Mefloquine: Side-effects ('Lariam') Minor side effects such as nausea, vomiting, heartburn and loose stools occur in about 20% of users, but this is no more frequent than with other antimalarials and usually subside with continued use. Taking ½ tab twice a week with food, and drinking copious water with medication will help reduce these.

Unfortunately mefloquine frequently produces annoying adverse neurological effects such as insomnia, vivid dreams, dizziness, mental clouding, anxiety and coordination problems. These are sufficient to interfere with daily activities in up to 10% of users and are probably aggravated by use of alcohol and cannabis.
Disabling side effects sufficient to recommend the cessation of mefloquine occur in 0.5% of users. However despite this, some  5 - 10% of users will stop the drug.  Consequently any person requiring a clear mind and good co-ordination should not use mefloquine. This may involve travellers to high altitude and definitely those contemplating aqualung (scuba) diving. Other contra indications include persons with a history of seizures, neuro-psychiatric disorder, the first 3 months of pregnancy and those with cardiac conduction problems. Women are advised to use contraceptives during and for 3 months after administration of mefloquine.

Mefloquine is probably best commenced 3 -4 weeks before entering a malarious area as it takes this long to build up to satisfactory blood levels. (half life of 21 days).  Also this provides time to detect those travellers that develop unacceptable side-effects and thus enable a change in medication. Commencing 1 or 2 weeks before departure is not adequate.
A recent report involving soldiers taking mefloquine 250mg daily for 3 days to enable a quick build up in blood levels was surprisingly well tolerated.
US authorities are prepared to use mefloquine for up to 2 years continuously in Peace Corps Volunteers overseas.

Serious neuropsychological side effects can occur when mefloquine is used in high doses. (click important note )
see also Malarial misconceptions: http://www.hc-sc.gc.ca/pphb-dgspsp/publicat/ccdr-rmtc/00vol26/26s2/26s2n_e.html or copy