How exactly do pain medications work in preventing pain?

Depends on the drug: Opoids effect certain receptors in the "pain pathway" and thus alleviate pain. Drugs like Ibuprofen reduce inflammation and reduce pain in other ways as well. Neuropathic pain is usually helped by drugs that work in the nervous system like antidepressants . Tylenol (acetaminophen) works on certain pain receptors but does not reduce inflammation. Again, it all depends on the drug.