Keep doing research on your country, because you might not want to trust the Peace Corps to take all the necessary health precautions. So, I got a prescription from my doctor and am taking it before I go. Also, there are reputed to be various horrible side-effects to mefloquine. You can follow this link to one of the pages devoted to the Lariam debate, and there are several such pages out there. It's really hard to say if these are psychosomatic or caused by the stress of Peace Corps life.
Some people do experience depression or disorientation, so keep an eye out for such symptoms.
There are alternatives to mefloquine if you discuss this with your medical officer. Most people do experience some sleeping disturbances, such as insomnia and vivid dreams, which can be alleviated by breaking the pill into halves or quarters and taking it several times over the week. Click here to see the official, bona fide Peace Corps Guinea and Stephanie Chasteen's Page guide to What To Bring. Note, however that what you should bring depends on where you're going, so ask around. You'll get the most accurate information from people who have been there.