Healing items

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
"Stop squirmin'. I'm gonna heal yo ass."

- Coach

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 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. 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.

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 can not 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 mouse1 / right trigger. A charging meter, which takes 5 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 before continuing. There will be a message on the left side of the screen informing everyone who brought whom back to life. The Defibrillator can become especially useful in Expert difficulty and/or Realism Mode, where a Witch is able to instantly kill a player. You can not revive a teammate who has fallen to their death. The defibrillator in action here, 6:00-6:10.

Pain pills
"Pills here!"

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 (and including) 40%.

Keep in mind that taking damage when both permanent health and temporary health exist, permanent health will be taken first. If while pain pills are active 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
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, and are immune to the horde's slowing melee attacks.

It also increases the speed of using health kits, reviving survivors, 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 the Adrenaline injector too much, as the monophonic penalty affects both players.

The Adrenaline injector 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 injector can be passed to a teammate by using the melee button.

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)

Trivia

 * Sprays of the first aid station have become widely available online and may look fairly convincing at a distance.
 * 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 advertises these specific pain pills, saying "Yes, we've got... Pills Here!"
 * The design of the bottle is based off of Target brand Ibuprofen.
 * Even whilst holding pain pills or a First-Aid kit, Survivors can still use their flashlight. This is odd as there is nowhere for the light source to emanate from.
 * Since hitting the shelves, Left 4 Dead's "Pills here!" became an instant Internet phenomenon/meme, millions of fans claiming that the Survivors were addicted to these over-the-counter pain relievers (or "peelz") with an extensive focus on Louis and Bill, with Louis being portrayed as overly obsessed with them, and often repeating the phrase "grabbin' peelz".
 * In the game, Survivors swallow the entire bottle of pain pills at once. However, if someone were to do this in real life, they would likely die from an overdose soon after, or choke on the pills considering the Survivors don't ingest them with water. Plus, Ibuprofen usually takes around 20-30 minutes to actually start relieving pain, with a recommended dose of no more than 6 tablets per day, where in Left 4 Dead, the pills take affect immediately.
 * It is possible for a merciful Director to change a bottle of pills into a first-aid kit, even if a Survivor has already seen the pills.
 * Pain pills are attached to the Survivors' belts with no visible strap holding them. Likewise, the first aid kit stays on the their backs with apparently no straps keeping them on.
 * After using a first-aid kit, small bands of gauze will appear on the right arm and left thigh or the target, but disappear a couple seconds later.
 * The First Aid Kit, like the new P220 pistol in Left 4 Dead 2, had no walking animation, staying still on the screen when moving.
 * The health pack is an Avatar Award for Xbox 360. Similar to achievements, awards present the player with a new item or article of clothing they can apply to their Xbox 360 Avatar. The requirement for the health pack is surviving each of Left 4 Dead 2's campaigns.
 * The defibrillator has no cords from the pads to the pack on a Survivor's back. This would make the defibrillator unusable in real life.
 * AI-controlled Survivors will never take the defibrillator, similar to their treatment of explosive items.