Healing items

"First to aid, last to die."

- Bill

Healing items are those used to restore lost health, be it a temporary health boost or a permanent restoration. They can be found in ambulances or randomly throughout the level, but are most often found inside of first aid stations. First aid stations may contain up to 4 bottles of pain pills, but there is also a chance that they contain first aid kits instead, depending on how well the Survivors are doing and the difficulty level. First aid stations found in safe rooms always contain first aid kits.

First aid kit
"Can everyone stop a sec? I'm gonna heal!"

- Zoey

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, the player has the choice to pick up a defibrillator or ammo upgrades instead, which can be used to revive dead team mates or ignite/explode Infected.

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. However in L4D2 if you use Adrenaline while healing yourself or another Survivor it will only take 3 seconds. 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 incapacitation work on the same rules, and thus, always receive 80 health points. 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.

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.

Defibrillator
"OW, whatcha shockin' me for?"

- Ellis

The Defibrillator, also referred by Survivors as "chest paddles", is an item used only in Left 4 Dead 2, symbolized by a lightning bolt. It can be used to revive dead teammate and can only be used once. For balancing purposes, the player cannot carry a defibrillator and health pack at the same time.

To use the defibrillator, players must walk up to a corpse, which will acquire a faint red glow, and hold either of the attack buttons. A charging meter, which takes 3 seconds to complete, is shown. Then, there is a short burst of electricity and the teammate has been successfully revived. He/she comes back at 50 health (which is permanent health that doesn't drop like temporary health gained when being revived after being incapacitated) and can pick up all dropped items except for secondary weapon before continuing. There will be a message on the left side of the screen informing everyone who brought whom back to life. It is recommended most Survivors pick up Health kits and just have one Survivor have a Defibrillator at a time. The Defibrillator can become especially useful in Expert difficulty and/or Realism Mode, where a Witch is able to instantly kill a player, and since in Realism Mode, there are no rescue closets. A defibrillator can be used on any Survivor corpse, even if they have already been saved from a Rescue closet (even your own corpse); the player's "consciousness" will then enter the body of the newly-revived teammate. It should be noted that you cannot revive someone as long as somebody is standing over the body. Also in single player, when a bot dies and is then kicked out of the game, if the player attempts to revive the playerless body, it shows "Healing yourself" in the middle of the screen and the reviving animation plays, but nothing happens and the defibrillator cannot be used again.

In reality, defibrillators cannot revive somebody from the dead - they only do so in fiction. Specifically, they are used to reset a heart's rhythm, and are not to be used during a flatline; in addition, gel must be applied to prevent burns and CPR must be used for several minutes before defibrillation occurs. Seeing as both games are "movies" in a sense, though, it makes sense to be able to revive one with a defibrillator.

If someone dies from a hanging death (holding onto a ledge), they cannot be revived at all.

Pain pills
"Pills here!"

- The Survivors

Pain pills, unlike the First aid kit, will only give the user 50 temporary points of health when taken. When carried by Survivors, it is held on the Survivors right hip, probably on the belt.

Pills can be given to teammates by pressing the melee button (right click on PC or left trigger on 360). When not facing a teammate, pressing the melee button will allow you to use your melee attack against enemies.

Since the health from Pain Pills is only temporary, it will slowly decrease over time, dropping one point every few seconds until all the temporary health is gone. The pills you take will restore your movement speed and stop you from limping for as long as your total health is above (or equal to) 40%. Since temporary health goes down 1 every five seconds, the effects of pain pills will last 4 minutes, 10 seconds, assuming no damage is taken.

Keep in mind that taking damage when both permanent health and temporary health exist, permanent health will be taken first. If you have temporary health and your actual health (read: permanent) drops below zero, your temporary health will continue dropping until it reaches 1%, where it will stay until you heal, take some more, etc.

Adrenaline Shot
"Grabbin' a shot!"

- Coach

Adrenaline is included in Left 4 Dead 2, occupying the pain pills' inventory slot. Upon injection, the user gains 25 temporary health; and for 15 seconds, they move faster, do not experience melee fatigue, are immune to the horde's slowing melee attacks and their speed isn't affected by water.

It also increases the speed of using health kits, reviving Survivors, and reloading and switching weapons and items. As temporary side effects, the user also experiences tunnel vision and sound becomes monophonic, hindering their ability to determine the direction from which a sound came. This can become annoying during a split screen campaign if one player uses Adrenaline too much, as the monophonic penalty affects both players.

The Adrenaline shot is always seen in the Crescendo event in "The Park". This item could become valuable at the end of a finale, where Survivors must reach a rescue vehicle while being impeded by the horde. Like Pain pills, the Adrenaline shot can be passed to a teammate by using the melee button.

The adrenaline shot is a very precious item during a scavenge game, as it can increase your speed and increase the speed of the gas pouring by roughly 50%. Generally 2 shots will be spawned during a scavenge game. Wasting them can be a big mistake.

The adrenaline shot is based on real-life emergency "EpiPen" auto-injectors, which are filled with synthesized epinephrine: literal adrenaline. The adrenaline acts as a vasoconstrictor, slowing the body's allergic response and delaying the victim from going into anaphylactic shock. Since as much as 15% of the United States population is considered 'at-risk' for anaphylaxis, it is not beyond the realm of plausibility for the adrenaline shots to be found as often as they are, particularly if they were included as part of CEDA's Emergency Relief Supplies.

Behind the Scenes
Early screenshots show that the First Aid Kit has a more boxlike shape, didn't look like it was made out of soft materials and had a carrying handle at one time. This design can be still be seen in the final version's First Aid Kit icon.

Early videos showed, it was possible to share medkits like pills instead of healing them. (As seen as in this video at 0:36)

The placeholder for the Adrenaline Shot was nothing like a syringe, rather a reskinned pain pills model with a yellow bottle. The texture can still be found in Left 4 Dead 2 files. Then it became an syringe similar to the one used in the HUD before the current look for adrenaline.
 * The Defibrillator world model was originally supposed to be a reskined first aid kit model. Its textures can still be found in Left 4 Dead 2 files.