Some pharmacies in northern Mexico that cater to U.S. tourists are selling medications that are laced with fentanyl and methamphetamine, according to a new study.

NPR reports researchers from the UCLA School of Medicine examined medications purchased legally in four cities in northern Mexico were U.S. tourists often buy low-cost health care and medications.

The researchers tested 27 pills sold as oxycodone, and found 10 or 11 of them contained either fentanyl or heroin.

“When I see there are fentanyl pills somewhere that look like [prescription drugs], I know there have to have been people who’ve died from that,” said researcher Chelsea Shover. “Similar products are available at a much lower price in Mexico, so Americans do travel to save money.”

Last week, Rep. David Trone (D-MD) and Sen. Edward Markey (D-MA) sent a letter to the U.S. State Department calling for a travel advisory to warn Americans about the danger of buying medications in Mexico.