One in every six deaths attributed to sudden cardiac arrest in San Francisco may have been triggered by a drug overdose, according to a new study.

The findings suggest drug-related deaths may be substantially underestimated, the researchers from the University of California, San Francisco said.

The researchers analyzed reports of 525 deaths between 2011 and 2014 labeled after autopsy as “out-of-hospital cardiac deaths,” HealthDay reports. A team of medical experts reviewed the reports, and compared them to 242 similar reports from 2014 to 2017. They found 15% of the “cardiac death” cases in the earlier group involved a drug overdose, compared with about 22% in the later group.

The researchers concluded that more than one in six deaths attributed to out-of-hospital cardiac deaths were actually due to overdose. Most of these deaths involved multiple drugs, including opioids. About half of the drugs were prescribed by a physician, the researchers reported in the Annals of Internal Medicine.