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.
If you touch anything or anyone while holding a charge, even unintentionally, the spell discharges.
Does Touch Injection only discharge willfully, or can it discharge unintentionally?