When traveling to tropical countries people tend to hear warnings of malaria, however, up to date, the USA CDC has no warnings about malaria in Costa Rica for all travelers, they do mention it in their website as one of the potential risks of traveling to any tropical country.
There are warnings to visit certain secluded districts in some of the rural areas of the country, in reality this disease is very strange even among locals that live in the supposed to be “risk areas”.
This success is due to the strong health system Costa Rica offers, one of the highest health systems. Statistics from the World Health Organization (WHO) frequently place Costa Rica in the top country rankings in the world for long life expectancy. WHO's 2000 survey ranked Costa Rica as having the 36th best health care system, placing it one spot above the United States at the time.
The public and private facilities and services are always being upgraded, for example the country has three JCI certified medical centers in the nation’s capital, San José. This is the highest global designation awarded in the healthcare industry.
In 2010 there were only eight registered cases. The vast majority of cases used to be in the province of Limon but new prevention measures in that province have resulted in there being only a single case in 2011. There have been no deaths in Costa Rica attributed to the strain of malaria that occurs in Costa Rica for more than a decade.
As far as medication goes, there is general agreement that Malarone has much fewer side effects and is just as effective as the older drugs such as chloroquin. Deciding whether or not you take the pills is essentially a risk/benefit analysis. In your position we would not take them, but if not doing so is going to mean that you or your traveling companion is going to be at all worried about the risk, then peace of mind might well be sufficient reason to take them.
Our Costa Rican doctor points out that while there is a slight chance of getting malaria in Costa Rica, there is also a slight chance of side effects from Chloroquin, which is at least as serious as the strains of malaria that have been recorded here. What we who live here do, if anything, is use insect repellant that contains DEET (diethylmethyltoluamide) when there is a likelihood of getting bitten. (Be sure to follow the precautions on the label.) Not only will this minimize the possibility of getting malaria, but also it will help protect you from getting other insect born diseases---not to mention itchy bites.
This all said, finally you have to make your own decision about whether to use malaria prophylaxis or not.
While we are talking about prevention of disease, make sure your tetanus inoculation is current.