Heartworm Prevention in Cats

AVRAM asks: "When I first got kittens, a coworker of mine pulled me aside and said "be sure to give them heartworm prevention meds." I've been reading up on this a bit and there seems to be no clear consensus about whether or not to feed the cat heartworm meds during mosquito season. My question: is there any harm in giving the meds? Does it have any side effects?"