Promethazine is used to prevent and treat nausea and vomiting related to certain conditions (such as before/after surgery, motion sickness). It is also used to treat allergy symptoms such as rash, itching, and runny nose. It may be used to help you feel sleepy/relaxed before and after surgery or to help certain narcotic pain relievers (such as) work better. It may also be used for a short time to treat a runny nose due to the common cold.

Promethazine is an antihistamine and works by blocking a certain natural substance (histamine) that your body makes during an allergic reaction. Its other effects (such as anti-nausea, calming, pain relief) may work by affecting other natural substances (such as acetylcholine) and by acting directly on certain parts of the brain.

Promethazine Hcl(NSC-231688) is the first-generation antihistamine; strong antagonist of the H1 receptor and moderate mACh receptor antagonist, moderate affinity for 5-HT2A, 5-HT2C, D2 and α1-adrenergic receptors.
As a sedative,Promethazine hydrochloride.To reduce nervousness, restlessness and agitation caused by psychiatric conditions (used for this purpose mainly in Europe)
As antiallergic medication to combat hay fever (allergic rhinitis), etc., or to treat allergic reactions, alone or in combination with oral decongestants such as 
As an adjunct treatment for anaphylactoid conditions (IM/IV route preferred)
Together with or dextromethorphan against cough
As a motion sickness or seasickness remedy when used with To combat moderate to severe morning sickness and hyperemesis gravidarum: In the UK, promethazine is drug of first choice, being preferred as an older drug with which there is a greater experience of use in pregnancy (second in line being metoclopramide or prochlorperazine).
Previously, it was used as an antipsychotic,although it is generally not administered for this purpose now; promethazine has only approximately 1/10 of the antipsychotic strength of chlorpromazine.
Treatment for migraines; however, other similar medications, like Compazine, have been shown to have a more favorable treatment profile,and are used almost exclusively over promethazine.