The First Aid mod adds a new system for how your health and damage is managed in Minecraft. It replaces the traditional system with the heart bar and adds a brand new one broken up over 8 sections of your player including the head, body, and limbs.
You can access the new health system by pressing H and you can also see the status of each part in the upper left corner of your screen. Each part can be damaged individually and can cause debuffs to occur depending on what was damaged. So if you take fall damage your feet or legs will take the hit and you’ll suffer slowness until you heal.
And if your head gets damaged you may suffer blindness or nausea for a time. But be careful because if your head or body damage drops to zero, you’ll die no matter how many hearts the rest of your body has.
You can heal yourself by sleeping to get some health back but this mod also adds morphine and plasters to get you back on your feet quicker. Morphine will dull the negative effects you’re feeling but won’t completely heal you.
You’ll then need to place plasters on the affected parts to bring everything back to full health. Just right-click with it in hand and click and hold it on the part that needs healing.
You’ll see your health slowly regenerate and that’s why the morphine will be needed to reverse the debuffs until your body fully regenerates. But one thing to note is that this mod will remain active even if you remove it from the mods folder so you’ll need to change it back in the config or by typing /gamerule naturalRegeneration true.