In my opinion, having horses accept administration of medicines, either orally or by injection, is important enough to deserve some discussion. Routine deworming, vaccinations and the occasional medical problem are part of a domesticated horse's existence. Having a horse accept these procedures well makes for a safe and efficient program.
I recently castrated a group of long yearling running Quarter Horse colts for a farm. I've been doing this for about 25 years. These colts have been handled on and off since they were babies, and are pretty well halter broke. But they run out on pasture most of the time, and good feeling yearling colts can get pretty excited and fractious if you were to just run them in, catch them and try to walk up and give them an injection in the jugular vein.
If they're excited or upset, they will be on an adrenaline rush. This situation makes it difficult to hold them still to give the injection, and it interferes with the desired effect of the drug. We've learned that it pays to bring these colts up a few days early so they can be handled in the same area you're going to do the surgery in. Conditioning these colts a little makes the procedure much easier and safer for both man and horse.
Another example of conditioning horses is getting them to accept oral medication. I think most horses that are difficult to administer oral meds to have been made that way by unwise approaches. One should never be in a hurry to push something into a horse's mouth. Have them in a confined area and let them get used to you running your hand over their nose and putting your thumb in the commisure of their lips (corner of their mouth) and onto their tongue.
When they have accepted having that done, administering the paste meds is just an extension. Never try to do it quickly or surprise a horse. You're not as fast as the horse, and you'll just make him suspicious or resentful of having his space invaded. This approach is really just a form of training similar to getting a colt to accept a bit in his mouth.
I also think that sometimes people can be nervous about administering drugs, and a horse can definitely pick up on that. If the human is nervous, the horse will be, too.
Personally, I don't use a lot of intramuscular, injectable antibiotics in horses anymore. Most of the time, the oral antibacterials are adequate, and you don't have to be sticking horses multiple times daily with large needles. Horses that are acutely ill should probably be in a medical facility where multiple drug administrations can be handled via an indwelling I.V. catheter.
I'd like to re-emphasize the point of getting that yearling colt up and handling him a little before you have the vet out to castrate him. He may not be as gentle as you thought he was when you turned him out last fall.