WASHINGTON D.C.: The US State Department has issued a travel warning regarding dangerous fake pills sold at pharmacies in Mexico, which could contain fentanyl.

Americans should "exercise caution when purchasing medication in Mexico," the alert said, adding that small pharmacies in tourist areas and border regions sometimes sell fake medications branded as OxyContin, Percocet, Xanax and others without the need of a prescription.

Also, the fake pills "may contain deadly doses of fentanyl," the State Department warned.

Some 68 percent of 40 Mexican pharmacies in four northern Mexico cities sold Oxycodone, Xanax or Adderall, and 27 percent of them sold fake pills, according to a study led by researchers at the University of California, Los Angeles published in January.

"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," the study said.

The State Department has not responded to requests for comment on whether any Americans had overdosed or died due to fake drug sold in Mexico.