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Promethazine – Uses, Dosage and Side Effects

Promethazine is a first-generation antihistamine and phenothiazine derivative used to treat allergies, nausea, and insomnia.

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Things worth knowing about "Promethazine"

Promethazine is a first-generation antihistamine and phenothiazine derivative used to treat allergies, nausea, and insomnia.

What is Promethazine?

Promethazine is a versatile medication belonging to the class of phenothiazine derivatives and first-generation antihistamines. In clinical use since the 1950s, it remains one of the most well-known drugs in its class. Promethazine exerts antiallergic, sedative, antiemetic (anti-nausea), and mild antipsychotic effects.

Indications – When is Promethazine Used?

Promethazine is prescribed for a wide range of medical conditions:

Allergic conditions: Hay fever, hives (urticaria), allergic skin reactions, and allergic conjunctivitis
Nausea and vomiting: Motion sickness, postoperative nausea, and pregnancy-related nausea (under medical supervision)
Sleep disorders: Short-term management of insomnia due to its strong sedative properties
Premedication: Used before surgical procedures to reduce anxiety and induce sedation
Pruritus: Relief of severe itching associated with various skin conditions

Mechanism of Action

Promethazine exerts its effects through several pharmacological mechanisms:

Antihistamine Effect

As an H1-receptor antagonist, promethazine blocks the action of histamine at H1 receptors throughout the body. Histamine is a chemical messenger released during allergic reactions, causing symptoms such as itching, redness, swelling, and runny nose. By blocking these receptors, promethazine effectively reduces these symptoms.

Sedative Effect

Unlike newer, non-sedating antihistamines, promethazine readily crosses the blood-brain barrier, allowing it to act on the central nervous system. This results in pronounced sedation and sleep-promoting effects.

Antiemetic Effect

Promethazine suppresses the vomiting centre in the brain by blocking dopamine D2 receptors and muscarinic acetylcholine receptors, thereby effectively reducing nausea and vomiting.

Dosage and Administration

Promethazine is available in several formulations, including tablets, oral drops, suppositories, and injectable solutions. The dosage depends on the indication, age, and body weight of the patient. Typical adult doses range from 10 to 25 mg, taken one to three times daily. The exact dosing regimen should always be determined by a qualified healthcare professional.

Important Notice: Promethazine is contraindicated in children under 2 years of age due to the risk of respiratory depression. Caution is also required in children aged 2 to 5 years.

Side Effects

Like all medications, promethazine may cause side effects:

Common: Drowsiness, fatigue, dizziness, dry mouth
Occasional: Blurred vision, constipation, urinary retention, light-headedness
Rare: Extrapyramidal symptoms (involuntary movements), blood count changes, photosensitivity of the skin
Very rare: Neuroleptic malignant syndrome, severe allergic reactions

Drug Interactions

Promethazine may interact with a number of other medications:

Enhanced sedation when combined with sleeping pills, tranquilizers, alcohol, or opioid analgesics
Reduced effectiveness of antiepileptic drugs
Increased risk of cardiac arrhythmias when combined with other QT-prolonging medications

Contraindications

Promethazine should not be used in patients with:

Known hypersensitivity to promethazine or other phenothiazines
Children under 2 years of age
Severe liver or kidney impairment
Narrow-angle glaucoma
Urinary retention due to prostate enlargement

References

Katzung B.G., Masters S.B., Trevor A.J.: Basic and Clinical Pharmacology. 14th Edition, McGraw-Hill Education, 2018.
World Health Organization (WHO): WHO Model Formulary 2008 – Antihistamines and medicines used in anaphylaxis. WHO Press, Geneva.
Simons F.E., Simons K.J.: Histamine and H1-antihistamines: Celebrating a century of progress. Journal of Allergy and Clinical Immunology, 128(6):1139–1150, 2011. PubMed PMID: 22094198.

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