Well, we used to not give it to them during the winter either. But I was talking to the vet & he was telling me that it's best because then they have "NO" chance of getting it, plus you don't have to have them tested every year if you keep them on it. I've found that it cheaper to go that route & I don't have to worry, what if I didn't get them tested & on the meds before the first mosquito came out? I personally just think it's best all around, and so do the dogs (they don't have to go get poked an extra time every year!)