It depends on what you mean by both "pain killers" and "harmful."
Just about any medicine can be harmful, if not taken as directed. Tylenol (acetaminophen) is available over the counter and is considered a pain reliever, but if you take more than the recommended dose can be hepatotoxic (poisonous to the liver). Narcotic pain relievers are quite effective, but become harmful if you take too many (the Tylenol thing again, and also the risk of addiction).
In the end it becomes a matter of following directions, and keeping in touch with the doctor to let him or her know what's working and what isn't.
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