Medical Mastery. One of the 8 available Masteries you can choose from.
Medical Mastery allows the ninja to learn Medical techniques such as healing and reviving fainted ninjas, as well as harness the deadliness of poisons and toxins in combat.
Medical Mastery is a pretty great mastery as it has decent fighting capabilities as well as support in buffs, healing and reviving. It has access to poison techniques allowing you to deal out nice damage since you can afflict a poison DoT(damage over time) allowing you to deal extra damage without having to do much work. You do get access to a self buff and a self heal later on allowing you to take your damage further and even allow you self sustain. Drawbacks to this are most of the damage techniques are pretty hard to land and you may struggle to hit targets. Another could include having to really split stats into either dealing more damage with Intellect or focusing on healing with Chakra.
For PvE it starts off pretty slow, and it gets a little faster once you get Poison Cloud. If you can time it right you can also use Poison Scalpel to do AoE damage without getting hit which makes it nice. You can take it further once you get self heal since you won't have to stop and wait to recover naturally. If you go Chakra focused you will most likely end up needing to be in a party and just be a healer.
For PvP it's considered top tier since you can deal out crazy damage with Cursed Seal boosted Senbon as well as heal yourself. This is only a 50 second burst damage though so without the boost you're almost useless and you will only be relying on senbon to hit targets. The Intellect path with poisons is pretty effective since you can place DoTs onto your enemies. It's pretty good in group fights as well since you can cast Poison Cloud and end up hitting multiple targets.