A horse tranquilizer drug often found mixed with opioids is increasingly involved in overdose deaths in some U.S.

illegal drug supply, frequently in combination with heroin or fentanyl (both types of opioids), a mixture sometimes referred to as "tranq dope," Live Science previously reported.

Now, Connecticut is the latest state to report a rise in overdose deaths involving xylazine, according to a report published Thursday (Sept. 16) in the Centers for Disease Control and Prevention (CDC) journal Morbidity and Mortality Weekly Report.

In the new study, researchers from the Connecticut Department of Public Health found that unintentional overdose deaths in the state that involved xylazine increased from 5.8% in 2019 to 11.4% in the first half of 2020.

That study found that, between 2010 and 2015, xylazine was detected in just 2% of unintentional overdose deaths involving heroin or fentanyl.

In Philadelphia, some focus groups have found that people who use illegal drugs report that xylazine makes the effects of opioids last longer, Live Science previously reported.

Still, the total number of overdose deaths involving xylazine in the U.S?

appears to be small — out of the more than 45,000 overdose deaths reported in 2019, only about 2% were positive for xylazine, the CDC researchers found.

While studies on the health effects of xylazine combined with opioids are limited, some research suggests that the mixture may increase the risk of opioid overdose death.

Indeed, xylazine may increase the toxic effects of opioids by "potentiating sedation and causing respiratory depression" along with low blood pressure and a slow heart rate, the Connecticut researchers said.

Labs should implement routine postmortem testing for xylazine, which could help researchers better understand the role of the drug in overdose deaths, they said.