The homatropine in hydromet and doxylamine are very similar. They can cause excess sedation, dry mouth, and possibly palpitations if taken together. Your nyquil contains dextromethorophan, a cough suppressant, and tylenol/acetaminophen for pain. They hydrocodone will tackle cough and pain. I would probably shy away from taking the nyquil unless you took a half dose of hydromet, the benefits of taking it are marginal at this point, since HC blows DM away for cough suppressant properties.