Except for the common ground of low NSC's, they eat completely different but it does sound as if you're not in the U.S., so I am of no help
What medications? For weight loss? What else?
If they are powder form (NOT granules like sugar is) or liquid, buy some 60CC syringes and cut the tips off so you have a hole that will be about 3/8" or 1/4" wide; I don't know what that is in centimeters but it's not quite as big as the eraser on a pencil
I put a little bit of 100% pure apple juice in some water, put the meds in the 60 CC syringe, pour the water/apple juice in,shake everything up, then syringe it down the horse about 10CC's at a time. Any more than that and the horse has trouble swallowing all that liquid.
I know apple juice is a no-no, especially if your horse is on some type of thyroid meds; you may have something else, just as tasty, that you could mix with water.
It also sounds as if your horse might be on Thyroxine (Thryo-L) to lose weight? If so, that is not something a horse should be on for more than a few months. The horse must also be taken off it gradually. Again, if you're not in the U.S., your horse may be on something else and I could be really wrong on my comment
There are a few different ways to approach feeding your horse but without knowing what country you're in, it's near impossible to help. If you don't want to mention the country on the forum, perhaps sending a private PM to someone of us who is in your country might be helpful??