Yes, Mucinex and Nyquil can be taken together because they contain different ingredients, but Mucinex DM and Nyquil should not be taken together because they share a common ingredient, dextromethorphan. Instead of taking Mucinex DM and Nyquil together you should just take Nyquil.

Mucinex contains guaifenesin which is an expectorant and its main purpose is to thin out the mucus in your airways and relieve congestion in your lungs and chest. Mucinex DM contains dextromethorphan in addition to guaifenesin. Dextromethorphan is a cough suppressant.

Nyquil contains acetaminophen (relieves pain and fever), doxylamine (an antihistamine that will dry up nasal secretions and may also cause drowsiness to help sleep) and dextromethorphan.