Kills Flea Infestation Quickly – My cat had a flea problem for around 8 months. I used drips from Advantage and Revolution, plus a tablet from the vet. All to no avail. Also used daily bathing with flea shampoo, but the infestation was just getting worse. In desperation, I tried this Seresto flea collar, and they cleared in days. I was aware of the side effects, so I checked him every day. Noticed a small skin rash where the collar was touching his skin, so I moved the collar to below his other collar, so that it didn't touch so much, and this seems fine now. I check him every day for side effects, but after a month he seems ok, and thank goodness no more fleas! Great product worked where others failed. Just watch the side effects. I don't think it can cause cancer quickly, but my cat is old, so just happy to get ride of those fleas.