Senbon

Senbon are metal needles with a point at both ends. They often serve a medical purpose, being used to strike acupuncture points. They have little killing power, but can be thrown with great accuracy. A user with proper medical knowledge or ANBU training however, can effectively use senbon in battle to incapacitate or even kill their target should they aim for vital spots. To make these needles more effective, the user can poison the tips of the senbon. An added benefit of throwing thses at the enemy is that thses are smaller then a kunai or a shuriken, making it harder to see and dodge.

Reference

 * Narupedia
 * See Narupedia's article here - Senbon