Two large sports nutrition companies struck out with the Federal Trade Commission yesterday over a purported muscle-building supplement made famous by baseball slugger Mark McGwire.

To settle complaints brought by the FTC, California-based MET-Rx USA Inc. and Colorado-based AST Nutritional Concepts & Research Inc. agreed to include safety warnings in their ads and on their labels for supplements that contain androstenedione (more commonly known as "andro").

The warnings will note that these over-the-counter supplements contain "steroid hormones that may cause breast enlargement, testicle shrinkage, and infertility in males, and increased facial and body hair, voice deepening and clitoral enlargement in females." Consumers who are at risk for prostate or breast cancer will be advised not to use the product at all.

Available for about four years, andro supplements gained popularity when it was disclosed that St. Louis Cardinals first baseman McGwire used them in 1998 when he made baseball history by hitting 70 home runs in a season.

With that revelation, sales soared to the point that some health stores reported empty shelves of andro supplements as bodybuilders, workout enthusiasts and weekend athletes bought the product in record numbers. According to Nutrition Business Journal, based in San Diego, sales of andro, which totaled only $2 million in 1995, climbed to $15 million in 1997 and $25 million in 1998.

Health store owners and sports nutrition companies say sales have leveled off considerably since 1998, perhaps partly because McGwire admitted this summer that he had quit using andro. McGwire, who had said he had taken andro to enhance his workouts, said he quit using the supplements because he didn't want to send the wrong message to kids. "Young kids take it because of me," McGwire said in one interview. "I don't like that. I discourage young people from taking it."

Under the Dietary Supplement Health and Education Act of 1994, the product can be sold as a dietary supplement because it occurs in nature and isn't already licensed as a drug. But the Drug Enforcement Administration is reviewing andro supplements to see if they should become controlled drugs. "It all hinges on whether it produces muscle growth," said DEA spokeswoman Rogene Waite.

Andro supplements are still permitted by Major League Baseball but have been banned by the National Football League, the National Collegiate Athletic Association and the International Olympic Committee. Major League Baseball officials did not return calls yesterday but have previously indicated that they are waiting for the results of a requested study being done at Harvard University before they decide whether to impose a similar ban.

Andro "sales peaked with the Mark McGwire hype and rapidly dissipated," said Scott Connelly, chairman and founder of MET-Rx. Connelly, who is also a physician, said he believed the andro supplements posed very little risk. But he said he agreed to the FTC warning because "as a physician, if there is any potential risk to anything, then I'm duty bound to declare it. The probability is not zero, but I would guess close to zero."

AST President Paul Delia said he believed the settlement "will pave the way for more in-depth research into the safety of sports nutrition supplements."

Androstenedione is a steroid hormone, one metabolic step away from testosterone. Made naturally in the body, it is converted in the liver to testosterone, which can boost energy, help build muscle and speed recovery from exercise and sports injuries.

There have been few studies about androstenedione, although a recent one by Iowa State University said it didn't increase strength but could increase the risk of heart disease, pancreatic cancer and breast enlargement.

The FTC accused the companies of making unsupported safety claims about andro. AST called its andro products "completely safe and very effective." MET-Rx said there was "little in the way of side effects." The FTC said both companies lacked scientific data to make those claims.

CAPTION: Cardinals slugger Mark McGwire used an andro supplement during his 70-home-run campaign in 1998.