Meclizine (INN, or meclozine) is an antihistamine considered to be an antiemetic. It is sold under the brand names Bonine, Bonamine, Antivert, Postafen, Sea Legs, and Dramamine (Less Drowsy Formulation). Emesafene is a combination of meclizine (1/3) and pyridoxine (2/3). In Canada, Antivert Tab (which is no longer available) was a combination of meclizine and nicotinic acid.
Meclizine is a first-generation antihistamine of the piperazine class. It is structurally and pharmacologically similar to buclizine, cyclizine, and hydroxyzine, but has a shorter half-life of six hours compared to cyclizine and hydroxyzine with about 20 hours (though half-life should not be confused with frequency of duration). It is used as an antivertigo/antiemetic agent, specifically in the prevention and treatment of nausea, vomiting, and dizziness associated with motion sickness.
Meclizine is less anticholinergic than many other antihistamines and other agents used for their antiemetic and antipruritic effects. Along with the aforementioned efficacy against nausea and itching, meclizine also shares the anxiolytic, analgesic-sparing (potentiating), sedative, and other effects of its chemical relatives cyclizine and hydroxyzine to varying extents. Related to this is the reported ability of meclizine to potentiate the antispasmodic, antidiarrhoeal, and other effects of diphenoxylate, loperamide, and difenoxin. Meclizine is sometimes combined with opioids, especially ones of the open-chain class like methadone, dextropropoxyphene, and dipipanone (originally combined with meclizine's parent drug cyclizine, the brand name of this combination is Diconal). The trend of mixing piperazine antihistamines has caught on in the United States, with some methadone users potentiating their doses with meclizine or hydroxyzine (cyclizine has only just recently been introduced to the U.S. OTC market in 2009, sold under the brand name Marezine at higher prices than generic equivalents of meclizine and hydroxyzine).
Mechanism of action
Meclizine is an antagonist at H1 receptors. It possesses anticholinergic, central nervous system depressant, and local anesthetic effects. Its antiemetic and antivertigo effects are not fully understood, but its central anticholinergic properties are partially responsible. The drug depresses labyrinth excitability and vestibular stimulation, and it may affect the medullary chemoreceptor trigger zone.
Meclizine is approved by the U.S. Food and Drug Administration (FDA) to treat symptoms of motion sickness and for management of vertigo that stems from diseases affecting the vestibular system. Meclizine's safety and efficacy in children younger than 12 years old has not been established, therefore use in this population is not recommended. Also, meclizine should be taken with caution in the elderly (older than 65 years) because of increased risk of confusion and amnesia.
Meclizine is effective in inhibiting the symptoms of motion sickness, such as nausea, vomiting, and dizziness. The recommended dose is 25–50 mg orally, taken one hour before travel. The dose may be repeated every 24 hours as needed.
Meclizine may be effective in relieving vertigo experienced as a result of inner ear infections or other conditions. The recommended dose is 25–100 mg per day orally, in divided doses.
Some common side effects such as drowsiness, dry mouth, and tiredness may occur. Meclizine has been shown to have fewer dry mouth side effects than the traditional treatment for motion sickness, transdermal scopolamine. A very serious allergic reaction to this drug is unlikely, but seek immediate medical attention if it occurs. Symptoms of a serious allergic reaction may include: rash, itching/swelling, severe dizziness, and/or trouble breathing.
Drowsiness may result as a side effect of taking meclizine. Users are advised not to operate heavy machinery while under the influence. The consumption of alcohol while under the influence of meclizine may result in additional drowsiness.
Special considerations in the elderly
As with any anticholinergic agent, meclizine may cause confusion or aggravate symptoms in those with dementia in the geriatric population (older than 65 years). Therefore caution should be used when administering meclizine to the elderly.
(4-Chlorphenyl)-phenylmethanol is halogenated with thionyl chloride before adding acetylpiperazine. The acetyl group is cleaved with diluted sulfuric acid. An N-alkylation of the piperzine ring with 3-methylbenzylchloride completes the synthesis.
Alternatively, the last step can be replaced by a reductive N-alkylation with 3-methylbenzaldehyde. The reductive agent is hydrogen, and Raney nickel is used as a catalyst.
Meclizine is obtained and used as a racemate, a 1:1 mixture of the two stereoisomers. Drug forms contain the dihydrochloride.