Spent some time on the mainland on our first trip and we took the anti-malarial, chloroquine. One pill a week, 1 week before departure, during the length of the stay, and , as I recall, two weeks upon return. None of us had any side effects whatsoever. Nothing.
I no longer take chloroquine for a trip just to the island, but irrespective of what you may hear, there are many documented cases of malaria in Belize. Belize has the highest incidence of malaria in the Americas.