Heartguard Plus Side Effects. The most common heartgard side effects are; Rarely, diarrhea or vomiting within 24 hours of administration were reported (1.1%).

We just started heartgard a month ago and through out the whole month she has. Which they were able to tolerate with few side effects. Some reported side effects of heartgard plus for cats include disorientation, blindness and dilated pupils, loss of appetite, agitation and tremors.