The US makers of products using the steroidal hormone androgen, or androstenedione, claimed it could be taken "safely and with minimal or no negative side effects".

However, the supplement - popular with young athletes - is suspected of causing breast enlargement, testicle shrinkage, and infertility in males.

It is also feared to cause increased facial and body hair, a deepening voice and clitoral enlargement in females.

The Federal Trade Commission (FTC) settled charges against MET-Rx USA, Inc, and AST Nutritional Concepts, who must now label their products to include all these risks.

In addition, the proposed warning, which must be approved by federal judges, says that people at risk for prostate or breast cancer should avoid the product.

Heart attack, death and stroke

Androgen supplements with ephedra, a stimulant also known as ephedrine, would have to contain a warning that taking extra amounts "may result in heart attack, stroke, seizure or death".

The head of the FTC Bureau of Consumer Affairs, Jodie Bernstein, said in a statement that teenagers and athletes who use such drugs "may be threatened by long-term use of steroid hormones" like androgen.

General Barry McCaffrey, the US equivalent of the UK's drug czar, said: "No victory is worth the physical harm these substances can bring."