|Systematic (IUPAC) name|
|Oral, rectal, IV, IM, topical|
|Bioavailability||88% absorbed but after first-pass metabolism reduced to 25% absolute bioavailability|
|Metabolism||Hepatic glucuronidation and sulfoxidation|
|Biological half-life||16-19 hours|
|Excretion||Renal and biliary|
|ATC code||D04AA10 (WHO) R06, R06|
|Molar mass||284.42 g/mol[[Script error: No such module "String".]]|
|Script error: No such module "collapsible list".|
Promethazine is a first-generation H1 receptor antagonist of the phenothiazine chemical class used medically as an antihistamine antiemetic. It has a strong sedative effect and in some countries is prescribed for insomnia when benzodiazepines are contraindicated. It is available over the counter in the United Kingdom, Australia, Canada, Switzerland, and many other countries, but by prescription in the United States (brand names Phenergan, Promethegan, Romergan, Fargan, Farganesse, Prothiazine, Avomine, Atosil, Receptozine, Lergigan).
Chemically, promethazine hydrochloride appears as a white to faint yellow crystlline powder that is practically odorless. Slow oxidation may occur upon prolonged exposure to air usually causing blue discoloration. Promethazine as the hydrochloride salt is freely soluble in water and somewhat soluble in alcohol. Promethazine is a chiral compound, occurring as a mixture of enantiomers (pictured).
- As a sedative.
- For preoperative sedation and to counteract postnarcotic nausea.
- As antiallergic medication to combat hay fever (allergic rhinitis), etc. To treat allergic reactions it can be given alone or in combination with oral decongestants like pseudoephedrine.
- As an adjunct treatment for anaphylactoid conditions (IM/IV route preferred).
- Together with codeine or dextromethorphan against cough.
- As a motion sickness or seasickness remedy when used with Ephedrine or Pseudoephedrine.
- 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 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.
- Also used to potentiate any opiates. Commonly combined with pethidine (AKA, meperidine, or Demerol) in a brand called Mepergan, a meperidine/promethazine combination. Also frequently used in conjunction with codeine, in a syrup form. The combination leads to more powerful euphoric effects than with codeine alone.
Mechanism of action
- Promethazine is a phenothiazine derivative that competitively blocks histamine H1 receptors without blocking the secretion of histamine. It also is a very weak dopamine antagonist.
- It has sedative, anti-motion-sickness, anti-emetic, and anti-cholinergic effects.
Some common side effects include:
- Confusion in the elderly
- Drowsiness, dizziness, fatigue, more rarely vertigo
- Dry mouth
- Respiratory depression in patients under age of 2 and in those with severely compromised pulmonal function
- Chest Discomfort/Pressure. (typically in cases when patient is already taking medication for high blood pressure)
- Euphoria (very rare, except with high IV doses and/or coadministration with opioids/CNS depressants)
- Restless legs 
- Short temper/Irritability
Extremely rare side effects include:
IV administration: Dilute with 0.9% NaCl or D5W. CONCENTRATION: Doses should not exceed a concentration of 25mg/ml. Administer through a large-bore vein through a running IV line into the most distal port. Slight yellow color does not alter potency. Do not administer if precipitate is present. RATE: Administer each 25mg slowly, over at least 10-15min. Rapid administration may produce a transient fall in blood pressure.  Serious complications including those listed above have resulted from improper parenteral administration, including those requiring surgical intervention and amputation.
Because of potential for more severe side effects, this drug is on the list to avoid in the elderly. (See NCQA’s HEDIS Measure: Use of High Risk Medications in the Elderly).
All patients should have their blood pressure measured frequently. During long-term therapy, blood cell counts, liver function studies, EKG, and EEG are recommended. The intervals should be determined according to the risk profile of the patient. In high doses, promethazine can create auditory and visual hallucinations causing panic and intense fear.
Product liability lawsuit
In 2009, the U.S. Supreme Court ruled on a product liability case involving promethazine. Diana Levine, a woman suffering from a migraine, was administered Wyeth's Phenergan via IV push. The drug was injected improperly resulting in gangrene and subsequent amputation of her right forearm below the elbow. A state jury awarded her $6 million in punitive damages.
The case was appealed to the Supreme Court on grounds of federal preemption and substantive due process. The Supreme Court upheld the lower courts' rulings stating that "Wyeth could have unilaterally added a stronger warning about IV-push administration" without acting in opposition to federal law. In effect, this means that drug manufacturers can be held liable for injuries if warnings of potential adverse effects (approved by the U.S. Food and Drug Administration, "FDA") are deemed insufficient by state courts.
On September 9, 2009, the FDA made the decision that a black box warning for injection be put on promethazine stating the contraindication for subcutaneous administration. The preferred administrative route is intramuscular (IM) which reduces risk of surrounding muscle and tissue damage 
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- "Promethazine". U.S. National Library of Medicine and National Institutes of Health.
- RxList: Promethazine
- http://www.rxlist.com/cgi/generic/phenergan.htm RxList: Promethazine Description
- RxList Indications for Promethazine
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- David J. McCann and Brett Roth, Toxicity, Antihistamine, eMedicine Toxicology, updated June 21, 2007
- Cordingley Neurology
- Davis Drug Guide for Nurses Eleventh Edition pgs. 1017-1020
- Liptak, Adam (2008-09-18). "Drug Label, Maimed Patient and Crucial Test for Justices". The New York Times. Retrieved 2008-10-31.
- Stout, David (2009-03-04). "Drug Approval Is Not a Shield From Lawsuits, Justices Rule". The New York Times. Retrieved 2009-03-04.