Left 4 Dead 2 from being the world's most comprehensive zombie apocalypse survival sim is its liberal use of pain meds and "health packs." In the real world, these things don't have the immediate recuperative effects they do in the video games. If you want a taste of how it'll truly be, you should probably check out this week's mutation: Healthpackalypse.
In this custom Versus play mode on both the PC and Xbox 360, the survivors get no health packs whatsoever. If you're on the Infected side, laugh maniacally as the humans try to make it through without the use of first aid. If you're on the survivor side, well -- better luck next time! It's called "the apocalypse" for a reason, muchachos.