1) Have a consumable item in your inventory, and an item of the same slot size in the vicinity (Whilst pressing tab)
2) click Use on the consumable item.
3) as soon as "Use" is clicked and the consuming animation begins, move the item that is in the vicinity, over onto the consumable item (Swapping the consumable for the item on the floor)
4) The consumable item is on the floor and remains at 100% (Or un-opened for cans) and the item that was previously on the floor is now in your inventory.