Promethazine is a synthetic antihistamine medication primarily used to relieve allergy symptoms, prevent motion sickness, and provide preoperative sedation. This medication has both anticholinergic and weak serotonin antagonist mechanisms, blocking histamine H1 receptors to alleviate allergic reactions such as redness and itching, while also suppressing the central nervous system to produce sedative and antiemetic effects. Clinically, it is widely used to control preoperative anxiety, nausea induced by chemotherapy, and to treat severe allergic rhinitis or urticaria.
The pharmacological properties of promethazine make it a versatile therapeutic tool, but 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 patients should not adjust the dosage or method of use on their own.
The generic name of this medication is promethazine, and common commercial brand names include Phenergan and Promethegan. Different countries or regions may have other brand names, but the active ingredient and mechanism of action remain the same. Patients should confirm that the active ingredient on the medication label is promethazine when using it.
Promethazine is classified as a first-generation antihistamine (H1 receptor antagonist) and also has anticholinergic and weak dopamine receptor antagonist effects. It is classified as an antiemetic and a central nervous system depressant, used to treat and prevent various symptoms. Due to its sedative effects, it is also categorized under anxiolytics.
The main indications for promethazine include:
In specific situations, this medication may also be used to treat seizures following epilepsy or as an adjunct treatment for seizures. However, the risks and benefits must be evaluated by a physician before use, especially for children or patients with neurological disorders.
Mild side effects include:
Serious side effects require immediate medical attention:
Contraindications and high-risk groups: The following conditions prohibit the use of this medication:
Pregnancy and breastfeeding: Use during pregnancy is only recommended when absolutely necessary, as it may pose risks to the fetus. Women who are breastfeeding may experience suppressed lactation, and the medication may affect the infant through breast milk. The U.S. Food and Drug Administration (FDA) classifies this medication as Category C, indicating that animal studies show risks, but there is insufficient data in humans.
The FDA has issued a black box warning for this medication, stating:
Combining with the following medications may increase the risk of side effects:
Using with monoamine oxidase inhibitors (MAOIs) may cause hypertensive crises; promethazine should not be used until at least 14 days after discontinuing MAOIs. Combining with antiepileptic drugs may reduce the efficacy of the latter, requiring dosage adjustments.
Dosing is adjusted based on age and symptoms:
The injectable form must be administered by healthcare personnel; intravenous injections should be given slowly (over 5 minutes). Extended-release capsules must be taken whole to maintain efficacy, as damaging the drug structure may cause fluctuations in blood concentration.
If a scheduled dose is missed, it should be taken immediately; however, if it is close to the next scheduled dose, skip the missed dose and resume the normal schedule. Never take a double dose to make up for a missed dose to avoid toxicity.
If missing a dose leads to recurring symptoms (e.g., severe nausea), contact a physician immediately to assess whether the medication plan needs adjustment. If the missed time exceeds 12 hours, resume the normal dosage at the next scheduled time and closely monitor symptom changes.
Overdosing may lead to severe central nervous system depression, with symptoms including:
Emergency measures include gastric lavage, activated charcoal to absorb unabsorbed medication, and providing respiratory support. Severe cases may require the use of antihistamine antagonists or intravenous calcium gluconate to alleviate seizures. Patients should not attempt to manage an overdose on their own and should go to the emergency room for treatment.
What are the differences in usage between the oral liquid and injectable forms of promethazine?
The oral liquid should be taken according to the dosage prescribed by the physician, usually before bedtime or after meals to reduce gastrointestinal discomfort. The injectable form must be administered by healthcare personnel in a medical facility and should not be self-administered. After injection, observe for any allergic reactions, such as rash or difficulty breathing.
What should I do if I experience drowsiness after using promethazine?
Drowsiness is a common side effect; avoid driving or operating machinery after taking the medication. If it persists for several days or severely affects daily life, contact a physician immediately to assess dosage adjustment. Avoid using with alcohol or sleeping pills to prevent exacerbating drowsiness.
What drug interactions should I be aware of when using promethazine with other medications?
Combining with other antihistamines, sedatives, or analgesics may increase the risk of drowsiness. Using with monoamine oxidase inhibitors (MAOIs) may lead to abnormal blood pressure. Inform your physician of all medications you are currently taking, including over-the-counter drugs or herbal supplements.
Can patients with glaucoma use promethazine?
Promethazine may worsen angle-closure glaucoma, so patients should avoid using it. For open-angle glaucoma, the physician will assess the eye pressure before deciding on use and recommend regular monitoring of eye health.
Is it safe to use promethazine during pregnancy or breastfeeding?
Use during pregnancy should be avoided unless necessary, as it may pose risks to the fetus. Use during breastfeeding may transmit through breast milk; it is advisable to consult a physician before taking the medication to assess its impact on the infant. If necessary, adjust feeding times or dosages.
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