Do I have to take medicine to prevent malaria?

We can't force you, but it's a good idea.  The Center for Disease Control recommends chemoprophylaxis with any of the following agents: atovaquone-proguanil, chloroquine, doxycycline, mefloquine, or primaquine.  The most commonly prescribed agent for Central America is Chloroquine phosphate 500 mg orally, once/week.

Medication instructions for adults:  Take 500mg once a week, beginning 1-2 weeks before travel, in the evening, with food. Take weekly on the same day of the week while in the malarious area and for 4 weeks after leaving such areas.

Common side effects include diarrhea, blurred vision, headache, nausea, loss of appetite, stomach irritation, strange dreams.  Chloroquine can exacerbate psoriasis, so considering another agent may be appropriate.  If choosing Doxycycline, just know that your skin will be more sensitive to sunlight.  

A prescription for Chloroquine can be obtained through your primary care provider or any number of private travel clinics around the U.S.