Many drug dealers use their customers to test the strength of the synthetic opioids they sell, the Associated Press reports. They want the drugs to be strong enough to keep their customers coming back, but not strong enough to kill them.

Local dealers take fentanyl made in Chinese labs and use powders such as baby formula to increase its volume and street value.

“It is sick and awful that dealers are treating people this way,” said Bradley Ray, Director of the Center for Criminal Justice Research at Indiana University-Purdue University in Indianapolis, who studies overdose prevention. “It is sad that things have come to this. (Testers’) addictions will push them to take that; they’re not thinking clearly.”