Mefloquine, sold under the brand name Lariam among others, is a medication used to prevent or treat malaria.

[4] Common side effects include vomiting, diarrhea, headaches, sleep disorders, and a rash.

[14][15] If a person becomes ill with malaria despite prophylaxis with mefloquine, the use of halofantrine and quinine for treatment may be ineffective.

[23] When some measure of subjective severity is applied to the rating of adverse events, about 11–17% of travelers are incapacitated to some degree.

[10] Available data suggests that mefloquine is safe and effective for use by pregnant women during all trimesters of pregnancy,[24] and it is widely used for this indication.

[27] Compared to other malaria chemoprophylaxis regimens, however, mefloqinone may produce more side effects in non-pregnant travelers.

Its elimination in persons with impaired liver function may be prolonged, resulting in higher plasma levels and an increased risk of adverse reactions.

The drug is currently manufactured and sold as a racemate of the (R,S)- and (S,R)-enantiomers by Hoffmann-La Roche, a Swiss pharmaceutical company.

[14] Mefloquine was formulated at Walter Reed Army Institute of Research (WRAIR) in the 1970s shortly after the end of the Vietnam war.

[5] Because of the drug's very long half-life, the Centers for Disease Control originally recommended a mefloquine dosage of 250 mg every two weeks; however, this caused an unacceptably high malaria rate in the Peace Corps volunteers who participated in the approval study, so the drug regimen was switched to once a week.

The incidence of these side effects is 1 in 13,000 with prophylactic use and 1 in 250 with therapeutic use.The first randomized, controlled trial on a mixed population was performed in 2001.

[38] Retired soldier Johnny Mercer, who was later appointed Minister for Veterans Affairs by Boris Johnson, told in 2015 that he had received "a letter about once or twice a week" about ill-effects from the drug.

[38] In autumn 2016, the UK military followed suit with their Australian peers after a parliamentary inquiry into the matter revealed that it can cause permanent side effects and brain damage.

[38] In early December 2016, the German defence ministry removed mefloquine from the list of medications it would provide to its soldiers.

An expert from Health Canada named Barbara Raymond told the same committee that the evidence she had read failed to support the conclusion of indelible side effects.

[38] Canadian soldiers who took mefloquine when deployed overseas have claimed they have been left with ongoing mental health problems.

[41] by acknowledging numerous instances of failure to assess the risks and warn of potential side effects of the drug.

In June 2010, the first case report appeared of a progressive multifocal leukoencephalopathy being successfully treated with mefloquine.