I had a thought for a Savage Left 4 Dead game I've been tooling around on. Notably, how to implement painkillers. After some thought, I determined the best way is the simplest: A bottle of painkillers converts one Wound into a level of Fatigue. It is possible to become Incapacitated from the use of painkillers.
I considered a mechanic to deal with addiction, but dismissed that as not in the spirit of things.
Remember that Wounds and Fatigue are additive, and there is no Edge to take away Fatigue penalties. Additionally, Hard to Kill only technically deals with Wounds and not Fatigue. I think it's not as overpowering as I worry about.
Potential other ideas:
1. Fatigue penalties don't kick in for 2d6 rounds.
2. Fatigue will be a greater presence in this game due to numerous other Special Infected I'm creating, so maybe I will create Edges that deal with Fatigue as well.
3. Perhaps painkillers just give a bonus to Vigor for a duration.
Why am I working in this rule? Because there's plenty of "survival horror" games and a serious lack of "action horror" RPGs. I don't know what your experience has been, but I have never creeped out my friends over the tabletop, and I can't sustain the mood that horror demands. What I can do is freak out the players with impossible odds, defy their expectations about the monsters' capabilities, and repeatedly cross the line between ally and enemy to my group.