TIJUANA, Mexico (AP) – At the Viva Tijuana Plaza, 100 yards from the U.S. border, Americans buy everything from steroids to Viagra, mostly without a prescription. But even in this pharmaceutical mecca, it’s nearly impossible to get a simple flu shot.

Americans, especially retirees living along the border, have long relied on easy access to Mexico’s cheap drugs. So when a contaminated batch of vaccines caused an unexpected shortage in the United States, Mexico was a natural place to look.

Pharmacies from Tijuana on the Pacific to Matamoros on the Gulf of Mexico have been scrambling to meet demand as Americans head south for their annual flu shot. Pharmacists say they sometimes get dozens of requests each day, but most can’t fill the orders.

Mexican health authorities said pharmacies and private clinics can sell the vaccine with a prescription from a Mexican doctor, although drugstores often ignore that rule and require no prescription. Customers must give themselves the shot or pay a doctor to do it.

But pharmacies weren’t expecting such high demand and are now having trouble getting more supplies from international drug makers.

Americans were having more luck crossing the other U.S. border – into Canada, where there was a much more abundant supply and where some Canadian cities were even holding flu shot clinics for their desperate American neighbors.

Flu shots are a relatively new idea in Mexico, where many people can’t afford the $20 shot. Mexican drug producers don’t generally make the vaccine, but the government decided this year for the first time to have a government-run lab produce 8.5 million doses to give out at government clinics.

The Mexican vaccine will be available to children from 6-23 months and for people 65 and over. But health officials said the Mexican lab didn’t produce enough vaccines to meet demand at private clinics or to sell outside Mexico.

That won’t help the millions of Americans still desperate for protection from the flu. Most border pharmacies that do sell the vaccine have already run out.

“We decided to order some (flu vaccines) because people have been coming in and asking to get them,” said Valdemaro Ascencio, a pharmacy clerk at Smart Buy Drug Store in Tijuana. “It’ll probably be a few days before we have them.”

Medicines in Mexico are generally 60 percent cheaper than on the U.S. side and are often sold without a prescription. While drugs are cheap and easy to get, pharmacies are often staffed by people without formal training.

That can mean customers sometimes get the wrong prescription. Last year, during a flu outbreak, some Americans bought vaccine for the wrong strain of flu.

Although there have been isolated reports of Mexican pharmacies selling fake medicines, that hasn’t stopped the flow south of people seeking cheaper health care, especially in the winter months when an estimated 15,000 people cross daily into Mexico.

U.S. officials have often warned that the effectiveness and safety of medicines sold in Mexico can’t be guaranteed.