One of the two anabolic steroids for which Alex Rodriguez tested positive, according to published reports, was Primobolan, a drug that is illegal to sell or market in the United States.

The United States Food and Drug Administration has banned the drug from entry into the United States. In July 1989, the F.D.A. sent out a border alert preventing a list of more than 40 anabolic steroids that can be used as performance-enhancers from being brought into the country. Primobolan was on that list.

“You can’t prescribe it in the United States, so if you test positive for it here, there’s no getting away with it,” said Dr. Gary I. Wadler, an antidoping expert and member of the World Anti-Doping Agency. “It’s also not manufactured by the human body, so if it shows up, it’s like flashing red lights going off. You’re caught with your pants down.”

In an interview Monday with ESPN, Rodriguez said he was unaware of the kinds of drugs he had taken during the 2001-3 seasons. A report released Saturday by SI.com said he tested positive in 2003 for testosterone and Primobolan.

Some anabolic steroids, like testosterone, may be prescribed by doctors as replacement therapy for men with a hormone deficiency. Others may be prescribed for women with gynecological conditions or breast cancer.

But Primobolan does not fit those categories. It is a black-market drug, taken primarily by injection but also in tablet form.

Wadler said the injectable form of the drug could be detected for just short of two weeks, and the oral drug might be detectable for less than a day, depending on the dose. The performance-enhancing effects can last for months, experts said.

Terry Todd, a former champion power lifter, said Primobolan had long been a popular drug among athletes. Todd is director of the H. J. Lutcher Stark Center for Physical Culture and Sports at the University of Texas.

Todd said that Primobolan was known for building strength without much muscle bulk, and also for having few side effects. He explained that male athletes using Primobolan would not have enlarged breasts, shrunken testicles or a higher voice because the drug, unlike some other steroids, did not convert to estrogen after it was taken.

“It’s a drug that you take when you don’t want all of the behavior changes that may go along with other steroids,” Todd said. “But still, you would also have to feel that you wouldn’t be tested, or that there would be no consequences to you getting caught. It’s not a miracle pill.”