Prepare the medication and have it ready to administer, but don't make the medicine the priority to begin with. Make the interaction the priority. If you cannot get your fingers in the horse's mouth, you will not get the medication in either. Below, I describe the procedure as I do it. I am right handed.

Halter the horse. Starting on the horse’s left side, and facing the horse's left eye, grasp the halter with the right hand but do not apply any pressure unless your horse moves its head.

At this point, begin to envision this task as separate small lessons, each of which needs to be successfully mastered before you proceed to the next step. The goal is to teach your horse to accept each individual step involved, and reward their attempts at compliance, however subtle.

Step 1: With your right hand on the halter, Put your left hand against the left side of your horse's muzzle – nothing more. If your horse resists, do not remove your hand. Instead put pressure on the halter encouraging them to lower their head. Upon the slightest "try", immediately release pressure. Do not remove pressure at all until the horse yields and never at the second your horse is exhibiting any resistance.

Step 2: Building on successful mastery of step 1, now try to insert your left thumb in the corner of the horse’s mouth – nothing more. Most horses will open their mouths at this point. Be careful not to get your thumb too far back as the molar teeth start about 3 centimeters behind the corner of the mouth. This is often the hardest part for horses that do not accept oral medications. If your horse doesn’t open their mouth, or throws their head or rears, put pressure on the halter encouraging them to lower their head. When there is a slight yield, immediately release pressure.

If your horse resists or raises its head, maintain or increase pressure. Only remove your thumb when you decide to, always when the horse is relaxed, and ideally after the horse has opened their mouth, and not while they are exhibiting any resistance.

Step 3: Building on successful mastery of step 1 and 2, now place the syringe in the back corner of your horse’s mouth – nothing more. Again, you are simply trying to desensitize your horse to this step.

Reward compliance by not placing any pressure on their head, and only remove the syringe when you decide to, and not while your horse is exhibiting any resistance.

Step 4: You get the idea… Building on successful mastery of steps 1, 2, and 3, you should be able to insert the syringe into your horse’s mouth and depress the plunger.

If you are afraid the horse will spit out the medication, follow step 4 by pushing your thumb into the roof of the mouth to stimulate chewing, which spreads the medication around in the mouth and makes it harder to spit out.

Stay calm and patient, and pay attention to detail and timing. If you take the time now to work through this issue, it will pay off in the future. If you are struggling, then stop and take a breath. Go back to the prior step and ensure mastery before proceeding to the next step.

Make sure that the horse's mouth is empty, and that there is not a wad of feed in your horse's mouth already. Unless instructed otherwise, give oral medications before feeding.

If the medication is bitter or you believe your horse dislikes its taste, you can mix some corn syrup, molasses or apple sauce with the medication. Some advocate performing the skill using nothing but apple sauce or other tasty treat in order to train the horse to accept the procedure. The idea is that following this, it is substituted with the medication. I believe that if your horse respects and understands what you are asking, this is not necessary.