To make it a bit more in line with other things, you could rename it, so someone with the power can create a "poisoned" aspect as a supplemental action on a successful attack. That aspect doesn't attack on its own, but it can be invoked (including a tag) to increase your rolls against the target. If the target already has a "poisoned" aspect, you can add a free tag if you use the supplemental action. Still powerful, but it's a completely active power now.
After that, you can simply roll against the fists level as is, I would interpret the penalty to doing first aid on yourself as taking more time. So the added difficulty should go towards increasing the time increment. That means if the roll fails, it can still work, just takes a lot of time, which the character might not have.