The man and woman, believed to be in their 30s, are originally from Dayton but had come back to buy drugs, Moraine police Sgt. Chris Selby said.
The truck had a Florida license plate.
While on the highway leaving Dayton, the pair decided to take the drugs, which police believe was either heroin or fentanyl.
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The man pulled off to the shoulder of the highway and parked near the Heidelberg Distributing facility around 5 p.m. to take the drugs with the woman, Selby said.
The drugs left them unconscious.
The incident resulted in backup on the highway, which included some accidents, the sergeant said.
The 3-month-old girl was not injured and was released to family member believed to be from Dayton.
Police will contact Montgomery County Children Services due to the incident, Selby said.
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The man and woman were taken to Kettering Medical Center.
They will likely face drug possession, possession of drug abuse instruments, inducing panic, OVI for the driver, and child endangering, Selby said.