-- Federal officials announced Thursday a crackdown on the supplement andro, which gained fame after baseball player Mark McGwire used the product in his record-setting 1998 season.

The Food and Drug Administration said that manufacturers of andro products will have to cease production unless the makers can prove it is safe. Although andro is not a steroid and is marketed as a dietary supplement, the FDA said the health risks are the same.

The agency sent letters Thursday to 23 companies that distribute andro products, warning they could face government action if they didn't follow the recommendations.

"While andro products may seem to have short-term benefits, the science shows that these same properties create real and significant health risks," said FDA Commissioner Mark McClellan.

"Anyone who takes these products in sufficient quantities to build muscle or improve performance is putting himself or herself at risk for serious long-term and potentially irreversible health consequences."

Andro, or androstenedione, is a hormone that is made naturally in the human body during the production of testosterone and estrogen. Some advocates say higher levels of testosterone will enable athletes to train harder, build bigger muscles and get results faster. Concentrated levels of andro are in the pills and supplements on the market.

But medical studies have shown it can have serious side effects. According to the Mayo Clinic, andro supplement use by males has been responsible for diminished sperm production, shrunken testicles, enlarged breasts, acne and decreased levels of the heart-friendly kind of cholesterol.

Officials said the move against andro also was prompted by concerns that children have been influenced by professional athletes' use of performance-enhancing supplements and the ease with which young people can buy them. Studies have shown such supplements can stunt children's physical and sexual growth.

About one out of 40 high school seniors reported that they had used andro in the past year, according to a 2002 Department of Health and Human Services survey.

"Young people, athletes and other consumers should steer clear of andro because there are serious, substantial concerns about its safety," Health and Human Services Secretary Tommy Thompson said in a statement Thursday.

"Young people should understand that there are no short cuts to a stronger body and that the best way to get faster and stronger is through good diet, nutrition and exercise."

Unlike drugs, supplements do not have to be proven safe before going on the market, but federal authorities can act to take them off the shelves if the product's safety is called into question. The FDA blocked the sale of ephedra last year, the first time U.S. officials had taken such action against a nutritional supplement.

The FDA's McClellan has said that his agency is stepping up its scrutiny of the health effects of supplements and that new manufacturing and labeling regulations are in the works.

Meanwhile, Sen. Joseph Biden, D-Delaware, has sponsored legislation that would ban the newly discovered steroid THG and andro, which baseball's McGwire admits he used the season he broke the long-standing single season home run record.

"[Drug companies] have these precursors to steroids that have the same properties and also diminish health," Biden said Thursday on CNN's "American Morning." "I have had a bill for the past two years with Sen. [Orrin] Hatch that says human hormone supplements, andro and all these things should be treated like steroids."

The FDA's action comes a day after a congressional hearing in which Republicans and Democrats alike sharply criticized Major League Baseball's lax steroid testing of athletes.

At the hearing, Major League Baseball Commissioner Bud Selig conceded that his sport has not done enough to curb use of steroids and other enhancement drugs. Selig told lawmakers he would be willing to reconsider current policy.