Each year, your vet requires your dog to undergo blood testing before she writes you a new prescription or supplies you with heartworm preventative medication. That's because your vet must make sure that your dog is still heartworm negative, even if you've given him heartworm tablets year round. Giving heartworm medication to an infected dog can result in serious complications.
The American Heartworm Society warns that giving heartworm medication to infected dogs can cause "rare but possibly severe reactions," that can kill the animal. Heartworm preventative works by killing off any larvae in the dog's system that enter via mosquito bites. It doesn't kill adult heartworms. An infected dog has microfilariae, or baby heartworms produced by the adults, circulating in his bloodstream. Giving him a preventative can cause a sudden microfilariae die-off, which can result in a dog going into shock and dying. If you think your dog may have heartworm, consult your vet immediately.