I found out my new rescue dog had roundworms after I gave him his first heartworm pill and found one in his poo. The vet said that the Sentinel also kills roundworms, and to give him another pill in 2 weeks instead of waiting 4 weeks. I haven't seen any worms since, but his poo has gotten darker and darker. It's really dark brown/blackish, but only on the outside. Inside its greenish. He hasn't had any change of food, and his poo has never been like this before... Is this some reaction to the Sentinel or the result of the worms dying? I have to take in another poo sample after he gets the second pill so if there's anything really wrong the lab will find it. But it's very strange.