For 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). For allergies such as hay fever and together with other medications in anaphylaxis
To aid with symptoms of the common cold
Motion sickness, including space sickness
Hemolytic disease of the newborn
Anxiety before surgery

Side effects
Some documented side effects include:

Tardive dyskinesia, pseudoparkinsonism, acute dystonia (effects due to dopamine D2 receptor antagonism)
Confusion in the elderly
Drowsiness, dizziness, fatigue, more rarely vertigo
Known to have effects on serotonin and dopamine receptors. Dry mouth
Nausea
Respiratory depression in patients under age of two and in those with severely compromised pulmonary function
Blurred vision, xerostomia, dry nasal passages, dilated pupils, constipation, and urinary retention. (due to cholinergic effects)
Chest discomfort/pressure (In children less than 2 years old)
AkathisiaLess frequent:

Cardiovascular side effects to include arrhythmias and hypotension
Neuroleptic malignant syndrome
Liver damage and cholestatic jaundice
Bone marrow suppression, potentially resulting in agranulocytosis, thrombocytopenia, and leukopenia
Depression of the thermoregulatory mechanism resulting in hypothermia/hyperthermiaRare side effects include:

SeizuresBecause of potential for more severe side effects, this drug is on the list to avoid in the elderly. In many countries (including the US and UK), promethazine is contraindicated in children less than two years of age, and strongly cautioned against in children between two and six, due to problems with respiratory depression and sleep apnea.Promethazine is listed as one of the drugs of highest anticholinergic activity in a study of anticholinergenic burden, including long-term cognitive impairment.
Society and culture
As of July 2017 it was marketed under many brand names worldwide: Allersoothe, Antiallersin, Anvomin, Atosil, Avomine, Closin N, Codopalm, Diphergan, Farganesse, Fenazil, Fenergan, Fenezal, Frinova, Hiberna, Histabil, Histaloc, Histantil, Histazin, Histazine, Histerzin, Lenazine, Lergigan, Nufapreg, Otosil, Pamergan, Pharmaniaga, Phenadoz, Phenerex, Phenergan, Phénergan, Pipolphen, Polfergan, Proazamine, Progene, Prohist, Promet, Prometal, Prometazin, Prometazina, Promethazin, Prométhazine, Promethazinum, Promethegan, Promezin, Proneurin, Prothazin, Prothiazine, Prozin, Pyrethia, Quitazine, Reactifargan, Receptozine, Romergan, Sominex, Sylomet, Xepagan, Zinmet, and Zoralix. It is also marketed in many combination drug formulations:

with carbocisteine as Actithiol Antihistaminico, Mucoease, Mucoprom, Mucotal Prometazine, and Rhinathiol;
with paracetamol (acetaminophen) as Algotropyl, Calmkid, Fevril, Phen Plus, and Velpro-P;
with paracetamol  and dextromethorphan as Choligrip na noc, Coldrex Nočná Liečba, Fedril Night Cold and Flu, Night Nurse, and Tachinotte;
with paracetamol, phenylephrine, and salicylamide as Flukit;
with dextromethorphan as Axcel Dextrozine and Hosedyl DM;
with dextromethorphan and ephedrine as Methorsedyl;
with dextromethorphan and pseudoephedrine as Sedilix-DM;
with dextromethorphan and phenylephedrine as Sedilix-RX;
with pholcodine as Codo-Q Junior and Tixylix;
with pholcodine and ephedrine as Phensedyl Dry Cough Linctus;
with pholcodine and phenylephedrine as Russedyl Compound Linctus;
with pholcodine and phenylpropanolamine as Triple P;
with codeine as Kefei and Procodin;
with codeine and ephedrine as Dhasedyl, Fendyl, and P.E.C. ;
with ephedrine  and dextromethorphan as Dhasedyl DM;
with glutamic acid as Psico-Soma, and Psicosoma;
with noscapine as Tussisedal; and
with chlorpromazine  and phenobarbital as Vegetamin. Recreational use
The recreational drug lean, also known as purple drank among other names, often contains a combination of promethazine with codeine-containing cold medication. Product liability lawsuit
In 2009, the US Supreme Court ruled on a product liability case involving promethazine.