Nitric oxide supplements: study doses of citrulline and arginine

Most nitric oxide boosters are built on two amino acids, L-citrulline and L-arginine, and the studies behind them used grams a day. Citrulline trials in a 2019 meta-analysis required at least 3 g, and the pooled effect on strength and power was small. An arginine meta-analysis found blood pressure changes at doses from 4 to 24 g a day. A product that hides its amounts inside a blend of a few hundred milligrams cannot show that it matches any of this.

This guide sets out what those studies did, what they found, and how to read a booster's label. It is general information, not medical advice. If you have blood pressure, heart or erectile problems, or you take medicine, ask a doctor or pharmacist before using an amino acid supplement.

How do L-citrulline and L-arginine relate to nitric oxide?

The body uses L-arginine as a raw material for nitric oxide, a molecule involved in relaxing blood vessels. Supplement makers reason that taking more arginine should raise nitric oxide. The catch, described in a 2011 erectile-function study, is that oral arginine is hampered by extensive first-pass metabolism, meaning much of it is broken down in the gut and liver. L-citrulline escapes that step and is converted to arginine, which is why citrulline became the favored ingredient.

That idea was tested in a 2008 double-blind, randomized, placebo-controlled crossover study of 20 healthy volunteers. Citrulline raised plasma arginine more effectively than arginine itself, in a dose-dependent way. The highest citrulline dose, 3 g twice a day, improved the ratio of arginine to ADMA (a natural inhibitor of nitric oxide production) from 186 to 278, and raised urinary nitrate and cGMP, two markers of nitric oxide signaling. But no treatment improved flow-mediated dilation, a test of blood vessel function, over baseline. So the chemistry moved in the expected direction, and the vessel test did not clearly follow.

What doses did the studies use?

The numbers vary, but they are all in grams:

• Citrulline, 1.5 g a day: the dose in a small 2011 study of 24 men with mild erectile dysfunction.
• Citrulline, 3 g twice a day: the top dose in the 2008 pharmacokinetic study above.
• Citrulline, at least 3 g, taken 30 minutes or more before exercise: the inclusion rule for the 2019 strength and power meta-analysis.
• Arginine, 4 to 24 g a day: the range in the 2011 blood pressure meta-analysis of 11 trials.
• Arginine, 0.15 g per kg of body weight: the acute dose that a 2020 review of athletic performance proposed, taken 60 to 90 minutes before. For a 70 kg adult, that is 10.5 g.

The same 2020 review suggested 1.5 to 2 g a day for 4 to 7 weeks for aerobic performance and 10 to 12 g a day for 8 weeks for anaerobic performance. Those are the authors' own conclusions from pooled studies, not approved recommendations.

How big were the results?

Small, and sometimes uncertain. The 2019 citrulline meta-analysis pooled 12 studies (13 samples, 198 participants) on high-intensity strength and power. It found a significant benefit over placebo, with a pooled standardized mean difference of 0.20 (95 percent confidence interval 0.01 to 0.39). The authors called the effect size small, and noted that the confidence interval for each individual study crossed the line of no effect. They suggested it might matter to high-level athletes, where small margins decide results.

The 2020 arginine review found a small anaerobic effect (0.24) and a large but highly inconsistent aerobic one (0.84, confidence interval 0.12 to 1.56, with high heterogeneity). The 2011 blood pressure meta-analysis found systolic pressure 5.39 mm Hg lower and diastolic 2.66 mm Hg lower than placebo. That is a measurable change, and a reason to speak with a pharmacist if you take blood pressure medicine.

The erectile-function study needs context too. In that single-blind study of 24 men with mild erectile dysfunction, everyone took placebo for a month and then citrulline for a month. Erection hardness improved from mild to normal in 2 of 24 men (8.3 percent) on placebo and 12 of 24 (50 percent) on citrulline. The authors described citrulline as less effective than prescription drugs of the PDE5 inhibitor class, and said its role deserves further research. It was a small, short study, so it cannot support a promise about any product.

How do booster labels compare with those doses?

Four of our reviews show the gap between a study and a scoop.

The CircO2 review covers a tablet whose four active ingredients share one 420 mg proprietary blend, while the trials on citrulline used 3 to 9 g a day. The headline numbers on its page came with no study attached.

The FLOW Nitric Oxide Booster review covers a capsule whose main ingredient is L-citrulline, and the company cites the 1.5 g erectile-function study. But FLOW's whole four-ingredient blend totals 850 mg, so even if citrulline were the entire blend it would fall short of the studied dose. The review also flags that the free bottle leads into a monthly subscription, with a price that differed between the sales page and the checkout page.

The Nitric Boost review covers a powder with eight ingredients and no stated amount for any of them. Its own centerpiece study used 6 g of L-arginine a day taken alone, and the powder splits an unknown, probably smaller amount of arginine across seven other ingredients. The VenoPlus 8 review makes a related point about a 5.4 g scoop holding ten ingredients: a formula can list the right names and still not reach the amounts researchers used.

What should you check on a nitric oxide booster?

1. Look for a milligram amount next to citrulline or arginine. A blend total does not tell you how much of each is inside.
2. Read the serving size. The panel amount is for the whole serving, which may be two capsules or a scoop.
3. Compare with the study doses above, and check that the study tested the same ingredient alone, not a mixed formula.
4. Watch for a free bottle. Read the terms for the monthly charge that follows, and see our ClickBank refund guide for how to claim.
5. Ignore percentage promises with no study named, such as figures for how much nitric oxide rises.
6. Ask a professional if you have a heart, blood pressure or erectile problem. A doctor can check for causes that no supplement addresses.

You can browse audited offers in the men's health category and the heart health category. Our how we review page explains how doses, price and refund terms feed the scores. We audit the offer, not our own experience with it.

FAQ

Is L-citrulline better than L-arginine?

In the 2008 crossover study, citrulline raised plasma arginine more effectively than arginine did, because oral arginine is largely broken down before it reaches the blood. That does not prove better results in a person, and neither one improved a vessel function test over baseline in that study.

How much L-citrulline was used in studies?

Between 1.5 g a day in a small erectile-function study and 3 g twice a day in a pharmacology study. The 2019 exercise meta-analysis only included studies with at least 3 g.

Do nitric oxide boosters lower blood pressure?

A 2011 meta-analysis of oral L-arginine found reductions of 5.39 mm Hg systolic and 2.66 mm Hg diastolic, at doses from 4 to 24 g a day. If you take blood pressure medicine, ask your pharmacist before adding arginine.

Can a booster replace erectile dysfunction treatment?

Nothing here says so. The one small study we cite found citrulline less effective than prescription PDE5 inhibitors, and erectile problems can signal other health issues. Talk to a doctor.