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). Pain pills are the only manner by which a survivor can restore their health back to 100 after taking damage, however this buff will still tick down as usual.

An interesting note is that in Team Fortress 2, another Valve game, pain pills make an appearance on an upgraded dispenser.