Promethazine

Promethazine is a first-generation H1 receptor antagonist of the phenothiazine chemical class used medically as an antihistamine. 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, Switzerland, and many other countries, but by prescription in the United States (brand names Phenergan, Promethegan, Romergan, Fargan, Farganesse, Prothiazine, Avomine, Atosil, Receptozine, Lergigan, and Sominex in the UK).

Chemistry
Chemically, promethazine hydrochloride appears as a white to faint yellow crystalline 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).

Promethazine, 10-(2-dimethylaminopropyl)phenothiazine, is synthesized by alkylating phenothiazine with 1-dimethylamino-2-propylchloride.
 * P. Charpentier, (1950).
 * S.B. Sidney, J.A. Nicholson, (1952).

Indications

 * 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 and potently blocks histamine H1 receptors without blocking the secretion of histamine. It also is a moderate muscarinic acetylcholine antagonist and a very weak dopamine antagonist.
 * It has sedative, anti-motion-sickness, anti-emetic, and anti-cholinergic effects.

Side effects
Some common side effects include:
 * Tardive dyskinesia
 * 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
 * Constipation
 * 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)
 * Akathisia
 * Paresthesia
 * Short temper/Irritability

Extremely rare side effects include:


 * Seizures
 * Neuroleptic malignant syndrome

IV administration: Dilute with 0.9% NaCl or D5W. CONCENTRATION: Doses should not exceed a concentration of 25 mg/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 25 mg 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).

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