Kotarō

is a form of saigenzai that was originally created by Tenma Kodon as a way to control Rōen. It can only be produced within the bodies of Kodon clansmen: by consuming a plant that grows in abundance around the Howling Wolf Village, the Kodon's body starts to produce Kotarō. The Kodon can exhale the Kotarō as a purple cloud that induces genjutsu-like effects in anyone who breathes it in. It smells like cape jasmines.

If Kotarō is mixed with other herbs and medicinal plants, a powerful medicine can be produced with different potencies depending on how it's prepared:
 * - 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 Kotarō 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.