Many drugs containing opiates are mixed with acetaminophen[1] by drug companies to reduce their abuse potential as acetaminophen is very toxic. The thought is that people that would abuse these medicines would think twice. All it really does is kill people[2] to the point where the government puts out information on how to spot someone going through such a poisoning.

[1] https://www.physicsforums.com/threads/why-some-opiates-mixed... [2] https://www.nlm.nih.gov/medlineplus/ency/article/002670.htm