My understanding is that diphenhydramine (generic name for the trade name "Benadryl") does not alter what some doctors call "the brain's architecture of sleep" the way some prescription sleep meds can.

If this individual takes more acetaminophen (aka, "Tylenol") during the day, then perhaps the bedtime dose may be complicating the picture. But if they are only taking the recommended dose at bedtime every night, it's unlikely that even that is dangerous to the liver.

This assumes that there isn't a pre-existing liver condition. It also assumes that the individual does not drink much alcohol.

I am not necessarily suggesting that it's a good thing that you need to take the diphenhydramine to get to sleep. The biggest issue for me might be that there's another reason you can't get to sleep.

There are many things that can cause this. Caffeine intake, other medications, depression, some health problems -- if you haven't had a medical exam, this might be a good time to do it, both because of your concerns about the Tylenol PM and to see if there's another sleep-related issue.

In the meantime, make sure you're only taking the recommended dose of the Tylenol PM, watch other intake of acetaminophen and goes easy on the alcohol. Definitely don't wash the Tylenol PM down with anything containing alcohol.

Before I forget: I do agree with what someone else suggested here about looking for diphenhydramine alone. In Tylenol PM, it's the ingredient that helps with sleep. The "Tylenol" part is help with any pain you might be experiencing.

I don't think it's necessarily worth throwing away any Tylenol PM you have unless any of the things I mentioned are problems (drinking, taking other acetaminophen, etc.). But if you're in the market for more, save some money and decrease your acetaminophen intake by picking up some generic/store-brand diphenhydramine. And, of course, take it as the directions indicate unless your physician should advise differently.