Horses can be masters of evasion when it comes to worming. If your horse doesn't like having a syringe in his mouth, follow this five-step plan from Vet Jo Gourlay.
Fill it with something tasty
To get your horse used to having a worming syringe in his mouth, get an empty syringe that's totally washed out and filled with something tasty like molasses or apple juice.
Get your timing right
To start with, hold the syringe close enough so that he can see it. He may show mild signs of slight concern, but not so much that he's pulling away.
A good place to hold it may be his lower neck or chest initially, but this will vary on depending on how reactive each individual horse is to the syringe.
Timing is critical, as soon as he stands still and doesn't react to the syringe in this location, it must be taken away.
Move up the neck
Gradually repeat this process moving up the neck towards the head a small step at a time, then onto the cheek and eventually touching the outside of the mouth.
If he starts to get too upset then go back a step and reinforce it again at this point.
From your horse's perspective, he doesn't want the syringe near his face, but has learnt that in order for you to take it away, all he has to do is stand still and not react.
Reward good behaviour
If you want to, in addition to removing the syringe, you could reward the correct behaviour of standing still with a treat.
To get the time of the reward exactly right, you could use clicker training. When he stands still with the syringe in place, you both click the clicker and remove the syringe simultaneously and then give a treat.
Take your time
Once he's happy with the worming syringe touching the outside of his mouth, the tip can be popped just inside his lips, preferably with something tasty on the outside of the syringe.
This allows him to learn that the syringe is removed once he stands still. He'll start to associate it with something positive.
From here you can progress to giving him a small amount of something tasty from the syringe, such as pureed apple.
When the time comes to use an actual wormer, he'll be used to the process so will accept you putting it in his mouth.