It is typically taken for one to two weeks before entering an area with malaria. Doxycycline and atovaquone/proguanil provide protection within one to two days and may be better tolerated. If a person becomes ill with malaria despite prophylaxis with mefloquine, the use of halofantrine and quinine for treatment may be ineffective. : 4

Malaria treatment
Mefloquine is used as a treatment for chloroquine-sensitive or resistant Plasmodium falciparum malaria, and is deemed a reasonable alternative for uncomplicated chloroquine-resistant Plasmodium vivax malaria. It is one of several drugs recommended by the United States Centers for Disease Control and Prevention. It is not recommended for severe malaria infections, particularly infections from P. falciparum, which should be treated with intravenous antimalarials. Mefloquine does not eliminate parasites in the liver phase of the disease, and people with P. vivax malaria should be treated with a second drug that is effective for the liver phase, such as primaquine. : 4

Resistance to mefloquine
Resistance to mefloquine is common around the west border in Cambodia and other parts of Southeast Asia. The mechanism of resistance is by increase in Pfmdr1 copy number. Adverse effects
Common side effects include vomiting, diarrhea, headaches, and a rash. Severe side effects requiring hospitalization are rare, but include mental health problems such as depression, hallucinations, anxiety and neurological side effects such as poor balance, seizures, and ringing in the ears. Mefloquine is therefore not recommended in people with a history of psychiatric disorders or epilepsy.
Canadian soldiers who took mefloquine when deployed overseas have claimed they have been left with ongoing mental health problems. Research
In June 2010, the first case report appeared of a progressive multifocal leukoencephalopathy being successfully treated with mefloquine. Mefloquine can also act against the JC virus. Administration of mefloquine seemed to eliminate the virus from the patients body and prevented further neurological deterioration.Mefloquine alters cholinergic synaptic transmission through both postsynaptic  and presynaptic actions. The postsynaptic action to inhibit acetylcholinesterase changes transmission across synapses in the brain. References
Further reading
Chen LH, Wilson ME, Schlagenhauf P (May 2007). "Controversies and misconceptions in malaria chemoprophylaxis for travelers". JAMA. 297 (20): 2251–63. doi:10.1001/jama.297.20.2251. PMID 17519415. Schlagenhauf P, Adamcova M, Regep L, Schaerer MT, Rhein HG (December 2010). "The position of mefloquine as a 21st century malaria chemoprophylaxis". Malar. J. 9: 357. doi:10.1186/1475-2875-9-357. PMC 3224336. PMID 21143906. External links

"Mefloquine". Drug Information Portal. U.S. National Library of Medicine.