The tag on her white lab coat read “professional pharmacist,” and the framed health and safety certificates lining the walls behind her gave the drugstore an air of legitimacy.

That pretense faded seconds later, when she was asked for controlled medications — and got on her hands and knees to pop open a hidden panel under the counter. She rooted around for a minute and emerged with two sealed bottles.

“These are from licensed laboratories,” she said. “The problem is when you’re buying from a laboratory that’s not certified.”

One of those bottles — sold as Adderall — tested positive for methamphetamine.

In pharmacy after pharmacy in this Mexican resort city, workers offered similar assurances, but time and again the pills proved to be fakes. There were oxycodone pills that tested positive for heroin and over-the-counter cough medicine, and Vicodin tablets that turned out to be fentanyl. Pills sold as Adderall were sometimes methamphetamine or caffeine, and sometimes simply an appetite suppressant.

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Aside from the glut of willing sellers and suppliers, another roadblock to reining in the sale of counterfeit pills in Mexico is the constant demand — often from Americans looking for medications that may be cheaper or easier to get than in the U.S., where opioid painkillers are tightly controlled and ADHD medications are scarce due to a years-long shortage. In recent months, several people prescribed ADHD drugs told The Times they’d purchased or considered purchasing their medications in Mexico.