First-aid kit

The first-aid kit, colloquially known as a med kit or health pack, is used to heal any damage the Survivors might take during a campaign. When carried by Survivors, it is held on like a backpack with no visible straps holding it.

First aid kits can always be found inside of safe rooms, and also during the finale of a campaign. They can also be found in first aid stations and in random places anywhere else throughout the campaign, though this is very rare. On expert difficulty, most first aid kits found in the level itself will be replaced by pills. The only sure way to get health packs on expert is to grab one in the safe room or at the finale. Finding one in the level itself is extremely rare and should not be wasted.

In Left 4 Dead 2, it is possible for no first aids kits to spawn in the safe rooms. Therefore, it is highly recommended to use Pain pills and Adrenaline injectors whenever possible. Also the player has the choice to pick up a defibrillator instead, which can be used to revive dead team mates.

Usage
It takes 5 seconds to use the first aid kit to heal yourself or another Survivor, during which time you cannot defend yourself in any way. Instead of restoring a set amount of health, first aid kits will only restore 80% of the damage the Survivor has taken. For example, if a Survivor has taken 70% damage, using a first aid kit will add 56% (80% of 70) to the Survivor's health, bringing the Survivor to 86% health. This healing is not based on any temporary health a Survivor might have after taking pain pills, and will remove any extra health gained by them. Survivors healing after an incapacitation work on the same rules, and thus, always receive 80 health points. Please note that first aid kits can never bring a Survivor back to 100% health, and using more to try to do so is a waste of a first aid kit. In Left 4 Dead 2, Survivors are able to walk slowly and heal at the same time.

As a weapon
A Survivor can use their melee attack while they are holding the first aid kit, though the only time you might consider doing so is if you were in the middle of healing yourself or another Survivor and you got attacked suddenly. If that happens, it would be best to use your melee attack to push the attackers away to give yourself time to switch to another weapon. Needless to say, using your melee attack while holding a first aid kit does the same amount of damage as any other melee attack, but it's easy to get caught up on trying to heal another Survivor (both the melee attack and healing another Survivor share the same button). Pistols would generally serve better in a (serious) fight.