Cyclizine is an antihistamine drug used to treat nausea, vomiting and dizziness associated with motion sickness, vertigo and post-operative following administration of general anaesthesia and opioids.
Pharmacology
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.
Some opiate-dependent patients combine cyclizine with their methadone  dosage, a combination that is known to produce strong psychoactive effects. It is also sold in combination with another opioid dipipanone under the brand name Diconal.
Side effects
Common (>10%) - Drowsiness, xerostomia (dry mouth)
Uncommon (1% to 10%) - Headache, psychomotor impairment, and antimuscarinic effects such as urinary retention, diplopia (blurred vision), dermatitis, and gastro-intestinal disturbances.
Rare - Hypersensitivity reactions (bronchospasm, angioedema, anaphylaxis, rashes and photosensitivity reactions), extrapyramidal effects, dizziness, confusion, depression, sleep disturbances, tremor, liver dysfunction.
Formulations
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).
References
 British National Formulary 53 2007
 Lexi-Online drug database http://online.lexi.com/crlsql/servlet/crlonline