L-Arginine

What it is

L-arginine is an amino acid that your body can make itself, but not always in sufficient quantities. It is therefore considered semi-essential. You take it in with every protein-rich food, such as nuts, pumpkin seeds, meat, fish and legumes.

As a supplement, arginine is usually sold as the free base or as the hydrochloride, as powder or capsules. It is often found in pre-workout blends, in blood pressure products or in potency preparations, there often together with citrulline or plant extracts.

How it is supposed to work

Arginine is the starting material from which the enzyme NO synthase makes nitric oxide. NO relaxes the smooth muscle in the vessel wall, the vessels widen, blood flows more easily and pressure falls. The same mechanism is behind an erection. This is well-understood physiology.

The catch lies in the gut. In a study with 10 healthy volunteers, only about 20 % of 10 g of swallowed arginine reached the circulation. A large part is already broken down in the intestinal wall by the enzyme arginase. This is exactly where citrulline comes in: a double-blind study with 20 volunteers showed that the precursor raises arginine levels in the blood more strongly than arginine itself, in a dose-dependent manner. If you are interested in the highest arginine level, citrulline is the more efficient choice according to these data.

That arginine works nonetheless is shown by the blood pressure data. Apparently the amount absorbed is enough to change something at the vessel wall. How much of this is due to NO and how much to other pathways has not been broken down in humans.

It is notable that the effect does not only show up in the doctor’s office. An analysis of 5 studies with 202 participants that included only 24-hour measurements found the reduction throughout the day and also at night: systolic minus 5.70 mmHg during sleep. Single measurements at the doctor’s can be distorted by nervousness; long-term measurements are less susceptible to this. The number of studies is small, however, and the authors themselves call for further confirmation.

Arginine or citrulline

The two substances are often lumped together, but they take different routes to the same goal. Arginine is the end product, but it is heavily broken down in the gut. Citrulline is the precursor, is well absorbed and is converted into arginine only in the kidney.

For blood pressure, arginine has the larger and more stable data base. For training performance it is the other way round: for citrulline there are several meta-analyses with a small but repeatedly found effect, for arginine a single, methodologically disputed one. Many combination products rely on both, but there are hardly any studies of their own on such blends.

What the studies show

2022 meta-analysis: 22 trials on blood pressure

Shiraseb and colleagues analyzed 22 placebo-controlled trials with 30 effect sizes, all with oral intake over at least 4 days. Systolic blood pressure fell by 6.40 mmHg, diastolic by 2.64 mmHg. The dose-response analysis showed an effective range for systolic pressure from 4 g daily. In individual subgroups, for example above 9 g daily or in obesity, the reduction was not significant.

2019 meta-analysis: erectile dysfunction

Rhim and colleagues pooled 10 randomized trials with 540 men who had mild to moderate erectile dysfunction. Arginine alone or in combination improved symptoms considerably more often than placebo. Side effects occurred in 8.3 % under arginine and 2.3 % under placebo, none of them serious.

VINTAGE MI 2006: trial stopped because of deaths

Schulman and colleagues gave 153 patients after a first heart attack 3 g of arginine 3 times daily or placebo for 6 months. Vascular stiffness and the heart’s pumping function did not improve. In the arginine group 6 patients died, in the placebo group none, and recruitment was ended for safety reasons.

2020 meta-analysis: athletic performance, disputed

Viribay and colleagues found a large effect on endurance performance and a small one on short, intense exertion in 15 studies. The endurance figure rests on studies that strongly contradict each other. In 2025, a commentary pointed out methodological errors that distort the pooled values.

Where the data stop

For blood pressure the direction is clear, but the individual studies are mostly small and short. Whether the lowered values also lead to fewer heart attacks or strokes has never been studied. For people after a heart attack there are even contrary findings: VINTAGE MI found a mortality signal, a meta-analysis of 2 studies with 927 participants no benefit. Anyone with heart disease belongs in a physician’s hands when it comes to arginine.

For sport there is only the one meta-analysis from 2020, and it is methodologically disputed. For the pump, reliable data are lacking. For erections, many studies come from combination preparations with differing doses and durations, and direct comparisons show PDE5 inhibitors alone to be more effective than arginine alone. Long-term data beyond months are largely lacking for healthy people.

Status, approval and legal

In Germany, arginine is sold as a food supplement. For a powder with 1.6 g of arginine and 65 mg of citrulline per day, the Federal Office of Consumer Protection and Food Safety issued a general ruling (Allgemeinverfügung) in 2013, with mandatory warnings for children, pregnant and breastfeeding women and people after a heart attack. There is no official maximum amount from EFSA or the BfR. For competitive athletes, the general advice of the National Anti Doping Agency applies: food supplements can be contaminated, and the Cologne List offers guidance without a guarantee.

Safety

For healthy people, arginine is well tolerated. A risk assessment considers safety up to 20 g daily to be well supported. The typical side effect is diarrhea: single amounts of 3 to 6 g rarely caused complaints; most occurred from single amounts above 9 g and disappeared when the amount was spread over the day. After an acute heart attack, arginine is not indicated. The German general ruling requires consultation with a physician in case of blood pressure medication, a low-protein diet and kidney or liver disease. Because arginine itself lowers blood pressure, it can enhance the effect of blood pressure medication.

Sources

• Shiraseb et al., Adv Nutr 2022 – dose-response meta-analysis, blood pressure
• Dong et al., Am Heart J 2011 – meta-analysis, blood pressure
• Shi et al., Nitric Oxide 2026 – meta-analysis, 24-hour blood pressure
• Rhim et al., J Sex Med 2019 – meta-analysis, erectile dysfunction
• Xu et al., Andrologia 2021 – arginine and PDE5 inhibitors compared
• Schulman et al., JAMA 2006 – VINTAGE MI
• Viribay et al., Nutrients 2020 – meta-analysis, athletic performance
• Trexler, Nutrients 2025 – methodological critique of Viribay 2020
• Tangphao et al., Br J Clin Pharmacol 1999 – pharmacokinetics
• Schwedhelm et al., Br J Clin Pharmacol 2008 – citrulline and arginine compared