The amino acid arginine can be synthesized by humans, making it nonessential. Its presence in the diet, however, may facilitate optimal protein synthesis. Most of the arginine is incorporated into proteins, but the free amino acid has several important functions. It is an intermediate in the metabolic cycle that leads to urea being excreted in urine.

The effect of arginine on growth hormone levels has interested body builders. In a controlled trial, when arginine and ornithine (500 mg of each, twice per day, five times per week) were combined with weight training, a greater decrease in body fat was obtained after only five weeks, than when the same exercise was combined with a placebo. In another study, however, 5 grams of arginine powder, taken orally 30 minutes prior to exercise, failed to affect growth hormone release and may have even impaired the release of growth hormone in younger adults.

Arginine is also needed to increase protein synthesis, which can in turn increase cellular replication. Therefore, arginine may help people with inadequate numbers of certain cells. For example, some, though not all, studies have found that men with low sperm counts experienced an increase in the number of sperm when they supplemented with arginine.

Arginine is also a precursor to nitric oxide, which the body uses to keep blood vessels dilated, allowing the heart to receive adequate oxygen. Researchers have begun to use arginine in people with angina and congestive heart failure.

Nitric oxide metabolism is also altered in people with interstitial cystitis, a condition of the bladder. Preliminary research found that supplementation with 1.5 grams of arginine per day for six months led to a significant decrease in most symptoms, including pain, though short-term supplementation (five weeks) has not been effective, even at higher (3–10 grams per day) intakes. In 1999, a double-blind study using 1.5 grams of arginine for three months in a group of women with interstitial cystitis, reported considerable improvement compared with the effect of a placebo in a variety of indices. Perhaps due to the small size of the study, some of these changes did not quite reach statistical significance.

Preliminary evidence suggests that arginine may help regulate cholesterol levels. Arginine also appears to act as a natural blood thinner by reducing platelet aggregation.

Normally, the body makes enough arginine, even when it is lacking in the diet. However, during times of unusual stress (including infection, burns, and injury), the body may not be able to keep up with increased requirements.

Arginine has so far appeared to be free of obvious side effects. However, longer-term studies are needed to confirm its safety.

People with kidney or liver disease should consult their doctor before supplementing with arginine. Some doctors believe that people with herpes (either cold sores or genital herpes) should not take arginine supplements, because of the possibility that arginine might stimulate replication of the virus.

Administration of large amounts of arginine to animals has been found both to promote and to interfere with cancer growth. In preliminary research, high intake (30 grams per day) of arginine has increased cancer cell growth in humans. On the other hand, in people with cancer, arginine has been found to stimulate the immune system. At this time it remains unclear whether arginine is dangerous or helpful for people with cancer.

Arginine works with ornithine in the synthesis of growth hormone.

Dairy, meat and poultry, and fish are good sources of arginine. Nuts and chocolate also contain significant amounts of this amino acid.