You don't mention when you plan to take your Belize vacation. When visiting parts of the world where malaria is a danger, it is important to assess the level of the threat, which can vary with the season. I've never taken anything to prevent malaria and never gotten the disease, but I am careful. When I traveled to southern India in the dry season, there was no risk of malaria because there were no mosquitoes, but most of the tourists I met were taking medication to prevent it.
Chloroquine, the drug you've been advised to take to prevent malaria, isn't benign. Its most common side effects include nausea, vomiting, headache, dizziness, blurred vision and itching. In addition, it can worsen the symptoms of psoriasis and carries a risk of liver toxicity. I checked with my colleague, pediatrician Sandy Newmark, M.D., about the wisdom of treating young children with chloroquine. He said that while the drug is considered safe for children at recommended dosages, it is best to be sure prevention is needed before giving it to youngsters. Dr. Newmark also cautioned that it is very important to keep chloroquine out of the reach of children because an accidental overdose can be very dangerous. I know of no effective anti-malarial drug that would be safer for young children. The adult dosage is 500 mg once a week. You're supposed to take the first dose a week before you arrive in a malaria-risk area and take subsequent doses on the same day during each week you spend in the area and for four weeks after you leave. If you take chloroquine on a full stomach, you'll lessen the risk of nausea.
From what I understand, the mosquito season in Belize runs from June through November, and the threat is worst in the western sections of the country. Belize City and the islands, such as Ambergris Caye, seem to hold little danger of an encounter with the mosquitoes that carry malaria, but if I were you, I would check to learn what the actual danger is during the time you expect to be vacationing. The U.S. Centers for Disease Control does recommend taking chloroquine when traveling in Belize outside Belize City.
If I were going to an area where risk of malaria was significant, especially a bad form of the disease (as in parts of Africa and southeast Asia), I would take chloroquine prophylactically (as a guard against the disease).
Andrew Weil, M.D.