Promethazine is a synthetic antihistamine primarily used to alleviate allergy symptoms, prevent motion sickness, and provide preoperative sedation. This medication exhibits both anticholinergic and anti-5-HT (serotonin) mechanisms, blocking H1 histamine receptors to reduce allergic reactions such as redness, swelling, and itching, while also suppressing the central nervous system to produce sedative and antiemetic effects. Clinically, it is widely used to control preoperative anxiety, chemotherapy-induced nausea, and severe allergic rhinitis or urticaria.
The pharmacological properties of promethazine make it a versatile treatment tool. However, due to potential side effects such as drowsiness, patients should avoid driving or operating machinery after use. This medication must be prescribed by a physician, and dosage or usage should not be adjusted independently.
The generic name of this drug is Promethazine. Common commercial brand names include Phenergan and Promethegan. Different countries or regions may have other brand names, but the generic ingredient and mechanism of action remain the same. Patients should verify that the active ingredient on the medication label is promethazine when using the drug.
Promethazine is classified as a first-generation antihistamine (H1 receptor antagonist), with additional anticholinergic and weak dopamine receptor antagonist effects. It is also categorized as an antiemetic and central nervous system depressant, used to treat and prevent various symptoms. Due to its sedative effects, it is also considered part of anxiolytic medications.
The main indications for promethazine include:
In certain cases, this drug may also be used to treat seizures following epilepsy or as an adjunct therapy for epilepsy. However, its use must be evaluated by a physician, especially for children or patients with neurological conditions.
Mild side effects include:
These symptoms usually resolve within a few hours. If they persist or worsen, medical advice should be sought.
Serious side effects require immediate medical attention:
Patients experiencing these symptoms should stop medication immediately and seek emergency medical assistance.
Contraindications and high-risk groups: Those with the following conditions should not use this medication:
Pregnancy and breastfeeding: Use during pregnancy should only occur if absolutely necessary, as it may pose risks to the fetus. During breastfeeding, it may inhibit milk production and the drug could pass into breast milk, potentially affecting the infant. The U.S. FDA classifies this medication as Category C, indicating animal studies show risk, but human data are insufficient.
The U.S. FDA has issued a black box warning for this drug, stating:
Prescribers must strictly adhere to age and dosage restrictions.
Using with the following medications may increase the risk of adverse effects:
Patients should inform their healthcare provider of all medications they are taking, including prescription drugs, over-the-counter medicines, and herbal supplements.
Combining with monoamine oxidase inhibitors (MAOIs) may cause hypertensive crises; at least 14 days should pass after stopping MAOIs before starting promethazine. Co-administration with anticonvulsants may reduce their efficacy, requiring dose adjustments.
Dosages are adjusted based on age and symptoms:
Oral formulations should be swallowed whole, not chewed or broken.
Injectable forms must be administered by healthcare professionals. Intravenous injections should be administered slowly (over 5 minutes). Extended-release capsules should be taken intact to maintain efficacy; breaking the capsule may cause blood concentration fluctuations.
If a dose is missed, it should be taken as soon as remembered. If it is close to the next scheduled dose, skip the missed dose and resume the regular schedule. Do not double doses to make up for a missed one, as this can lead to toxicity.
If missed doses cause symptoms to recur (e.g., severe nausea), contact a healthcare provider to evaluate whether medication adjustments are necessary. If more than 12 hours have passed since the missed dose, resume the normal dosing schedule at the next dose and monitor symptoms closely.
Overdose may cause severe central nervous system depression, with symptoms including:
If overdose is suspected, seek immediate medical attention and bring the medication container for reference.
Emergency treatments include gastric lavage, activated charcoal to adsorb unabsorbed drug, and ventilatory support. Severe cases may require antihistamines or intravenous calcium gluconate to relieve seizures. Patients should not attempt to treat overdose themselves; go to the emergency room immediately.
What are the differences between oral syrup and injectable promethazine in usage?
Oral syrup should be taken according to the physician's instructions, usually before bedtime or after meals to reduce gastrointestinal discomfort. Injectable forms must be administered by healthcare professionals in medical settings and cannot be self-administered. After injection, observe for allergic reactions such as rash or difficulty breathing.
What should I do if I feel drowsy after taking promethazine?
Drowsiness is a common side effect. Avoid driving or operating machinery after taking the medication. If drowsiness persists for several days or significantly affects daily life, consult a healthcare provider for dosage adjustment. Avoid alcohol or sleeping pills to prevent worsening drowsiness.
What drug interactions should I be aware of when using promethazine?
Using with other antihistamines, sedatives, or painkillers may increase drowsiness. Combining with MAOIs may cause blood pressure abnormalities. Inform your doctor of all current medications, including over-the-counter drugs and herbal supplements.
Can patients with glaucoma use promethazine?
Promethazine may worsen closed-angle glaucoma and should be avoided by such patients. For open-angle glaucoma, the doctor will evaluate intraocular pressure before deciding on use and recommend regular eye monitoring.
Is promethazine safe during pregnancy or breastfeeding?
Use during pregnancy should be avoided unless absolutely necessary, as it may pose risks to the fetus. During breastfeeding, the drug may pass into breast milk and affect the infant. Consult a healthcare provider before use to assess potential impacts on the baby. If necessary, adjust feeding times or dosage accordingly.
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