While the spell lasts, you can deliver the substance with a mere touch. To do so to an opponent, you must make a successful melee touch attack. If you hit, the substance takes effect immediately, despite any onset period, and that opponent receives the normal saving throw (if any) against the substance.
http://www.d20pfsrd.com/magic/all-spells/t/touch-injection
If you touch anything or anyone while holding a charge, even unintentionally, the spell discharges.
http://www.d20pfsrd.com/gamemastering/combat#TOC-Cast-a-Spell
Does Touch Injection only discharge willfully, or can it discharge unintentionally?