There's a bunch of chemicals in nyquil and most other cold medicines that can be abused.
diphenhydramine
pseudoephadrine
dextromethorphan (DXM)
and, of course, the alcohol
I relapsed on benadryl (diphenhydramine) and have abused all sorts of OTC cold medicines. I also know others who have done the same.
For me, taking the "recommended" dosage is a risk. So I don't take any of the stuff. If someone I know in recovery were taking a half a bottle a night (sick or not), I would be concerned.