Guide

The first seventy-two hours

Hour zero: the drop lands between the shoulder blades and starts spreading with the skin's own oils. The dog feels nothing but mild indignity.

Hour 12: fleas already on the dog are dying — the label's first checkpoint. Existing infestations start collapsing here, because adults die before laying the next generation.

Hour 24: the waterproof mark. From here the month's protection survives swimming, rain and the occasional roll in something regrettable.

Day 30: the drop's month is up — fleas, ticks and mosquitoes have been repelled and killed on contact the whole way. Reapply and the clock restarts; skip a week and the tall grass notices before you do.