The number of successful saving throws needed to cure a poison increases by one. If the poison deals ability damage on a failed saving throw, that ability damage increases by 1.

Augmented (6th): If you expend two uses of mythic power, the target's penalty on saves against poison increases to –8, and the penalty for attempting to cure the target with skill or magic increases to –8.

If you expend three uses of mythic power, the poison bypasses the creature's poison immunity (this doesn't affect poison immunity from not having a Constitution score).