Cyclizine, sold under a number of brand names, is a medication used to treat and prevent nausea, vomiting and dizziness due to motion sickness or vertigo. It may also be used for nausea after general anaesthesia or that which developed from opioid use. It is taken by mouth, in the rectum, or injected into a vein. Common side effects include sleepiness, dry mouth, constipation, and trouble with vision. More serious side effects include low blood pressure and urinary retention. It is not generally recommended in young children or those with glaucoma. Cyclizine appears to be safe during pregnancy but has not been well studied. It is in the anticholinergic and antihistamine family of medications. Cyclizine was discovered in 1947. It is on the World Health Organization's List of Essential Medicines. In the United States it is available over the counter. Medical uses Primary uses include nausea, vomiting and dizziness associated with motion sickness, vertigo and post-operatively following administration of general anesthesia and opioids. It is sometimes given in hyperemesis gravidarum, although the manufacturer advises that it be avoided in pregnancy. Off-license use often occurs with specialists in hospitals to treat inpatients who have become severely dehydrated in pregnancy. An off-label use is as an opioid/opiate potentiator. The drug Diconal is a combination of cyclizine and the opioid dipipanone. Dipipanone is a schedule I controlled substance in the US. Contraindications Its antimuscarinic action warrants caution in patients with prostatic hypertrophy, urinary retention, or angle-closure glaucoma. Liver disease exacerbates its sedative effects. Adverse effects Common (over 10%) — Drowsiness, dry mouth. Uncommon (1% to 10%) — Headache, psychomotor impairment, dermatitis, and antimuscarinic effects such as diplopia (double vision), tachycardia, constipation, urinary retention and gastro-intestinal disturbances. Rare (less than 1%) — Hypersensitivity reactions (bronchospasm, angioedema, anaphylaxis, rashes and photosensitivity reactions), extrapyramidal effects, dizziness, confusion, depression, sleep disturbances, tremor, liver dysfunction, and hallucinations. 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 vestibular system and on the chemoreceptor trigger zone. Cyclizine exerts a central anticholinergic (antimuscarinic) action. Synthesis Cyclizine may be prepared by the Eschweiler–Clarke methylation of diphenylmethylpiperazine or by reaction of benzhydryl bromide with 1-methylpiperazine in acetonitrile to form the hydrobromide salt of the drug. History Cyclizine was developed in the American division of pharmacy company Burroughs