A growing number of overdose deaths in the United States involve counterfeit pills, health officials reported Thursday.

Overdose deaths involving counterfeit pills were twice as common in the latter half of 2021 as they were in the last six months of 2019, accounting for about 5% of overdose deaths, according to a new study from the U.S. Centers for Disease Control and Prevention. In Western states, those rates tripled during the same period.

The drugs are made to look like prescription opioids or stimulants to treat ADHD. About 75% were made to look like oxycodone, the researchers said.

Most were transported to this country from Mexico, “infiltrating drug markets in western U.S. states,” the U.S. Drug Enforcement Agency said in a public safety alert issued two years ago. The alert warned of fake prescription pills laced with fentanyl and methamphetamine. In 2021 alone, law enforcement seized more than 9.5 million pills, a steep increase.

In the latest study, about two out of five overdose deaths were caused by illicitly manufactured fentanyl. The drug was found in nearly all overdose deaths that contained evidence of counterfeit pills. Methamphetamine was also found in about 25% of deaths involving counterfeit pills. Other drugs, such as cocaine and benzodiazepines, were found in one in eight cases.

While deaths from these pills were more common in the West, Hispanic people and those younger than 35 were more at risk.

The findings were published Sept. 1 in the CDC publication Morbidity and Mortality Weekly Report.

Fentanyl test strips and similar products can help identify pills' contents, while the overdose antidote naloxone may also help save lives.

People should only use pills prescribed to them and received from a pharmacy or healthcare provider.