I've seen about 20 dogs do heartworm treatworm, two of which where almost emaciated and in horrible shape. The emaciated ones had to gain a little bit of weight, but we did do the treatment when they were under weight. I have never seen a dog die from the treatment. I know it happens, as they are injected with immiticide, a form of arsenic, but it dosen't happen all the time.

But then again, we had a 9 year old Golden boarding with us, and had had heartworm treatment early in his life. He passed away overnight due to a weak heart, from the heartworms and treatment. I believe the treatment does take years off the dogs life, so you have to weigh in that option.