Medicine is a craftable syringe in Far Cry 3 and Far Cry 4 made out of green leafs. Upon using it, it refill some of the players health. The amount of health restored varies on what skills the player has.
In Far Cry 3 it can be crafted in the Crafting menu, using one green leaf.
In Far Cry 4 it will be auto-crafted using two green hurks if there is enough in the inventory and the Medicine slot are not full.