Promethazine, an antihistamine used to treat allergies, nausea, and motion sickness, was developed in the mid-20th century and approved for medical use in the 1950s. Its history is marked by its effectiveness in managing allergic reactions, sedation, and nausea, but also by the recognition of potential side effects such as drowsiness and, in rare cases, severe respiratory depression. Promethazine, a phenothiazine derivative with antihistaminic and antiemetic properties, has been widely used in clinical practice and is included in multiple therapeutic regimens for allergy relief and nausea control. Its development involved early clinical adoption and expanded medical use across various indications, allowing for broader monitoring of its safety and effectiveness.
BRAND NAMES
Phenergan (most widely known)
Avomine (commonly used for motion sickness)
Promethegan (rectal suppository form)
Phenadoz (another suppository brand)
MECHANISM OF ACTION
Promethazine acts primarily by blocking histamine H1 receptors, thereby preventing the effects of histamine released during allergic reactions and reducing symptoms such as itching, swelling, and runny nose. It also exerts antiemetic effects by acting on the chemoreceptor trigger zone in the brain, helping to control nausea and vomiting. In addition, promethazine has significant sedative properties due to its ability to cross the blood–brain barrier and depress central nervous system activity. Its anticholinergic action further contributes to its effectiveness in managing motion sickness and drying secretions, making it a versatile drug with antihistaminic, antiemetic, and sedative functions.
PHARMACOKINETICS
Absorption
Promethazine is well absorbed after oral administration, though its bioavailability is reduced due to first-pass metabolism in the liver. It is also effectively absorbed when given via intramuscular or rectal routes, with onset of action typically occurring within 20 minutes.
Distribution
Promethazine has a large volume of distribution, indicating extensive penetration into body tissues, including the brain. This reflects its high lipid solubility and strong binding to tissues outside the bloodstream.
Metabolism
Promethazine is extensively metabolized in the liver, primarily by cytochrome P450 enzymes, into inactive metabolites. This first-pass metabolism contributes to its reduced oral bioavailability.
Elimination
Promethazine is eliminated mainly through the kidneys in the form of inactive metabolites. A small amount may be excreted in bile. Its elimination half-life typically ranges from about 10 to 19 hours, allowing for sustained therapeutic effects.
PHARMACODYNAMICS
Promethazine produces its effects mainly by blocking H1 histamine receptors, which reduces allergic symptoms like itching, swelling, and runny nose. It also has antiemetic effects by inhibiting the chemoreceptor trigger zone in the brain, helping control nausea and vomiting. In addition, it causes sedation by depressing central nervous system activity and has anticholinergic properties that contribute to its usefulness in motion sickness and drying of secretions.
ADMINISTRATION
Promethazine can be administered orally (tablets or syrup), intramuscularly, intravenously (with caution due to risk of severe tissue injury), or rectally as suppositories. The route depends on the clinical use, such as allergies, nausea, or sedation, with parenteral routes providing faster onset than oral administration.
DOSAGE AND STRENGTH
Promethazine is commonly available in strengths such as 10 mg, 12.5 mg, 25 mg, and 50 mg tablets or suppositories, and as 25 mg/mL injection for parenteral use. Typical adult doses vary by indication: for allergies, 10–25 mg orally every 4–6 hours; for nausea or motion sickness, 12.5–25 mg every 4–6 hours; and for sedation or preoperative use, higher doses may be given under medical supervision. Doses in children are lower and strictly weight-based due to risk of respiratory depression.
DRUG INTERACTIONS
Promethazine interacts with alcohol, opioids, and sedatives, increasing drowsiness and risk of breathing problems. It also adds to effects of anticholinergic drugs and may have stronger effects with MAO inhibitors.
FOOD INTERACTIONS
Promethazine has no major direct food interactions. However, alcohol should be avoided with meals or at any time, as it can greatly increase drowsiness and sedation. Heavy or fatty meals may slightly delay its absorption, but this is usually not clinically significant.
CONTRAINDICATIONS
Promethazine should not be used in children under 2 years, in patients with severe CNS depression or coma, or in those allergic to it or phenothiazines. It is also avoided in severe respiratory problems and used with caution in glaucoma and liver disease.
SIDE EFFECTS
Drowsiness and sedation
Dizziness
Dry mouth
Blurred vision
Constipation
Low blood pressure
Confusion (especially in elderly)
Rare: respiratory depression (mainly in children)
Rare: allergic reactions
Rare: movement disorders (extrapyramidal symptoms)
OVERDOSE
Overdose causes severe drowsiness, confusion, dry mouth, dilated pupils, and may lead to breathing problems, seizures, or low blood pressure. Treatment is mainly supportive in a hospital.
TOXICITY
Promethazine toxicity is mainly due to strong CNS depression and anticholinergic effects. It can cause severe sedation, confusion, hallucinations, hyperthermia, dry skin, and dilated pupils. In severe cases, it may lead to respiratory depression, seizures, coma, or cardiovascular collapse, especially in children and overdose situations. Treatment is supportive and requires urgent medical care.
