Meclizine

Meclizine is a long-acting piperazine H1 antihistamine with antimuscarinic and central anti-vertigo activity, used for motion sickness and vertigo of vestibular origin. It is sedating and anticholinergic, so it is poorly suited to older adults.

Classification. First-generation H1 antihistamine; vestibular suppressant; antiemetic.

Mechanism. Reduces excitability of vestibular nuclei and labyrinthine function by H1 and muscarinic blockade, and depresses the vomiting centre.

Pharmacodynamics. Onset about 1 hour; duration 8 to 24 hours. Prolonged use blunts central vestibular compensation.

Pharmacokinetics. Absorption is variable. Half-life about 5 to 6 hours.

Metabolism. Hepatic, mainly by CYP2D6. Metabolites are excreted in urine and faeces.

Adult oral doses unless stated.

Motion sickness prevention. 25 to 50 mg 1 hour before travel, repeated once every 24 hours for the duration of the journey.

Vertigo from vestibular disease.

• 25 to 100 mg daily in divided doses.

• Short courses only (days, up to 2 weeks) because long use impairs central compensation.

• Positional vertigo from benign paroxysmal positional vertigo needs a repositioning manoeuvre, not a vestibular suppressant.

Nausea and vomiting of pregnancy (second-line). 25 mg up to three times daily when dietary measures and pyridoxine are insufficient; evidence is limited.

Children. Not advised under 12 years; from 12 years use adult doses.

Kidney and liver adjustment. No fixed adjustment; use the lowest effective dose in severe impairment and in older adults.

Contraindications. Hypersensitivity to meclizine or other piperazine antihistamines. Caution in narrow-angle glaucoma, prostatic enlargement, urinary retention, asthma, pyloric obstruction and older adults.

Common adverse effects. Drowsiness, dry mouth, blurred vision, headache and fatigue.

Serious adverse effects.

• Confusion, delirium and falls in older adults.

• Urinary retention and acute glaucoma in susceptible patients.

• Tachycardia.

• Paradoxical excitation in children and with overdose.

• Impaired driving and psychomotor performance.

Major interactions.

• Alcohol, benzodiazepines, opioids, sedating antihistamines: additive sedation.

• Other anticholinergic drugs (tricyclic antidepressants, antipsychotics, hyoscine): additive anticholinergic effects.

• CYP2D6 inhibitors (fluoxetine, paroxetine, bupropion): raised meclizine exposure.

• May mask early signs of ototoxicity with aminoglycosides.

Special groups.

• Pregnancy: no consistent teratogenic signal; acceptable when needed.

• Breastfeeding: may reduce milk supply and cause infant drowsiness; use occasional low doses only.

• Children: not advised under 12 years.

• Older adults: avoid or use 12.5 to 25 mg once daily; strongly anticholinergic.

• Kidney impairment: use the lowest dose.

• Liver impairment: use the lowest dose.

• Regional: sold without prescription in many markets; ask about self-medication in patients with dizziness who present with confusion or retention.

Formulations. Tablets 12.5 mg, 25 mg and 50 mg; chewable tablets. Availability varies by market.

Administration. Take with water, with or without food. For motion sickness take 1 hour before travel.

Baseline. Medication review for anticholinergic burden and sedatives. Cause of vertigo: peripheral or central.

Monitoring.

• Sedation, confusion and falls.

• Urinary retention and intraocular pressure symptoms.

• Review the need at 1 to 2 weeks.

Therapeutic drug monitoring. Not applicable.

Stopping. After short use, no taper is needed.

Referral and escalation. Urgent assessment for central causes of vertigo: vertical or direction-changing nystagmus, severe headache, new neurological deficit, inability to walk, or vascular risk factors. Vestibular rehabilitation for chronic imbalance.