5-HTP (5-Hydroxy-Tryptophan) is sold over-the-counter across the United States and can be purchased in a variety of supplements. Its sale is weakly controlled by the US laws regarding 'nutritional supplements' (primarily the 1994 "Dietary Supplement Health & Education Act") and is included in a wide variety of nutritional "health" supplements.

L-Tryptophan now appears to be legal to sell as a supplement in the US. L-Tryptophan was controlled for sale in the United States (for human consumption only) by the FDA in 1989, due to association with a series of deadly and disabling symptoms. See Ceri.com's overview of this. Pure L-Tryptophan has been available in the United States as a veterinary supplement and in other forms not intended for general human consumption. Since 2001, more and more human-consumption-targeted products have shown up on the U.S. market and the defacto status of L-Tryptophan supplements appears to have changed.

There is some confusion added by a statement by the FDA from February, 2001 which states:

Although FDA continues to enunciate its concern about the safety of dietary supplements containing L-tryptophan and related compounds such as L-5-hydroxytryptophan, this does not mean that FDA prohibits the marketing of dietary supplements that contain L-tryptophan. Under the Food, Drug and Cosmetic Act (the Act), as amended by the Dietary Supplement Health and Education Act of 1994 (DSHEA), the manufacturer is responsible for ensuring that its products are safe. A firm is not required to obtain premarket review or approval from the FDA of its products before marketing them as dietary supplements. Moreover, a firm is not required to submit scientific evidence to FDA of the safety of its products or ingredients. While we are unaware of conclusive scientific data that would establish that a dietary supplement L-tryptophan would be safe, if a firm has information that it believes establishes that a product containing L-tryptophan is safe within the meaning of the Act, it could market such a product as a dietary supplement. The burden and responsibility for assuring that such a product is not adulterated under the Act is with the firm and not FDA.

This may mean that the FDA has softened its prohibition on the sale of L-Tryptophan containing supplements and these may now be considered 'legal' in the US. Importation of L-Tryptophan is still blocked under this rule.