To get this achievement you need to obtain and use 50 Medkits. This is an accumulative achievement, and will more than likely be obtained by just playing the game.
The Medkits are medium sized red boxes, and you can stack about 10 in your inventory. Any time you find a Medkit and you already have 10, simply hurt yourself in some way, use a Medkit that you have, then pick up the extra one.
(Thanks to Althir1990 who says you can actually stack up to 10 Medkits)