Hi, I haven't any experience with heart worm medication giving a bad reaction, or making my bullies ill. But it is very possible, as any medication can cause a bad reaction, or the potential to make your dog sick, just as every person is different, and reacts differently to prescription medications, dogs can have the same reactions. We use Revolution from June to November, and it is administered by a vial that we break open and place the drops in between the shoulder blades, it covers fleas, ticks, and heartworm. This is our second season using this medication and we haven't had any reactions to it so far. Which medications are you using?