I give it to all of mine and have never noticed any side effects. I do have to hide it in a yummy piece of food in order to get them to take though
If they know it is there they will spit it out... the little turkeys!
During the fall/winter months I often use Revolution for flea/tick and heartworm control. But I don't feel Revolution has strong enough tick control for spring and summer months so I use Frontline Plus during those months. Frontline doesn't contain the heartworm preventive so, while using the Frontline Plus I use Interceptor also.