By Toni Clarke

July 26 (Reuters) - A vitamin B dietary supplement called B-50 contains two anabolic steroids and has been cited as causing unusual hair growth in women and impotence in men, U.S. health regulators said on Friday, warning consumers to avoid the product.

The product, sold by Healthy Life Chemistry By Purity First, contains the steroids methasterone and dimethazine, both of which are potentially dangerous. The ingredients are not listed on the label and should not be in a dietary supplement, the Food and Drug Administration said.

The FDA said it had received 29 reports of problems associated with use of the product, including fatigue, muscle cramping and muscle pain. They also included reports of abnormal liver, thyroid and cholesterol tests.

Women who used the product reported unusual hair growth and missed menstruation, and men reported impotence and low testosterone, the FDA said.

The FDA said Healthy Life Chemistry has refused to voluntarily recall the product, which is manufactured by Mira Health Products. Both companies are based in Farmingdale, New York. It is sold through various websites and retail stores.

The company could be liable to criminal prosecution if it continues to sell B-50, the FDA said.

A person who answered the phone at Healthy Life Chemistry said she could not to speak about the FDA's letter, but said she would ask a supervisor to return the Reuters call.

The FDA said it first became aware of the problem from a physician who treated 20 patients with similar symptoms of fatigue and muscle pain as well as abnormal lab tests. All the patients had been referred by another physician who had been prescribing a regime of dietary supplements. The one common product was the Healthy Life Chemistry product.

The FDA collected samples of the product and discovered the steroids. (Reporting by Toni Clarke in Washington; Editing by Leslie Adler)