Pain pills

Pain pills can be found throughout the game and in first aid stations. Unlike the First aid kit, they will only give the user 50 temporary points of health when taken. Temporary health, like the name suggests, is health that wears off after it is received. Pills can also be given to teammates by pressing the melee button (right click on PC or left trigger on 360 version).

Trivia

 * An interesting note is that in Team Fortress 2, another Valve game, pain pills make an appearance on an upgraded dispenser.
 * A poster in the terminal of the airport advertises these specific pain pills.
 * Ibuprofen was apparently the most often used pain reliever during the development of L4D, which may be why it is used as the label in-game. This is probably a reason why they appear in the TF2 upgraded dispensers as well.