TIJUANA, Mexico— As tourists cross the border from San Diego, they are likely to be given fliers urging the aspiring Olympian or All-America athlete to stop by the nearest pharmacy and choose from what is advertised as ''Steroids: Largest Selection Anywhere, at Wholesale Prices!''
''No prescriptions necessary at Tijuana's most prestigious drugstore chain, '' proclaims an advertisement of the Roma and Regis pharmacies offering Anavar, Methandrosten and Maxibolin, payable with Visa or Mastercard. ''It is legal to bring back drugs to the U.S. if bought for your personal use only.''
United States Customs Service officials say that, to the contrary, steroids are a prohibited substance that cannot be legally imported in any amount without a prescription. But the openness with which the kind of drug that led to Ben Johnson's downfall at the Seoul Olympics is hawked here demonstrates the dimensions of a problem that increasingly worries authorities on both sides of the border: the production, sale and smuggling of anabolic steroids in Mexico, largely for consumption by Americans intent on beefing up their bodies.
''Mexico is obviously a very crucial source of steroids, because a lot of manufacturing is going on there, not just of the real thing, but phony, adulterated stuff as well, '' said Michael Fleming, a United States Customs Service spokesman in Los Angeles. ''The traffic has increased because the market is very lucrative, and our concern has led us to take a very aggressive enforcement position.'' Enforcememt Is Often Lax
Over the last three years, more than 150 people have been arrested trying to smuggle steroids from Mexico into Southern California and several million dollars in steroids have been seized. United States Customs officials said that about 30 people, all reported to be involved in large-scale steroid smuggling and trafficking, are expected to be indicted shortly.
On Mexico's side of the border, the National Chamber of the Pharmaceutical Industry recently issued a statement that said, ''Mexico has seen itself invaded by these products, which are being commercialized in an irresponsible fashion.'' The association also called on authorities to ''carry out more of an in-depth effort to avoid indiscriminate sale of anabolic steroids.''
In fact, Mexico's General Health Code prohibits over-the-counter sale of restricted drugs like steroids. But enforcement is usually lax, and many Americans have taken advantage of that to cross the border to stock up, either for personal use or, more commonly, for resale back home.
''What do you prefer, tablets or injectable liquid solution?'' a saleswoman at Le Drugstore asked a prospective customer one recent Saturday morning. Profits for Smugglers
At a curio shop a block farther down Avenida Revolucion, a woman called to a passer-by, ''Hey, Senor, I know what you're looking for, '' apologized for having sold out her own stock of steroids, and then suggested several other locations where the drug could be obtained.
A pharmacist at one Regis Pharmacy here, who identified himself only as Armando, said: ''You're supposed to have a prescription, but many people take them across the border even though they don't have one. Guys have come into this store and bought $400 worth of steroids at one time.''
Even for such amateur, small-scale smugglers, there are substantial profits to be made. A 100-milliliter bottle typically sold over the counter here for less than $10 is likely to cost 10 times as much by the time it is bought by the user across the border, often a young athlete or body-builder hoping to improve performance.
Mexican and American officials say the main trafficker in steroids over the last three years has been Juan Macklis, owner of record of the Milano Pharmaceutical Laboratories here. They said he not only manufactured much of the drug in Tijuana, but also used a large network of couriers to take the drug across the border, often in false compartments of automobiles. Evidence of Links to Noriega
''Between 1987 and 1989, Laboratorios Milanos has been the largest single distributor of steroids in the U.S. marketplace and perhaps in the world, '' said Phillip Halpern, an assistant United States attorney in San Diego. ''I was arresting people of his who were trafficking in the ton-loads, so we are conservatively talking about a business that involved tens of millions of dollars.''
In April, the police here raided the laboratory and arrested Mr. Macklis. Several company employees, including Hector Salgado Herrera, chairman of the chemistry department at the University of Baja California, were also apprehended, as were two Americans who had just bought 4, 900 steroid tablets from United Pharmaceuticals of Baja California, a Macklis-affiliated company that operates out of a hotel suite here.
A senior Government official in Mexico City said the Mexican police had uncovered evidence establishing a longstanding business relationship between Mr. Macklis and Panama's military leader, Gen. Manuel Antonio Noriega. ''Here in Mexico, Noriega is an owner of a pharmaceutical company in Tijuana that is poisoning people, '' the official said.
According to Mexican and American officials, however, Mr. Macklis was released from custody and has dropped out of sight. It was not possible to determine what charges, if any, were filed against him in Mexican courts, and though he remains under indictment on drug trafficking charges in San Diego, Mexico's aversion to extraditing its nationals to the United States makes it unlikely he will ever stand trial there. Less-Stringent Export Curbs
''I've indicted him in more than one case, '' Mr. Halpern said. ''I could probably have indicted him 50 times, but why waste my time?''
American pharmaceutical companies have also emerged as an important source of the steroids sold over the counter here. While restrictions on the sale of steroids in the United States are stringent, export regulations are less so, and several laboratories in California and Arizona have been selling the drug to clients here, who then sell them to American customers, often without even bothering to remove the American price tag or warning that says, ''Federal law prohibits dispensing without prescription.''
''They know for sure what they are doing, but even if they do know, it is not a crime, because you would have to show they conspired with a person to evade the law, '' Mr. Halpern said. ''I'm not aware of any prosecutor who would try and make such a case.''