If you agree with the guide, please rate up, help spread the information about Proper Kit Usage to make those public games safer and more enjoyable!

For my guide on Throwables:
Proper Throwables Usage

Для русских игроков сделан перевод данного руководства:
(Russian Translation by l!me)
Russian Translation

The following rules apply to Campaign/Realism 4 player Co-Op modes, as well as some 4 player Mutations. These rules can still apply to Easy/Normal difficulties, but are targeted at players in the Advanced/Expert level due to the lack of first aid kits on higher difficulty settings. Versus/Scavenge and other 8 player Mutations tend to vary so much between groups that kit usage should really be discussed as a team. The general rule regarding healing in versus mode is to keep your health above 40 permanent points to maintain speed.

Knowing when to use kits will set you apart from the newbies in the game. Proper Medkit use not only allows you to finish your current stage but also sets your next stage up for success. Because first aid kits are typically hard to come by, it is important to use them wisely or else you risk having multiple party members black and white, or worse, dead.

Things to keep in mind:

• Left click heals yourself
  
• Right click heals survivor in front of you or shoves whatever is ahead of you. When under attack, you don't want to use your kit to shove because you might accidentally start a heal on an ally instead, fail to shove, and resulting in extra damage taken. Don't shove with your pills or shots either, for similar reasons.
  
  
• Upon loading a map, all locations of pills are already determined. No further first aid or pain pills will spawn, but if your party is doing very poorly then the director may have mercy on you and decide to "upgrade" an item. Pills/Shots can turn into a Defib/Kit. This depends on how much damage you have taken and how many times you have been incapacitated, which are tracked by the game.
  
  
• Most players consider it friendly, and good karma, to heal others before healing yourself. Some believe it appeases the director, but at Advanced and Expert difficulties, the rarity of First Aid Kits does not change. Healing each other with First Aid Kits is not tracked by the game.
  
  
• First Aid Kits always restore 80% of a survivor's missing permanent health. Using a kit while at 0 permanent health brings you to 80HP. Using another kit brings your HP to 96 because 80% of 20 is 16.
  
  
• You lose 1 temporary health every 4 seconds, but sometimes it bugs out and will not bleed.
  
  
• First Aid Kits take 5 seconds to use, 3 seconds if under effects of an Adrenaline Shot.
  
  
• Closing the safe room door to end the round will cause first aid to finish casting instantly.
  
  
• The two major sources of damage onto survivors are zombie hordes and Special Infected. Special Infected can respawn in as quickly as 30 seconds though in campaigns they can take anywhere between 40-60. This fact alone means that the longer you spend on a map, the more damage you risk taking. Move through maps quickly, scavenge items and communicate well with your team. Do not spend time looking for items you don't need, or waiting for teammates to turn around to pick up items or guns.
  
  
• The third major source of damage onto survivors is friendly fire. Do not walk in between people shooting at infected. Consider it as in real life, you would never purposely want to walk downrange of an AK-47 or Hunting Rifle. If you wish to progress forward, crouch and/or walk and proceed slowly, wait for things to die and your allies to move, or otherwise communicate. Be sure to provide cover for each other by shooting zombies that are outside of an ally's field of vision, but be careful not to injure your allies in doing so. Effective campaign play is not about racking up common kills, so do not go out of your way to kill common, including stopping to snipe zombies on the outskirts, or firing through teammates in order to get kills. Have faith that your teammates can handle the common they see on their own, and instead shoot the ones approaching from the sides or behind.
  
  
• You want to be careful not to be TOO injured going into the Finales. However, Finales are generally considered to be the easiest stages because it usually boils down to knowing one gimmick spot to hold up at. There are also always 8 or more first aid kits on each Finale map (4 in safe room, 4 at rescue).
  
  
• Remember, there is no use to clear to a health item if you will sustain more damage because of the trip. If your teammates are already very far ahead of you, calling them back for an unnecessary healing item can lead to an extra horde and SI spawn, and more damage taken.
  
  
• A First Aid Kit is much more useful than a defib, and while you may choose to carry a defib on your team at all times, it is really only necessary on Realism mode, due to the lack of survivor closets.
  
  
• Never leave a First Aid Kit behind for special ammo. If you must, just deploy the ammo on the spot.
  
  
• If someone is 1HP, they will be moving at an extremely slow pace. It is usually worth it to take the few seconds to shoot them down and revive them assuming they're not black and white, of course.
  
  
• You can use a kit on someone while they're helping someone up. It is also possible to heal yourself and simultaneously help someone up if you press the buttons fast enough.
  
  
• Killing allies in the saferoom upon reaching the end of a level is acceptable ONLY if that player
  
  A) Agrees
  AND
  B) Has less than 50 permanent health.
  
  If you kill them (note: not incapacitate), they will drop all of their items and their primary weapon, and respawn the next round with 50 permanent health. Many times, groups having trouble with Expert mode will use this tactic to give themselves the best chance at completing the next round.

Common times to use a First Aid Kit:

1. If a survivor has been incapacitated twice and their screen is now black and white.
   
   Possible exceptions: there is a defib nearby, or there is a saferoom nearby.
   
   
2. If your team has a full inventory (either 4 kits or 3 kits and 1 defib), and comes across, or will come across an extra kit. For example, on Dead Center_1, there is always a kit directly after the elevator. You can use a first aid kit in the elevator and just grab the new one downstairs without having to stop in a hot zone.
   
   Possible exceptions: You are all very healthy already (all above 80 HP), Horde/SI are in the way and it is not worth the time to clear. For example, there is often a kit in the back of the army vehicle right outside the saferoom on The Parish_2, which many teams will skip because everyone is still healthy and there are a lot of zombies to clear.
   
   
3. You know a tank is nearby, and you are limping (below 40HP). This is often a good time to heal so that you may properly kite the tank punches, dodge the rocks, and kill him without taking damage. If you hear a tank coming and you are limping, you have choices:
   1. Use a kit, save the pills or shot in case you get hit by something because you wont be able to use a kit during the tank fight.
   2. Use the pills, kite and kill the Tank quickly, save the kit for later, or ideally, the end of the round.
   3. Do not heal at all, kill the Tank with teamwork and good baiting by healthy party members to keep the Tank off of you.
   With new players, I suggest using option #1. With higher ingroup coordination you can rely on option #3, and with high individual skill in the group, option #2 works.
   
4. You know a horde or panic event is soon, and you are limping. During places where you need to rush to a spot, or run past hordes, you don't want to be limping. Note that the game will almost always provide players with health items before these runs. ie: Dark Carnival_3(coaster), Dark Carnival_4(barns), Dead Center_3(mall), The Passing_2(underground).
   
5. A witch is blocking your path, you're limping and you cannot crown or kill her for whatever reason. Use the kit if and ONLY if you know you can run past her, and couldn't if you were limping.
   
   Possible exceptions: You have a good position, and your teammates can kite the tank, kill the witch, clear the horde without you needing to run. Another possible exception is if you are sacrificing yourself by taking all the hits from a horde and you know your teammates will be able to help you up or retrieve your kit if you die.
   
   
6. You are in the safe room at the end of a level. There will always be 4 kits waiting for you at the end of each level. Making it into the saferoom with a kit in your inventory will allow you to start the next stage with at least 80 health and a first aid kit.
   
   Possible exceptions: If multiple party members need multiple kits and you don't really need to heal. Don't feel obligated to take a medkit just because there are 4.
   
   Uncommon times to use a First Aid Kit:
   
   
7. Your teammates have died or rushed up several screens ahead. You are either alone or maybe paired with someone who hasn't been the fastest at clearing SI off of others. In this situation you usually just want to play on the safe side and use kits earlier than normal (perhaps after being incap'd only once). Making it to the end of the level is ultimately more important than being efficient with your med kits, and even if you die, you'll return to the saferoom with whatever supplies you originally had.
   
   
8. If your entire party has found itself low on health and limping, one or two players may opt to heal preemptively in order to prevent a possible wipe. Like #5, you want to be concerned with finishing the level, although ideally your party has measures to prevent this scenario from happening in the first place. This tends to occur after a poorly handled tank or SI attack.
   
   
9. During a finale, as rescue is arriving. No use having extra kits just lying there. Sometimes you may want to use a kit to ensure you can reach the rescue vehicle. Just make sure that using the kit doesn't delay you or your team! Usually as the second tank of the finale is dying.
   
   
10. Parkour, shortcuts, jumps, or just catching up. There are some areas in the game where having a running jump will allow you to take a shortcut or reach a more defendable position. Also, if you're ever abandoned by your team and they refuse to heal you, then you should just heal yourself so you can catch up since its doubtful they'll go back to save you if you go down.

The points illustrated below may have extremely rare exceptions.

1. Using a Kit immediately before or after being incap'd once, especially early in the level. You see this happen...a LOT. This is mostly because new players receive bad cues from the bots about when to use first aid kits. When you see this happen in Expert and/or Realism mode, that person usually gets kicked immediately. If you use First Aid Kits as often as bots do, most players would rather have a ♥♥♥. Stay with the ticking 30HP and..
   (Option 1): Play more cautiously until you find pills or reach an appropriate time to use a kit.
   (Option 2): You can also offer to be the lead position/bait, risking damage and possibly another incap so that the rest of your team remains healthy. Having a "damage sponge" is an excellent tactic. You rarely want to put yourself in a position where you deliberately take damage, but in a lot of situations, some chip damage is inevitable and will enable you to progress through the map with some speed.
   
   If you use a Kit before or after only 1 incap you essentially risk running out of first aid kits, leaving the survivors in a position where they are a man down, or have multiple black and whites.
   
   Do not listen to the bots when they tell you to heal! They are scripted to tell you to heal whenever you are low health. Also, bots are notorious for being kit sponges. They use them frivolously and they should not be used as reference on when to use first aid kits.
   
   
2. Using a Kit while in sight of, but before entering the safe room.
   There are maps where horde can spawn very close to the safe room door, and SI are known to hide in some safe rooms. Wait until the safe room is cleared and you are inside to heal. Related, it is generally extremely unwise to heal in the safe room first and then leave in order to rescue a fallen survivor, unless they too have a first aid kit on them and you're confident in your ability to save them.
   
   If you use a Kit before the safe room, and then take damage from something while on a heroic but unnecessary rescue, you've wasted your kit.
   
   
3. Using a Kit on someone else during a firefight. Not only do you freeze your target in place but you've also taken your guns out of the fight. Clear the zombies first.
   
   
4. Using a Kit after just having used pills, or using a Kit in the inventory instead of pills from the ground. No need to waste items. Think before you use 'em! Taking pills before you use your first aid kit will not grant you any extra health. Having 99 temporary health is better than having 80 permanent health. Also remember that temporary health isn't always "temporary" and will not always bleed. When that happens, your temporary health is as good as permanent.
   
   
5. Using a Kit because you hear SI. Do not panic at the sound of an SI. If you want to heal, be sure you are a safe distance from the SI. In most cases it is still better to attempt to kill it or shove it away. Many waste First Aid Kits by getting taken by an SI while they are foolishly trying to heal themselves in a bad position.
   
   
6. Survivors clustering to heal or check for items. It is important to leave one or two people on "lookout" in order to block spawns. AI SI and infected hordes follow the same spawn rules as players do in Versus mode, regarding line of sight and proximity. If everybody goes into a store to check for items, then you may find yourself ambushed out the front door.
   
   
7. Hard Rain. During this campaign, the survivors will be revisiting locations they travel through. Act 1 and 2 mirror Act 5/4 and 3 respectively. At first glance, there is an abundance of supplies, leading some newbies to overconsume kits. If you take all the items during one trip, you risk not having enough items on the return trip. Take kits from medicine cabinets first as they are believed to respawn sooner than ones taken from the table.
   
   
8. The biggest mistake of all is those that abandon their teammates and use all the first aid kits. Whether accidental or trolling, these things will get you kicked, killed, banned, ignored, or left with no game to play and nobody to play with. While it is true that the director will adapt against uncooperative players, human players have a much lower tolerance to unpleasant trolls.

Notes on Pain Pills/Adrenaline Shots

• Pain Pills grant 50 temporary health
  
• Adrenaline Shots grant 25 temporary health and a 15 second buff. This buff increases your movement speed, negates the slow from infected hitting you, negates shoving fatigue, and negates movement penalties associated with treading water. This buff also filters the colors on your screen and turns your sound from stereo into mono, reducing your ability to tell the originating direction of sounds.
  
• If you come across a crate of these, use them to cap your party's health.
  
• If you have both permanent health and temporary health and then take damage, you will lose permanent health first.
  
• Temporary health does not always bleed.
  
• Save Adrenaline Shots for moments of running, getting multiple teammates up, or pouring in consecutive scavenged gas cans.
  
• Save Pain Pills for those who are limping, use extra ones freely to cap your health as you run through the map.
  
• Dire situations may require you to use these items, so do not be caught without one in a clutch moment. Treat these as outs for tough situations, not as a grab bag for health points.
  
• Being low HP is not an automatic reason to use your med. Do not use pills or shots just to gain movement speed/health unless you really need that speed/health. More often than not, it does not help you if you get into a saferoom 5 seconds faster than you would have. It isn't worth wasting a medical item, especially if you're just about to use a First Aid Kit anyway.
  
• If somebody is limping and doesn't have meds, toss some to them so they at least have the option of using something if trouble should arise.
  
• Using meds before a First Aid Kit does NOT increase the amount you are healed. You simply waste your meds. Meds do not give you ANY permanent health! Save the shots and pills for times you'll really need them (or if you find extras).
  
• All that said, meds are fairly common to acquire outside of Realism mode, so don't feel TOO restricted on using them in casual campaigns. As you improve your game you'll suddenly feel like you are constantly surrounded by extra pills!

• Using a Defib on a dead teammate will revive them with 50 permanent health. They will be able to pick up everything from around where they died except for a secondary weapon.
  
• Defibrillating takes 5 seconds, but can be shortened to 3 seconds while under the effects of an Adrenaline Shot.
  
• If you cannot find the location of someone's body, hold the defib out and the corpse will be outlined in red.
  
  Generally speaking...
  
• In Campaign Mode, the max you want to carry is 1. Survivors rarely die and there are always rescue closets if they do.
  
• You receive a 25 point penalty for using one but you get to keep a multiplier, so it is likely beneficial to use one if you have the opportunity.
  
• In Realism Mode, you'll likely just take whatever you can get your hands on. There are no rescue closets, so Defibrillators become the only way of getting a player back in the game before reaching the safe room.
  
• If somebody is black and white and limping and you can spare approximately 5-10 seconds, then the team may agree to kill and then defib that person instead of using a kit (or if one is not available).

Occasionally you'll come across these yellow boxes of ammo. They share the same slot as your First Aid Kit and Defibrillator so generally you want to deploy them on the spot or skip them all together. They are a nice help in Realism Mode but entirely unnecessary in Campaign Mode.

The fire caused by Incendiary Ammunition can create confusion and vision problems for some players and the extra friendly fire damage from Explosive Ammunition can lead to the downfall of other groups.

Explosive Ammunition
vs Common:
You can aim at the ground to kill large areas of common at a time. They take damage from the shrapnel.
vs SI:
Each hit will free a captured survivor and stumble the SI.
vs Witch:
Each hit will stumble her.
vs Tank:
Each hit does more damage, does not stumble.

Incendiary Ammunition
vs Common:
One hit each is enough.
vs SI:
Extra damage, sets them on fire for 5 seconds.
vs Witch:
One hit will permanently light the witch.
vs Tank:
Each hit ignites him for 5 seconds. Known to sometimes override original fire source (Molotov, Gas can) thereby causing flames to go out.

I covered much of the basics of First Aid Kit usage that I learned from Left 4 Dead. Kit efficiency was important in that game due to a lack of defibs, but nevertheless remains vital in Left 4 Dead 2 as well, especially in Realism mode.

I am sure there are other circumstances I have not listed, but I feel like I hit most if not all of the important ones. Hopefully players can learn to use their own judgment based on the knowledge provided here, but if you think something should definitely be added I'd love to hear it in the comments below, along with any feedback!

I understand that a bulk of L4D2 players nowadays may not use English as their primary language. Anyone interested in translating this guide can feel free to do so if they contact me first.

If you agree with the guide, please rate up, help spread the information about Proper Kit Usage to make those public games safer and more enjoyable!

8/08/2015 version 1.07 (light updates, formatting)
9/13/2014 version 1.06 (general updates)
2/28/2013 version 1.05 (changed title from Kit Usage to Item Usage, added Defib, SpcAmmo, edited Meds)
2/27/2013 version 1.04 (added meds section)
2/23/2013 version 1.03 (added link to Throwables guide)
2/18/2013 version 1.02 (minor edits)
2/17/2013 version 1.01
2/16/2013 version 1.00 (Proper Kit Usage)