The best nitric oxide supplement depends on your goal, but L-citrulline is the strongest all-around choice for most people. It raises blood levels of arginine (the raw material your body uses to produce nitric oxide) more effectively than taking arginine itself, and it does so with fewer digestive side effects. Beetroot juice concentrate is a solid second option, working through a completely different pathway, and some people benefit from combining the two.
Why L-Citrulline Beats L-Arginine
This surprises most people: if you want more arginine in your blood, taking citrulline works better than taking arginine directly. When you swallow L-arginine, your gut and liver break down a large portion of it before it ever reaches your bloodstream. L-citrulline bypasses that breakdown. Your kidneys convert it into arginine efficiently, giving you a sustained rise in the amino acid your body needs to make nitric oxide.
A pharmacokinetic study comparing citrulline formulations found that citrulline HCl (a salt form) converted to arginine even more efficiently than free-form L-citrulline, with the conversion ratio nearly tripling at lower doses. At a 2-gram dose of citrulline HCl, relative arginine bioavailability was over 200% greater than an equivalent amount of free-form L-citrulline. The practical takeaway: you don’t necessarily need massive doses if you pick the right form. That said, free-form L-citrulline at higher doses (6 grams or more) still produces strong results and is the most widely studied version.
L-arginine also causes more stomach problems. Single doses above 9 grams commonly trigger nausea, cramping, and diarrhea, especially in healthy, active people. L-citrulline rarely causes GI distress at the doses used in research.
Beetroot Juice: A Different Pathway
Your body makes nitric oxide two ways. The first is the arginine pathway that citrulline feeds into. The second converts dietary nitrate (found in beets, spinach, and arugula) into nitric oxide through bacteria on your tongue and chemical reactions in your stomach. Beetroot juice supplements tap into this second pathway.
An umbrella review of beetroot juice studies found it significantly improved time to exhaustion during exercise, with a small but consistent effect. It also reduced oxygen cost during exercise by about 4% compared to an equivalent dose of sodium nitrate alone, likely because the natural antioxidants in beets help protect the nitric oxide once it’s produced. The effect on VO2max was statistically significant but negligible in size, so beetroot is better thought of as an endurance aid than a raw fitness booster.
Beetroot supplements come as concentrated juice shots, powders, or capsules. Look for products that list the nitrate content in milligrams or millimoles. Most positive studies used around 6 mmol of nitrate, which translates to roughly 400 mg of dietary nitrate per serving.
Doses That Actually Work
For L-citrulline, the effective range is 3 to 6 grams per day. Here’s how that breaks down by goal:
- Exercise performance: 6 to 8 grams about an hour before training. One study found that 6 grams daily for seven days allowed participants to sustain higher-intensity exercise significantly longer. Another showed 2.4 grams daily for eight days improved cycling speed and reduced fatigue.
- Blood pressure support: 3 to 8 grams daily. Studies on blood pressure have used doses across this range, with reductions of roughly 6 points systolic and 6 points diastolic observed in people with elevated readings.
- Erectile function: Even 1.5 grams daily for a month showed promising results in a small study, though higher doses are more common in practice.
If you’re using citrulline malate (citrulline bonded to malic acid), note that about 40% of the weight is malic acid. So a 6-gram dose of citrulline malate delivers roughly 3.6 grams of actual citrulline. Adjust accordingly.
Combining Citrulline With Glutathione
One of the more interesting findings in nitric oxide research is that pairing citrulline with glutathione, your body’s main internal antioxidant, produces better results than citrulline alone. The logic is straightforward: citrulline helps you produce more nitric oxide, while glutathione protects that nitric oxide from being destroyed by oxidative stress before it can do its job.
Four double-blind, placebo-controlled trials found the combination enhanced and sustained nitric oxide production and improved blood vessel dilation significantly better than citrulline by itself. If you’re looking to maximize your supplement’s effect, adding 200 to 1,000 mg of reduced glutathione (sometimes labeled GSH or Setria glutathione) is worth considering.
What to Avoid and Watch For
Nitric oxide supplements interact with medications that also lower blood pressure or affect blood vessel dilation. The most important ones to know about are PDE5 inhibitors used for erectile dysfunction (sildenafil and tadalafil) and prescription nitrates used for chest pain. Combining these with a nitric oxide booster can cause a dangerous drop in blood pressure. There are 14 major drug interactions documented with nitric oxide, and people with heart failure should be especially cautious.
At reasonable doses (under 9 grams in a single sitting), side effects from citrulline and arginine are uncommon. If you do experience stomach upset, splitting your daily dose into two or three smaller servings typically resolves it. Beetroot supplements can turn your urine and stool reddish, which is harmless but startling if you’re not expecting it.
Choosing a Supplement: What to Look For
The supplement market is crowded with nitric oxide products that combine five or six ingredients at doses too low to do anything individually. A better approach is to pick one or two proven ingredients at clinical doses. Here’s a practical framework:
- For exercise and general vascular health: 6 grams of L-citrulline (not citrulline malate unless the label specifies 6 grams of citrulline content) taken about 60 minutes before training, or split into two daily doses on rest days.
- For endurance sports: Beetroot juice concentrate providing at least 400 mg of dietary nitrate, taken 2 to 3 hours before competition. This can be stacked with citrulline since they work through separate pathways.
- For sustained nitric oxide levels: Citrulline plus glutathione, based on the clinical evidence showing better durability of nitric oxide when both are present.
Third-party testing matters here. Look for products certified by NSF International, Informed Sport, or USP. These certifications confirm that the product contains what the label claims and isn’t contaminated with banned substances or heavy metals. Unflavored L-citrulline powder is typically the most cost-effective option, running $15 to $25 for a month’s supply at 6 grams daily.