nvm, I did the thing!
public boolean onItemUse(ItemStack itemStack, EntityPlayer player, World world, int x, int y, int z, int par7, float par8, float par9, float par10)
{
if(player.getCurrentEquippedItem() != null)
{
ItemStack currentItem = player.getCurrentEquippedItem();
player.addPotionEffect((new PotionEffect(Potion.regeneration.getId(), 100, 1))); //4 seconds of Regen II
if (currentItem.stackSize == 1)
{
player.inventory.decrStackSize(player.inventory.currentItem, 1);
}
else
{
ItemStack itm=player.inventory.getStackInSlot(player.inventory.currentItem);
player.inventory.decrStackSize(player.inventory.currentItem, 1);
player.inventory.setInventorySlotContents(player.inventory.currentItem,itm.copy());
}
}
return true;
}