"Drug makers add a small amount of antihistamine (usually 25 mg or 50 mg of diphenhydramine, the active ingredient in Benadryl) to pain killers and call the resulting concoction the "PM" version of the drug.

For example, Tylenol PM is simply tylenol with a pinch of antihistamine added to induce sleepiness. Midol PM is Midol with a pinch of antihistamine added to cause drowsiness.

There is also Excedrin PM, Anacin PM, Legatrin PM, etc."
http://www.coreynahman.com/ura.html

"Current CDER Considerations when Evaluating a Modifier
• Does the root name exist as a currently marketed product?
• Has the applicant provided a rationale for the modifier?
– Eg, ‘PM’ used to represent a base product plus the addition of diphenhydramine and help differentiate that product from the base product with out the added ingredient) [...]
http://www.fda.gov/downloads/AboutFDA/CentersOffices/Officeo...

--------------------------------------------------
Note added at 1 hr (2014-05-03 11:46:18 GMT)
--------------------------------------------------

I don't know what "PM" actually stands for when it is used as a modifier with this meaning. Donal suggests (by implication) that it stands for "post meridian", as in 5 pm, meaning that this version of the drug is administered in the evening or at night to help people sleep. I don't know whether this is correct or not.

--------------------------------------------------
Note added at 1 hr (2014-05-03 11:49:10 GMT)
--------------------------------------------------

By the way, here is Bayer's own page on their aspirin PM.
http://www.wonderdrug.com/products/es/esbpm.htm