Cyclizine is a piperazine derivative with histamine H1-receptor antagonist (antihistamine) activity. The precise mechanism of action in inhibiting the symptoms of motion sickness is not well understood. It may have effects directly on the labyrinthine apparatus and on the chemoreceptor trigger zone. Cyclizine exerts a central anticholinergic (antimuscarinic) action.
Contraindications
Its antimuscarinic action warrants caution in patients with prostatic hypertrophy, urinary retention, glaucoma. Liver disease exacerbates its sedative effects. It cannot be combined with diclofenac, for example in the same syringe or bag of fluids, as a crystallisation reaction occurs.
Uncommon (1% to 10%) - Headache, psychomotor impairment, and antimuscarinic effects such as urinary retention, diplopia (blurred vision), dermatitis, and gastro-intestinal disturbances.
As cyclizine hydrochloride 50mg tablets and cyclizine lactate solution for intramuscular or intravenous injection (Brand names: Valoid® in UK and Marezine®, Marzine® and Emoquil® in US)).
Abuse
In the UK the drug was removed from the shelves as an OTC preparation in the mid 80s because of it's interaction with opiates. It served to increase the 'flash' of injected opiates. Since opiate users would regularly use high doses, the number of ER room admissions due to seizures became a notable public health risk. Even so, the drug remains available on the internet and pockets of abuse still exist in the UK.
References
British National Formulary 53 2007
Lexi-Online drug database http://online.lexi.com/crlsql/servlet/crlonline
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