Worming a horse can be tricky business. First of all, you have to choose a horse wormer, and then you have to make sure your horse gets all of it in his mouth - and not all over your clothes!
When I am ready to worm my horses, I don’t just shove the tube into their mouths – that’s guaranteed to make them throw their heads up and try to avoid me.
I hold their halter underneath their chin, show them the tube of wormer and then just lay it alongside their lips. As soon as they relax - which they do quickly, sensing that there’s nothing to be afraid of - I slide the tube as far up alongside their tongue as I can reach. Pushing the end of the plunger, I keep it inside their mouth long enough that I know they’ve gotten all the wormer.
Every now and again one of the horses won’t open his mouth and in that case I just insert the wormer between his cheek and teeth – no matter what, he'll get it. You can even try feeding your horse his favorite treat after worming him. If it's a favorite food, the odds of him spitting out the wormer are slim, and you'll (hopefully) accomplish 2 things: your horse will swallow the remainder of the worming paste and he'll start to associate worming as a positive thing. Don't underestimate the power of a treat!
As a final hint, when you are ready to worm your horse, make sure that she hasn’t had anything to eat in a while. If she has a wad of half-chewed hay in the back of her mouth, the paste will stick to that and then it’s easy for her to spit out the wormer.