Kotarō

is a medicinal form of saigenzai that can only be prepared within the bodies of Kodon clansman. It can be prepared with three levels of potency:
 * - its weakest form but which has zero side effects when taken.
 * - its middle form which has no side effects if taken in moderation.
 * - its strongest form but with guaranteed and powerful side effects, with symptoms varying depending on how it is taken.

Itachi Uchiha used the coarse form of Kotarou as an eye drop in order to extend his life, though it caused temporary blindness. Sasuke Uchiha also uses this to eliminate eye irritation as his Mangekyō Sharingan awakens.