Commonly called andro, the product is a steroid precursor — the body uses it to make testosterone.
That means it poses the same health risks as directly using an anabolic steroid, the Food and Drug Administration says in warnings telling 23 manufacturers to cease their production.
"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," said FDA Commissioner Mark McClellan.
The FDA's newest crackdown comes as it is facing a legal challenge to its pending ban on another controversial dietary supplement, ephedra. That herbal stimulant, widely used for weight loss, is linked to 155 deaths and dozens more heart attacks and strokes.
The maker of the Stacker 2 brand of ephedra supplements, NVE Pharmaceuticals, filed suit this week in federal court in New Jersey seeking to block FDA's sales ban, which is set to begin April 12.
"We're confident that we do have a clear legal basis" for the ban, FDA spokesman Brad Stone said Thursday.
Anabolic steroids, which build muscle, are controlled substances. But andro — because it is a precursor, not the steroid itself — has long been marketed as a dietary supplement, selling over the counter. U.S. law lets dietary supplements sell with little oversight to ensure they're safe.
But the FDA is citing a seldom-used provision of that law that defines as a dietary supplement natural ingredients that were on the market before 1994 — and says manufacturers must prove that any new ingredients are safe before selling them as supplements.
Andro wasn't on the market in 1994 and thus its makers failed to follow the law's safety provision, said a spokesman for the Department of Health and Human Services.
The FDA expects its action to at least temporarily halt andro sales. Meanwhile, Congress is considering legislation sponsored by Sen. Joe Biden, D-Del., that would permanently end over-the-counter sales of andro, as well as a new steroid named THG, and subject them to the same prescription restrictions as apply to anabolic steroids.
At a Senate hearing Wednesday that addressed the use of performance-enhancers in sports, Sen. John McCain, R-Ariz., took dead aim at major league baseball's current testing policy.
"Your failure to commit to addressing this issue straight on and immediately will motivate this committee to search for legislative remedies," McCain, chairman of the Senate Commerce Committee, told players' union chief Donald Fehr.
"I don't know what they (the remedies) are. But I can tell you, and the players you represent, the status quo is not acceptable. And we will have to act in some way unless the major league players union acts in the affirmative and rapid fashion," the senator said.
McCain made the threat after Fehr refused to accept the senator's challenge to agree to the more comprehensive policy found in the NFL. McCain said sports such as baseball are "aiding and abetting cheaters" with a weak testing policy.
Andro's use skyrocketed after McGwire said he used it in 1998, the year he hit a record-setting 70 home runs for the St. Louis Cardinals. He has said he later quit the supplements.
Medical studies show andro does raise testosterone above normal levels. Side effects of elevated testosterone include acne, baldness, and a drop in the so-called good cholesterol that could lead to heart disease.
Critics are especially concerned about andro's effects if taken by children while they're undergoing puberty.