was less taxing. The soreness reduction at 48 hours was less consistent across studies, but still showed a positive trend. For people training frequently, that faster bounce-back in the first day after hard sessions could add up over weeks of training.
Erectile Function
Because erections depend heavily on blood flow and nitric oxide signaling, L-citrulline has been studied as a natural option for mild erectile dysfunction. In a trial of 24 men with mild ED, half of those taking 1.5 grams of L-citrulline daily improved from a hardness score of 3 (mild dysfunction) to 4 (normal function). Only 8.3% of men taking the placebo saw the same improvement. That’s a significant difference, though the study was small and focused specifically on mild cases. L-citrulline is not a replacement for prescription ED medications in moderate or severe cases, but it may offer a meaningful benefit for men at the milder end of the spectrum.
Food Sources
Watermelon is by far the most concentrated natural source. Both the flesh and the rind contain L-citrulline, with the rind carrying slightly more, around 45 mg per gram of rind compared to about 44 mg per gram of flesh. A large wedge of watermelon might provide somewhere in the range of 1 to 2 grams of citrulline, depending on the size. That’s a meaningful amount, but well below the 3 to 6 grams typically used in clinical studies. Other foods contain citrulline in smaller quantities, including cucumbers, pumpkins, and other gourds, but none come close to watermelon’s concentration.
L-Citrulline vs. Citrulline Malate
Supplements come in two main forms: pure L-citrulline and citrulline malate. Citrulline malate combines L-citrulline with malic acid, typically marketed at a 2:1 ratio (two parts citrulline to one part malic acid). The malic acid plays a role in energy production, which is why citrulline malate has been the form most studied for exercise. However, independent testing has revealed that many supplement manufacturers don’t actually deliver the advertised 2:1 ratio. Nuclear magnetic resonance analysis found most products provided closer to a 1.6:1 ratio, with some as low as 1.1:1. At the lowest ratio, an 8-gram dose of citrulline malate delivers only about 4.2 grams of actual citrulline instead of the expected 5.3 grams.
Cleveland Clinic dietitians recommend choosing pure L-citrulline over citrulline malate, noting that the evidence base for L-citrulline is stronger and the research on citrulline malate is more conflicting. If you do choose citrulline malate for exercise, you may need a higher total dose to get enough actual citrulline.
Dosage and Timing
For general cardiovascular and blood flow benefits, clinical trials have used 3 to 6 grams of pure L-citrulline per day. The blood pressure meta-analysis suggests that doses at 6 grams or above are more effective, particularly for lowering diastolic pressure. For exercise performance, 6 to 8 grams of citrulline malate (or 3 to 5 grams of pure L-citrulline) taken 40 to 60 minutes before training is the most studied protocol. The ED study used a lower dose of 1.5 grams daily, taken consistently over several weeks rather than as a single pre-activity dose.
Side Effects and Cautions
L-citrulline is generally well tolerated, but because it’s an amino acid hitting your gut in concentrated form, it can cause digestive issues. Starting at a lower dose and working up helps you gauge your tolerance. Reported side effects include bloating, stomach cramps, diarrhea, and sweating.
Because L-citrulline affects blood vessel dilation, it has the potential to interact with medications that also lower blood pressure or affect blood flow, including nitrate drugs. People with kidney disease should be aware that citrulline-to-arginine conversion happens primarily in the kidneys, and this pathway is disrupted when kidney function is impaired. As with any supplement, the products themselves are not tightly regulated in the U.S., so choosing a brand that uses third-party testing adds a layer of quality assurance.