Promethazine is a versatile phenothiazine-derived medication primarily indicated for the management of allergic conditions, nausea, vomiting, and motion sickness. It functions as both a histamine H1-receptor antagonist and an antiemetic agent, offering multi-symptom relief through central and peripheral mechanisms. Its sedative properties also make it valuable in preoperative sedation and adjunctive therapy. Available in oral, rectal, and injectable formulations, promethazine is widely prescribed under medical supervision to ensure safe and symptom-targeted use.

Features

• Chemical class: Phenothiazine derivative
• Mechanism: Antagonizes histamine H1 receptors; possesses anticholinergic, antiemetic, and sedative effects
• Bioavailability: Approximately 25% orally due to first-pass metabolism
• Half-life: Ranges from 9 to 16 hours
• Forms: Tablets, syrup, suppositories, and injectable solution
• Prescription status: Available by prescription only in most jurisdictions

Benefits

• Provides rapid and effective relief from symptoms of allergic reactions, including urticaria and rhinorrhea
• Effectively prevents and treats nausea and vomiting associated with motion sickness, surgery, or illness
• Offers sedative effects useful for preoperative preparation or adjunctive therapy in calming agitated patients
• Reduces severity and duration of itching in dermatological conditions
• Supports management of migraine-related nausea when other antiemetics are unsuitable
• Cost-effective compared to newer generation anti-allergy and antiemetic agents

Common use

Promethazine is commonly prescribed for the management of allergic conditions such as allergic rhinitis, conjunctivitis, and mild allergic skin reactions. It is also indicated for preventing and treating nausea and vomiting—particularly postoperative nausea, motion sickness, and nausea associated with gastroenteritis or migraine. In clinical practice, it is sometimes used off-label for its sedative effects in managing anxiety or insomnia, though this is not a primary indication. Its use in children is generally reserved for severe cases and under strict medical guidance due to safety considerations.

Dosage and direction

Dosage must be individualized based on indication, patient age, and clinical response. For adults, the typical antihistaminic dose is 25 mg orally at bedtime, or 12.5 mg before meals and at bedtime. As an antiemetic, 25 mg orally may be taken initially, followed by 12.5 to 25 mg every 4–6 hours as needed. For motion sickness, 25 mg taken 30–60 minutes before travel is recommended. Pediatric dosing is weight-based and should be carefully calculated; promethazine is not recommended for children under two years of age. Always use the lowest effective dose for the shortest duration necessary.

Precautions

Promethazine may cause drowsiness, impaired alertness, or reduced coordination; patients should avoid driving or operating machinery until their response is known. Caution is advised in elderly patients due to increased risk of sedation, confusion, and anticholinergic effects. It should be used carefully in patients with respiratory conditions, including asthma, due to potential suppression of cough reflex. Avoid concurrent use with other CNS depressants. Long-term use may lead to tolerance or dependence. Patients with cardiovascular disease, glaucoma, prostate hypertrophy, or liver impairment should use promethazine only under close medical supervision.

Contraindications

Promethazine is contraindicated in individuals with known hypersensitivity to promethazine or other phenothiazines. It should not be used in comatose patients, or in those with significant hepatic impairment. Use is contraindicated in neonates and infants younger than two years due to risk of fatal respiratory depression. Concomitant use with monoamine oxidase inhibitors (MAOIs) is contraindicated. It is also not recommended in patients with bone marrow depression, narrow-angle glaucoma, or severe CNS depression.

Possible side effect

Common side effects include drowsiness, dry mouth, blurred vision, dizziness, and tinnitus. Less frequently, patients may experience hypotension, tachycardia, extrapyramidal symptoms (e.g., dystonia), or photosensitivity. Rare but serious adverse effects include agranulocytosis, neuroleptic malignant syndrome, seizures, and respiratory depression, especially in vulnerable populations. Allergic reactions such as rash or anaphylaxis may occur. Pediatric patients may exhibit paradoxical excitability. Report any persistent or severe side effects to a healthcare provider promptly.

Drug interaction

Promethazine may potentiate the effects of CNS depressants, including alcohol, opioids, benzodiazepines, and sedative-hypnotics. Concurrent use with anticholinergics may increase the risk of adverse effects. It may reduce the effectiveness of levodopa and other dopamine agonists. Caution is advised with antihypertensives, as promethazine can enhance hypotensive effects. MAOIs may intensify anticholinergic and sedative actions. Epinephrine should be avoided in promethazine-induced hypotension due to risk of reversed epinephrine effect.

Missed dose

If a dose is missed, take it as soon as remembered unless it is nearly time for the next scheduled dose. In that case, skip the missed dose and resume the regular dosing schedule. Do not double the dose to make up for a missed one. Consistent timing is advised to maintain therapeutic effect, particularly for motion sickness prevention. If unsure, consult a pharmacist or healthcare provider.

Overdose

Overdose may manifest as severe CNS depression, hypotension, respiratory failure, agitation, hallucinations, or convulsions. Anticholinergic symptoms such as dry mouth, fixed dilated pupils, and flushed skin may occur. In children, excitability and incoordination may be noted. Treatment is supportive and may include activated charcoal if ingestion was recent, along with monitoring of vital signs. There is no specific antidote; manage symptoms and provide respiratory support if needed. Seek immediate medical attention in suspected overdose situations.

Storage

Store promethazine at room temperature (15–30°C), away from light, moisture, and excessive heat. Keep the medication in its original container, tightly closed, and out of reach of children and pets. Do not use beyond the expiration date printed on the packaging. Dispose of unused or expired medication through a pharmacy take-back program or according to local guidelines to prevent accidental ingestion or environmental contamination.

Disclaimer

This information is intended for educational purposes only and does not replace professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before starting, changing, or stopping any medication. Individual medical conditions and responses to therapy may vary. Never disregard professional medical advice or delay seeking it based on information provided here.

Reviews

Clinical studies and patient reports generally affirm promethazine’s efficacy in managing allergies, nausea, and vomiting, with particular praise for its rapid onset of action. However, sedation is frequently cited as a limiting factor. Many users find it highly effective for motion sickness and postoperative nausea. Healthcare professionals emphasize its value in clinical settings but caution against overuse without medical supervision. Long-term user experiences highlight the importance of dose titration to balance benefits and side effects.