At least one person died and eight more were treated in a San Francisco emergency room late last year after taking counterfeit Xanax tablets cut with fentanyl, according to HealthDay.

The tablets looked exactly the same as prescription Xanax. “The users that were exposed to these tablets had no idea it was anything other than what they thought they were buying,” said Dr. Ann Arens, who reported the cases in JAMA Internal Medicine.

Three of the people suffered heart attacks, and one had heart failure. Others experienced dangerous drops in the functioning of their respiratory and nervous systems. One patient was an infant who took a fake Xanax pill he found on the floor in his home.