Injection
This weapon or its ammunition can be filled with a drug; a contact, ingested, inhaled, or injury poison; or a medicinal compound. On a successful attack with the weapon (either the first attack if it’s a melee weapon or an attack with the relevant piece of ammunition if it’s a ranged weapon), the weapon automatically injects the target with the substance. Refilling the weapon with a new substance acts as reloading it and is a move action. Each different injectable material must be bought separately and can be used in any weapon with the injection special property. See page 231 for rules and prices for drugs, medicinals, and poisons.
Source
Core Rulebook pg. 181