I've been using Frontline and now the Frontline Plus for several years with good success. Seems to last a full month even if I swim the dogs and so far (crosses fingers) no allergic or other negative reactions from 3 different Labs.
It's what my Vet partner with 5 of our dogs uses in Georgia too. If it's his choice, it's my choice.

Just watch for any reaction to it from the dog the first few times you use it. All dogs are not the same.