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They have 20% chance to proc on light attacks , heavy attacks and weapon skills . You can read it in the description of any poison .
Some skills like the throwing blade (not sure about the correct name) from the dual wield skilline ignore´s poison cooldown, making it possible to proc the selected poison on every hit/throw. "Old" bug that still hasn´t been fixed.
A common build that utilize this is when someone using widowmaker set and just throw daggers to proc poison into widowmaker proc (Since Widowmaker doesn´t have a cooldown you can theoreticly proc widowmaker on every usage of throw dagger)