I haven't heard of many cases of bad reactions (no more than the other brands), but it doesn't seem to work as well as far as heartworm prevention. I think since it covers so many things, it doesn't quite do as good of a job as Interceptor or Sentinel or other heartworm meds that are only for heartworm prevention.
There have been several cases at the clinic where I work that an animal on Revolution got heartworms. I think its better to do the flea and heartworm meds seperately. Also, I notice Frontline and Advantage seem to be much quicker at killing fleas. We don't even sell Revolution at the clinic anymore since it doesn't do a great job.
Shiloh, Reece, Lolly, Skylar
and fosters Snickers, Missy, Magic, Merlin, Maya