What you need to do first is get an actual diagnosis. Because you describe what could be 3 different injuries that have 3 different treatments.
The first and what you are most likely dealing with:Sprain of the Distal Sesamoid ligament-The onset of lameness is sudden with swelling over the area. Treatment, confine horse to stall for 6 weeks and wrap with a support bandage, return horse to work very slowly as area will always be prone to re-injury.
Second-Sesamoiditis, this occurs following a sprain. Signs vary in degree of pain/swelling visible over the back of the leg. Treatment-Ice packs followed by compression therapy, If severe cast up to knee for 2-3 weeks. Corrective shoeing, with a very long rest period[months] Gradual return to light work.
And third, A fracture of the sesamoid- Horse immobilized in a sling with a compression bandage, followed by a hard cast to knee for 12 weeks. Then standing bandages and stall rest, possibly blistering of joint. Treatment however is rarely successful and as a rule they are unfit for work, have permanent lameness,stiffness, arthritis and calcification.
Bute is the drug of choice for these injuries, but there are other options.
The only way to get a proper diagnosis is to have x-rays and/or ultrasound to confirm which one your actually dealing with.
Good Luck