Nyquil has dextromethorphan (DXM) in it as well, which is a very abusable dissociative. I used to abuse it when I couldn't get any other drugs. Drinking a whole bottle will give you some very "drunk-like" feelings (the kids call using DXM "robo-tripping"). Nyquil also contains doxylamine succinate which is a very powerful sedative, actually the most powerful OTC sedative available in the US. When I was drinking it was actually pretty nice to get a good buzz off the DXM and the have no problems falling asleep. I will say for myself that I never used them for the small alcohol content, I wanted the DXM buzz more than anything.
The only time I used an entire bottle of any DXM containing medicine it was to get messed up.