Pet shop clerks throughout Tijuana acknowledge that foreigners regularly inquire about the drug. “We’ve probably had 100 people come in asking for the drug in the last couple years,” said Pepe Velazquez, a veterinarian and owner of El Toro pharmacy.

Until El Norte, a regional newspaper, published an article recently that detailed how easy it was to buy pentobarbital  and how foreigners intended to use it  many store owners and clerks said they assumed the customers were using the drug to end the lives of their animals.

“We didn’t have any idea what they were doing,” said a sales clerk at a pet shop called California. “It’s for animals. Everything here is for animals. We thought they were giving it to their animals.”

It turns out that they were buying it for human consumption. Mr. Nitschke estimates that 300 members of his group, most of them from Australia but some from the United States and Europe, have bought the drug in Mexico in recent years. Some save it for when their health fails to the point that they no longer wish to live. In a few instances, buyers took the drug while in Mexico.

“To witness it, it looks as peaceful as can be,” Mr. Nitschke said of death by pentobarbital. “I usually recommend that they take it with their favorite drink since it has a bitter taste. I’ve never seen anyone finish their whiskey or Champagne. There isn’t enough time to give a speech. You go to sleep and then you die.”

But now that word is out that the drug is being used for human consumption, local authorities are seeking to clamp down on unauthorized purchases. Shops are now supposed to sell the drug only to licensed veterinarians who present a prescription.