Nyquil is a brand of over-the-counter cold medicine that contains a combination of antihistamines. Nyquil contains d hydrocodone bitartrate, doxylamine succinate, and pyrilamine maleate. It is intended for short-term use as an analgesic (an agent that relieves pain). Nyquil reportedly is over 60% as potent as codeine but has fewer side effects. The codeine and acetaminophen in Nyquil are used to treat pain due to the common cold, flu, and other illnesses.

The antihistamines in Nyquil are used to relieve the symptoms of colds, including nasal congestion and runny nose, by blocking the effects of histamine.

Pyrilamine maleate is also an anti-cholinergic agent. This means that it blocks acetylcholine, a chemical which transmits nerve impulses. Blocking acetylcholine prevents the action of the impulses and relieves allergy- or cold-induced congestion by preventing drainage from occurring.

Doxylamine succinate is also an antihistamine (most commonly used to treat allergies) that acts as a sedative.