Left 4 Dead

There is a container of Pain Pills in the house right near the start of Death Toll - Boathouse Finale (I loaded it up on Normal..not sure if that effects whether they spawn or not). Exit the initial building and go grab the pipe bomb on the Humvee. Throw it so that you have a clear run into the house. The pills will be right near where you exit the house, next to the sink. Now once you've grabbed them, run up to one of the other survivors and hold down LT to give them the pills. This should take you no longer than 30 seconds per go so if you count in the time it takes to reload the level, I guess it should take you around 10 minutes.

The way that I got this achievement was to start up "Last Stand" in survival mode (if you do not have survival mode, then you can download it for free here: http://marketplace.xbox.com/en-US/games/offers/0ccf0001-0000...) locally with two controllers. Player one was getting the achievement and player two (who was a guest) simply got hurt and popped pills. When the game starts, you already have a bottle of pills. Just grab the assault rifle and shoot up controller two until his health is low. Have controller two use their pills, then give yours to them and have controller two pop the bottle you just gave him. You can only get one towards the achievement progression per round, so after controller two pops the second bottle of pills, hit start and call a vote to start a new round. Have both controllers vote yes and rinse and repeat. An easy 20G.

Its not just pain pills picked up off the ground, the player you give them to has to actually use them for it to count. Thats why you cant just pass them back and forth.