I don't think anyone has suggested using Provent a Mite directly on the snake - in fact, most people will tell you precisely NOT to do that.
We recommend Provent a Mite for the cage, and reptile relief to treat the snake itself. The Reptile Relief is a specific type of poison dilute enough to be safe to use on your snake in small amounts. Obviously, overdoing it can cause problems, like with any parasite treatment.
However, spraying provent a mite directly on the snake is not something we recommend at all. I've heard of keepers who do so, but it's risky and can cause neurological damage with your animals. I always let cages air out for at least half an hour, if not longer, with provent a mite, and soak the snake(s) in water for 5 to 10 minutes before and after treatment with Reptile Relief to ensure they are fully hydrated.
-Jen