The drug, stanozolol (sta-NOZ-ah-lol), is an anabolic steroid that has been used for decades. "It's potent. It's up there in the major leagues of anabolic steroids," said Dr. Gary Wadler of New York University, a leading expert on steroid use.
The drug is easily detected through urine testing. "Anyone who is going to be tested would have to be an utter moron to use it," said Charles Yesalis, a steroid expert at Penn State University.
He and other observers expressed surprise that a contemporary professional athlete would risk using it.
Stanozolol, occupies a familiar place in the history of sports doping.
Pending confirmation of reports that stanozolol was the muscle-building drug that the Orioles slugger used, the pattern of embarrassment and denial visits Major League Baseball.
The most famous stanozolol user was Ben Johnson, the Canadian track star who was stripped of his gold medal in the 1988 Seoul Olympics after testing positive for the drug.
"I have never knowingly taken illegal drugs nor have had illegal drugs administered to me," Johnson said a week later. A few days after that, he said that he had "never, ever knowingly taken illegal drugs."
His trainer, Charlie Francis, acknowledged that he advised Johnson to take steroids to keep up with rival athletes who were doing the same.
Though Palmeiro hasn't named the drug that was found in a urine sample taken in May, he said he never intentionally took steroids. The first baseman, who became the fourth player in history to get 3,000 hits and 500 home runs, hinted that a steroid may have contaminated a supplement he was taking.
"It was an accident," Palmeiro said in a news briefing Monday. "It was not an intentional act on my part."
Two years ago, two-time middleweight champion Fernando Vargas was charged by the Nevada Athletic Commission with taking stanozolol after it turned up in a post-fight urinalysis.
"I did not knowingly take any prohibited substance, much less a steroid," Vargas wrote in a two-page letter to the commission.
In 2003, when Major League Baseball began testing for steroids, the drug was found in 26 samples. Last year, it turned up in 11 of 12 positive tests.
Like all anabolic steroids, stanozolol is a synthetic testosterone that increases strength and decreases fat. Short-term side effects include acne, testicular shrinkage and sometimes aggressive behavior. Over longer periods, steroids may accelerate hardening of the arteries and cause liver damage.
Stanozolol differs from prohormones, which convert to steroids once ingested. Prohormones such as androstenedione had been legally made in the same factories as dietary supplements until Congress passed a ban last fall.
Wadler said he can't imagine a scenario in which stanozolol, a prescription medication, could contaminate supplements made in different factories.
Stanozolol, which is also known by a trade name, Winstrol, is approved for the treatment of angioedema, a condition in which large welts develop below the surface of the skin. But since 1991, the federal government has classified it - and all other anabolic steroids - as a Schedule III drug, subject to strict penalties for abuse.
Experts say the drug is widely available on the black market. "Winstrol is a commonly used steroid in the bodybuilding community," says Harvard University psychiatrist Harrison Pope, who has studied steroid use for almost two decades.
For users, stanozolol has several appealing features. Unlike many steroids, it can be taken orally as well as injected, eliminating the need for needles. It also clears from the body relatively quickly when taken orally. In that form, Pope said, it might disappear within two months.
Experts say the drug is also relatively easy to get because it is used as a veterinary medicine. In that form, Winstrol-V, it is less tightly controlled.