Instruct: Classify Amazon reviews into positive or negative sentiment
Query: Doesn't work anymore

We've always used Frontline on our dog but this Summer it doesn't seem to be working at all. After 2 weeks of putting it on our dog is scratching and has irritated skin. We talked to our vet and he said Frontline isn't working as well anymore because it is no longer sold by prescription and has saturated the market and fleas have built up an immunity to it.