Promethazine

• Nausea and vomiting prevention and treatment
• Allergic rhinitis and other allergic conditions
• Motion sickness prevention
• Sedation (preoperative and postoperative)
• Adjunct in anaphylaxis treatment

Promethazine is a first-generation antihistamine with additional antiemetic, sedative, and anticholinergic properties. Originally developed as an antihistamine, it has found widespread use for prevention and treatment of nausea and vomiting, as a sedative, and as an adjunct in pain management. Its versatility makes it valuable in multiple clinical settings, though safety considerations limit its use in certain populations.

Mechanism of Action

Promethazine exerts its effects through multiple receptor interactions. As a histamine H1 receptor antagonist, it blocks the effects of histamine on capillaries, bronchial smooth muscle, and the gastrointestinal tract. Its antiemetic effect results primarily from blocking H1 receptors and muscarinic receptors in the vomiting center and chemoreceptor trigger zone. The sedative effects are mediated through central H1 blockade. Promethazine also has significant anticholinergic activity and weak antidopaminergic properties, contributing to both therapeutic effects and side effects.

The medication has local anesthetic properties due to its membrane-stabilizing effects, which may contribute to its ability to relieve itching and irritation associated with allergic reactions.

Available Formulations

Promethazine (Phenergan) is available in multiple formulations including oral tablets (12.5 mg, 25 mg, 50 mg), oral solution (6.25 mg/5 mL), suppositories (12.5 mg, 25 mg, 50 mg), and injectable solution (25 mg/mL, 50 mg/mL) for deep intramuscular or intravenous use. The variety of formulations allows administration even when nausea prevents oral intake. Combination products with codeine (Phenergan with Codeine) are available for cough suppression.

FDA-Approved Indications

Promethazine is FDA-approved for allergic conditions (perennial and seasonal allergic rhinitis, allergic conjunctivitis, uncomplicated allergic skin manifestations of urticaria and angioedema), prevention and control of nausea and vomiting associated with certain types of anesthesia and surgery, therapy adjunct in anaphylactic reactions (after acute symptoms controlled with epinephrine), preoperative, postoperative, or obstetric sedation, and prevention and treatment of motion sickness.

Dosing Guidelines

For allergic conditions, the typical adult dose is 25 mg orally at bedtime, or 12.5 mg before meals and at bedtime. For nausea and vomiting, 12.5-25 mg every 4-6 hours as needed is typical. For sedation, 25-50 mg at bedtime or preoperatively. For motion sickness, 25 mg 30-60 minutes before travel, repeated every 8-12 hours as needed. Doses should be reduced in elderly patients and those with hepatic impairment.

Boxed Warning for Respiratory Depression in Children

Promethazine carries a boxed warning contraindicated in children less than 2 years of age due to the potential for fatal respiratory depression. In children 2 years and older, use the lowest effective dose and avoid combination with other respiratory depressants. Promethazine should not be used for treatment of uncomplicated vomiting in children; use should be limited to prolonged vomiting of known etiology.

Important Safety Considerations

Beyond the pediatric concerns, promethazine causes significant sedation and impairs mental and physical abilities required for tasks such as driving. Anticholinergic effects include dry mouth, urinary retention, constipation, and blurred vision. The medication can lower the seizure threshold and should be used cautiously in patients with seizure disorders. Neuroleptic malignant syndrome has been reported. Avoid concurrent use with other CNS depressants including opioids, sedatives, and alcohol.

Injection Site Concerns

Intravenous administration of promethazine has been associated with severe tissue injury including gangrene requiring amputation. The preferred route is deep intramuscular injection. If IV administration is necessary, it should be given through a running IV line at the port furthest from the patient's vein, at a concentration no greater than 25 mg/mL, at a rate not exceeding 25 mg/minute. Subcutaneous and intra-arterial injection are contraindicated.

Where Promethazine Sits Among the Antiemetics

Promethazine works against nausea, but it is no longer an automatic first choice, and the reason is that its useful effect and its sedation cannot be separated. For nausea in an ambulatory adult who needs to work or drive, ondansetron is usually preferred because it does not sedate; promethazine is kept for situations in which sedation is acceptable or actively wanted, such as nausea at night, or for nausea that a serotonin antagonist has not controlled. Motion sickness is the exception that runs the other way. The vestibular pathway responds to antihistamine and anticholinergic blockade rather than to serotonin blockade, so a first-generation agent taken before travel is genuinely the more effective option there. Acute vertigo of inner-ear origin is treated along the same lines, but only for the first few days of an episode, because prolonged vestibular suppression delays the brain's own compensation and prolongs the illness.

For allergy, promethazine has been largely displaced. Cetirizine, loratadine, and fexofenadine control allergic rhinitis and hives (urticaria) without impairing daytime function, and an intranasal steroid outperforms any oral antihistamine for nasal symptoms. Promethazine retains a role where the sedation itself is the point, such as itching that repeatedly wakes a patient at night, and as an adjunct after epinephrine in anaphylaxis, where it addresses residual hives and never the reaction itself.

Sedation, Driving and Adults Over Sixty-Five

The sedation from a single dose frequently outlasts the nausea it was given for. Patients should be told plainly not to drive, operate machinery, or make consequential decisions until they know how a dose affects them, and that alcohol, opioids, benzodiazepines, and over-the-counter sleep aids compound the effect. The impairment is often worst the morning after an evening dose, which patients rarely anticipate and which shows up as a near miss on the drive to work.

In adults over sixty-five, promethazine is generally avoided altogether. It combines heavy sedation with anticholinergic activity, meaning confusion, dry mouth, constipation, urinary retention, and blurred vision, and both properties raise the risk of falls and delirium in precisely the population most likely to be handed something for nausea or itching. When an antiemetic is needed in an older patient, an agent without those properties is chosen first; if promethazine is used at all it is used briefly, at the lowest effective dose, with the rest of the medication list reviewed for other sedating and anticholinergic drugs. Extrapyramidal reactions, meaning restlessness, muscle stiffness, or involuntary movements of the face and tongue, are uncommon but well described and are sometimes mistaken for anxiety or for worsening of the underlying illness. The MedlinePlus promethazine monograph sets out the patient warnings, and the National Institute on Aging guidance on taking medicines safely as you age explains why an older patient's list is reviewed rather than simply added to. Our article on benzodiazepines in older adults and deprescribing applies the same reasoning to another sedating class.

Drug Interactions

CNS depressants (opioids, benzodiazepines, alcohol) have additive effects and may cause profound sedation or respiratory depression. Anticholinergic drugs increase the risk of adverse effects. MAO inhibitors may prolong and intensify anticholinergic effects. Promethazine may enhance the hypotensive effects of antihypertensive agents. It may interfere with pregnancy tests and glucose tolerance tests.

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