1. Clinical Overview
Meclizine is a first-generation piperazine-derivative antihistamine (H1-receptor antagonist) with pronounced antimuscarinic and central nervous system depressant properties. It is primarily used for the prevention and treatment of nausea, vomiting, and dizziness associated with motion sickness and vertigo of various etiologies. Its efficacy stems from its ability to suppress labyrinthine excitability and depress conduction in the vestibular-cerebellar pathways.
| Onset | Duration | Bioavailability |
|---|---|---|
| Approximately 60 minutes after oral administration. | Up to 24 hours, allowing for once-daily dosing for prophylaxis. | Well absorbed from the gastrointestinal tract, but precise percentage data is not well-established due to extensive first-pass metabolism. |
2. Mechanism of Action
Meclizine competitively antagonizes histamine H1 receptors in the central nervous system, particularly in the vestibular apparatus and the chemoreceptor trigger zone (CTZ). It also possesses significant anticholinergic (antimuscarinic) activity. This dual action reduces the sensitivity of the labyrinthine apparatus, inhibits the vestibular-cerebellar pathways, and diminishes the excitability of the CTZ, thereby alleviating nausea, vomiting, and dizziness.
3. Indications & Uses
- Prophylaxis and treatment of nausea, vomiting, and dizziness of motion sickness
- Management of vertigo associated with diseases affecting the vestibular system (e.g., Ménière's disease, labyrinthitis, vestibular neuronitis)
4. Dosage & Administration
Adult Dosage: For Motion Sickness: 25-50 mg orally, 1 hour before travel. May repeat 24-hourly if required. For Vertigo: 25-100 mg daily in divided doses, as per physician's advice. Typical maintenance: 25 mg three times daily.
Administration: Administer orally with or without food. For motion sickness prophylaxis, take at least 1 hour before anticipated travel. Tablet can be broken for dose titration. Advise adequate hydration.
5. Side Effects
Common side effects may include:
- Drowsiness/sedation
- Dry mouth
- Blurred vision
- Fatigue
- Headache
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Alcohol, Benzodiazepines, Opioids | Potentiation of CNS depression, leading to severe drowsiness, impaired coordination, and respiratory depression. | Major |
| Other Anticholinergics (e.g., Atropine, TCAs, Antipsychotics) | Additive anticholinergic side effects (dry mouth, constipation, urinary retention, confusion). | Major |
| CNS Depressants (Barbiturates, Sedative Hypnotics) | Increased sedative effect. | Moderate |
| Monoamine Oxidase Inhibitors (MAOIs) | Exaggerated anticholinergic and CNS depressant effects. | Contraindicated |
| CYP2D6 Inhibitors (e.g., Fluoxetine, Paroxetine, Quinidine) | May increase meclizine plasma levels, increasing risk of side effects. | Moderate |
7. Patient Counselling
- DO take the first dose for motion sickness at least 1 hour before travel.
- DO inform your doctor about all other medicines you are taking, including OTC drugs.
- DO stay well-hydrated to counter anticholinergic drying effects.
- DONT consume alcohol while on this medication.
- DONT drive, operate machinery, or engage in hazardous activities until you know how it affects you.
- DONT break or crush the tablet unless advised (for dose titration).
8. Toxicology & Storage
Overdose: Symptoms are primarily extensions of its pharmacological effects: Severe CNS depression (coma, respiratory depression), intense anticholinergic effects (hyperthermia, tachycardia, dilated pupils, flushed dry skin, urinary retention, ileus), hallucinations, seizures (especially in children), and cardiovascular collapse.
Storage: Store at room temperature (15-30°C), protected from light and moisture. Keep out of reach of children. Do not use after the expiry date printed on the pack.