Some Mexican pharmacies that cater to U.S. tourists are selling medications that appear safe but are laced with deadly fentanyl and methamphetamine.

That's the conclusion of new research that examined medications purchased legally in four cities in northern Mexico where travelers from the U.S. often seek low-cost health care and pharmaceuticals.

"For pills sold as oxycodone, we tested 27 and found 10 or 11 of them contained either fentanyl or heroin," said Chelsea Shover, a researcher at the UCLA School of Medicine.

She said the behavior by retail pharmacies in Mexico puts unsuspecting people at high risk of overdose and death.

"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," Shover said.

Her team also found medications sold at Mexican pharmacies laced with methamphetamines.

While these drug stores sell medications to Mexican consumers, Shover says their main customers appear to be Americans.

"Similar products are available at a much lower price in Mexico, so Americans do travel to save money."

Two Democratic lawmakers sent a letter to the U.S. State Department calling for a travel advisory to warn Americans of the danger of purchasing medications in Mexico.

"We should be absolutely very concerned," said Rep. David Trone (D-Md.), one of the authors of the letter. "We have almost 12 million Americans visiting Mexico every year."

According to Trone, pharmacies boosting profits with the high-risk practice are located in communities where Americans travel seeking relief from high-cost prescription medications sold in the U.S.

"There's literally a pharmacy on every corner, they're everywhere down there, because the price of drugs is cheaper."

On Saturday, the Los Angeles Times reported State Department officials apparently knew about the danger posed by Mexican pharmacies as long ago as 2019 but failed to issue a high-profile alert to travelers.

According to the newspaper's investigation, at least one U.S. traveler is known to have overdosed and died after taking medications purchased at a drug store in Cabo San Lucas, Mexico, in 2019.

Rep. Trone said if U.S. officials knew about unsafe medications being sold at legal outlets in Mexico, they should have warned travelers sooner.

"We've heard nothing back [from the State Department] and it's very frustrating," he added.

The State Department sent a statement to NPR saying it wouldn't comment on the letter from lawmakers.

On background, an official pointed to an advisory included in the State Department's standard on-line information about Mexico that urges travelers to "exercise caution when purchasing medications overseas."

"Counterfeit medication is common and may prove to be ineffective, the wrong strength, or contain dangerous ingredients," the advisory reads.

There's no reference, however, to the specific risks of dangerous drugs laced with fentanyl sold at legal pharmacies.

During a press briefing Monday, spokesman Ned Price said American officials constantly update safety advisories issued for Mexico.

"We are always looking at information to determine whether it is necessary to move our travel warnings in one direction or another," he said.

Earlier this month, four Americans were kidnapped by gunmen while traveling to Mexico to seek low-cost medical care. Two of them were killed.

That case had already raised concerns about the safety of medical tourism in the country.