MEXICO CITY (AP) — Mexico has shuttered 23 pharmacies at Caribbean coast resorts, six months after a research report warned that drug stores in Mexico were offering foreigners pills they passed off as Oxycodone, Percocet and Adderall without prescriptions, authorities said Tuesday.

A four-day inspection raid targeted drugstores in Cancun, Playa del Carmen and Tulum.

In March, the U.S. State Department issued a travel warning about sales of such pills, and the practice appears to be widespread.

The Navy Department said Tuesday that irregular sales were found at 23 of the 55 drug stores inspected.

The Navy said the pharmacies usually offered the pills only to tourists, and the drugstores advertised such pills, and even offered home delivery services for them.

The Navy said it found outdated medications and some for which there was no record of the supplier, as well as blank or unsigned prescription forms.

In February, the University of California, Los Angeles announced that researchers there had found that 68% of the 40 Mexican pharmacies visited in four northern Mexico cities sold Oxycodone, Xanax or Adderall, and that 27% of those pharmacies were selling fake pills.

UCLA said the study, published in January, found that “brick and mortar pharmacies in Northern Mexican tourist towns are selling counterfeit pills containing fentanyl, heroin, and methamphetamine. These pills are sold mainly to US tourists, and are often passed off as controlled substances such as Oxycodone, Percocet, and Adderall.”

“These counterfeit pills represent a serious overdose risk to buyers who think they are getting a known quantity of a weaker drug,” Chelsea Shover, assistant professor-in-residence of medicine at the David Geffen School of Medicine at UCLA, said in February.

And the U.S. State Department travel warning in March said the counterfeit pills being sold at pharmacies in Mexico “may contain deadly doses of fentanyl.”

The Mexican Navy did not confirm that any fentanyl-laced pills had been found in last week’s raid, but said medications had been seized to test whether they contained fentanyl.

Fentanyl is a synthetic opioid far more powerful than morphine, and it has been blamed for about 70,000 overdose deaths per year in the United States. Mexican cartels produce it from precursor chemical smuggled in from China, and then often press it into pills designed to look like other medications.