Medical-nin (医療忍, Iryō-nin) are ninja who specialise in medical treatment and use of medical-oriented techniques to heal others. Becoming a medical-nin requires high intelligence and excellent chakra control. A medical-nin is trained to avoid getting injured in any way, for the reason that if they are injured, then there would be nobody to heal the other members of their team. Medical-nin accompany other ninja on high ranking missions to increase the success rate of the mission and overall survival rate of the squad.
Medical-nin make use of the Mystical Palm Technique — a technique used to heal comrades and can also be used in surgery. It allows them to make precise incisions and then doubles as a means to heal a wound. For more serious injuries, multiple medical-nin can combine their efforts with techniques such as the Healing Resuscitation Regeneration Technique which uses a portion of the patient's own body, as a medium to help the healing process.
The knowledge of a medical-nin can be used for a variety of purposes; for example, Hana Inuzuka is a veterinarian medical-nin that specialises in the treatment of ninken. Chiyo, in the past, used her medicinal knowledge to create deadly poisons. However, her efforts were often counteracted by Tsunade, renowned as the greatest medical-nin in the world.
Field medical-nin (Konoha ones at least), when in the field or on the battlefield, are equipped with medical pouches in order to carry medical supplies as well as their regular weapons.
According to Madara Uchiha, the First Hokage, Hashirama Senju, was also able to use medical ninjutsu with such skill that he could heal himself instantly without the need of forming hand seals; Madara concluded that Tsunade, who is heralded as the greatest medical-nin in the world, pales in comparison — something Tsunade initially herself admits — but after seeing her Strength of a Hundred Technique, Madara deemed Tsunade's healing abilities similar to that of her grandfather.
Tsunade has also been credited with creating the official system of the medical ninja along with which she created laws that her students and other medical-nin alike, should abide by. The four rules that govern medical-nin are:
- First clause (第一項, Daiikkō): "No medic ninja shall ever stop medical treatment until the lives of their party members have come to an end." (「医療忍者は決して隊員の命尽きるまで治療を諦めてはならない」, "Iryō ninja wa kesshite taiin no inochi tsukeru made chiryō o akiramete wa naranai.")
- Second clause (第二項, Dainikō): "No medic ninja shall ever stand on the front lines." (「医療忍者は決して最前線に立ってはならない」, "Iryō ninja wa kesshite saizensen ni tatte wa naranai.")
- Third clause (第三項, Daisankō): "No medic ninja shall ever die until they are the last of their platoon." (「医療忍者は決して小隊の中で最後まで死んではならない」, "Iryō ninja wa kesshite shōtai no naka de saigo made shinde wa naranai.")
- Fourth clause (第四項, Daiyonkō): "Only those medic ninja who have mastered the Strength of a Hundred Technique of the ninja art Creation Rebirth are permitted to discard the above-mentioned laws." (「忍法創造再生の百豪の術を極めし医療忍者のみ 上記の掟を破棄できる」, "Ninpō Sōzō Saisei no Byakugō no Jutsu o kiwameshi iryō ninja nomi, jōki no okite o haki dekiru.")
Since Tsunade is the only person who is able to use the Strength of a Hundred Technique, this effectively means she is the only medical-nin allowed to break the above rules. This hails as a testament to her reputation as the greatest known medical-nin in the world.