Following a four-day raid, Mexico has closed 23 pharmacies in Caribbean resorts of Cancun, Playa del Carmen and Tulum for irregular pill sales.
Last spring, the United States warned of dangerous pill sales to foreigners and tourists where counterfeit drugs contained fentanyl, heroin and methamphetamine.
Mexican investigators went to 55 drug stores, finding irregular sales at 23 of them, according to the Mexican Navy Department.
In a January study from researchers at the University of California, Los Angeles (UCLA), the scientists said they visited 40 pharmacies in four Northern Mexico cities, finding that 68% sold oxycodone, Xanax or Adderall.
About 27% of those pharmacies were selling fake pills, according to the report, which said "brick and mortar pharmacies in Northern Mexican tourist towns are selling counterfeit pills containing fentanyl, heroin and methamphetamine. These pills are sold mainly to U.S. tourists, and are often passed off as controlled substances such as oxycodone, Percocet and Adderall."
In the raids, the Navy said it found outdated medications and those with no supplier record, the Associated Press reported. Also found were blank and unsigned prescription forms.
The Navy did not confirm finding fentanyl-laced pills, but it did say that the medications would be tested for the presence of fentanyl.
Fentanyl is a powerful synthetic opioid responsible for about 70,000 deaths each year in the United States, the AP reported, noting that Mexican cartels produce it from chemicals smuggled in from China.
"These counterfeit pills represent a serious overdose risk to buyers who think they are getting a known quantity of a weaker drug,"researcher Chelsea Shover, an assistant professor-in-residence of medicine at the David Geffen School of Medicine at UCLA, told the AP in February.
More information
The U.S. Centers for Disease Control and Prevention has more on fentanyl.
SOURCE: Associated Press