Arginine and citrulline are amino acids that work together to produce nitric oxide, a gas your body uses to relax blood vessels, improve blood flow, and support everything from cardiovascular health to exercise performance. They’re often discussed as a pair because they feed into the same biochemical loop: your body converts arginine into nitric oxide (plus citrulline as a byproduct), and then converts that citrulline back into arginine to keep the cycle going.
How the Nitric Oxide Cycle Works
Arginine is the direct fuel for nitric oxide production. An enzyme called nitric oxide synthase breaks arginine apart, releasing nitric oxide gas and producing citrulline as a leftover. That nitric oxide signals your blood vessel walls to relax and widen, which lowers blood pressure and increases blood flow to muscles, organs, and tissues throughout the body.
Citrulline doesn’t just sit idle after it’s produced. Your kidneys convert it back into arginine, replenishing the supply so the cycle can continue. This is why taking citrulline as a supplement can be surprisingly effective at raising arginine levels, sometimes more effective than taking arginine itself.
Why Citrulline Raises Arginine Better Than Arginine Does
This is one of the more counterintuitive findings in supplement research. When you swallow arginine, roughly 70% of it gets broken down in your gut and liver before it ever reaches your bloodstream. Only about 30% makes it into general circulation. Citrulline, on the other hand, passes through the gut almost entirely intact. In animal studies, essentially 100% of supplemental citrulline reached the bloodstream, where it was then converted into arginine.
The practical result: citrulline supplementation raised plasma arginine concentrations about 35% more than the same amount of arginine supplementation. Citrulline also increased the overall rate at which arginine was being produced and used throughout the body, something arginine supplements failed to do. This is why many supplement formulas now favor citrulline, or combine the two.
Blood Pressure and Heart Health
Both amino acids can meaningfully lower blood pressure, with reductions comparable to what you’d expect from dietary changes or starting an exercise program. A pooled analysis of 11 placebo-controlled trials found that arginine supplementation (typically 4 to 24 grams per day for two to four weeks) lowered systolic blood pressure by about 5.4 mmHg and diastolic by about 2.7 mmHg. Daily doses used in cardiovascular studies generally range from 3 to 12 grams.
Citrulline’s blood pressure effects are a bit less consistent across studies but still notable. One meta-analysis of 15 trials found reductions of about 7.5 mmHg systolic and 3.8 mmHg diastolic, while a more conservative analysis of eight trials put the numbers closer to 4.1 and 2.1 mmHg. Doses in these studies ranged from about 3 to 9 grams daily over one to 17 weeks. The variation likely reflects differences in study populations, with larger effects showing up in people who had elevated blood pressure to begin with.
Exercise Performance and Recovery
Citrulline malate (citrulline bonded with malic acid) is the form most studied for exercise. In one well-known trial, participants taking citrulline malate before a resistance training session completed significantly more repetitions as the workout progressed, with the gap widening set by set. By the final set, the citrulline group performed nearly 53% more reps than the placebo group. Muscle soreness at 24 and 48 hours after training dropped by about 40%.
The mechanism here ties back to blood flow: more nitric oxide means more oxygen and nutrient delivery to working muscles, plus faster clearance of metabolic waste products like ammonia and lactate. For athletic use, studies have tested acute doses of 8 grams of citrulline malate before training, or daily doses of 2.4 to 6 grams of citrulline over one to two weeks. Results with acute dosing have been mixed, suggesting that loading up over several days may work better than a single pre-workout dose.
Arginine on its own has been less impressive for exercise. Studies using doses up to 6 grams in recreational and trained athletes generally haven’t shown clear improvements in physical performance or nitric oxide production, likely because so much of it gets broken down before reaching the bloodstream.
Erectile Function
Because erections depend heavily on blood flow, and nitric oxide is the key signal that relaxes the smooth muscle in penile blood vessels, both amino acids have been studied for erectile dysfunction. A double-blind trial of 100 men with blood-flow-related erectile dysfunction found that high-dose arginine supplementation over three months improved erectile function scores by about 20% compared to baseline, while the placebo group saw no change. Men with mild to moderate dysfunction saw the most reliable improvements.
Citrulline has also shown promise in smaller studies, again likely because it raises arginine levels more efficiently. The effect isn’t as strong as prescription medications, but it represents a meaningful improvement for men with mild symptoms or those looking for a complementary approach.
Typical Doses
For cardiovascular benefits, most studies have used 3 to 12 grams of arginine daily or 3 to 10 grams of citrulline daily, taken for at least four weeks. For exercise performance, 6 to 8 grams of citrulline malate is the most common pre-workout dose, though taking 3 to 6 grams daily for a week or two before relying on acute dosing appears to produce more consistent results.
Higher doses of arginine (above roughly 10 grams at once) tend to cause gastrointestinal side effects: nausea, bloating, abdominal pain, and diarrhea. Citrulline is generally better tolerated at equivalent doses, partly because it bypasses the gut metabolism that arginine goes through.
Safety Considerations
Because both amino acids lower blood pressure through nitric oxide, combining them with blood pressure medications can cause blood pressure to drop too low. The same risk applies when pairing arginine with nitrate medications (used for chest pain) or with erectile dysfunction drugs like sildenafil, which also work through the nitric oxide pathway. If you’re taking any of these, the interaction is worth discussing before adding either supplement.
One specific caution: arginine can potentially reactivate the herpes simplex virus, the one responsible for cold sores and genital herpes. The virus uses arginine to replicate, so significantly boosting arginine levels may trigger outbreaks in people who carry the virus. Citrulline raises arginine indirectly, so the same concern applies in theory, though it’s less well studied.