In every Fallout game to date, chem addiction has been based on chance. In Fallout3, Bethesda added the Usage Monitor, which has the net effect of increasing the chance of addiction with each successive use of a chem, but it still depends on a random number generator.
Personally, I don't think that's very fun or interesting. I take this chem, and I might get a debuff to my stats after it wears off. I could either pay a doctor to get rid of it, or I could just reload a save. It's an annoyance more than anything, and it's miles off from reality.
This mod adds chem levels and chem tolerances. Each dose of a chem raises your chem level and contributes to to a running average that constitutes your tolerance to the drug. Higher levels of a chem relative to your tolerance result in more pronounced effects, but if your tolerance is high enough, you will inevitably suffer withdrawal as the chem level decays. There is also a hard level limit for each chem, which, when hit, can cause lung failure, heart attacks, brain bubbles, 'roid rage, Mobius loops, or permanent addiction (depending on the chem). It also changes the effects of certain chems, so taking Psycho no longer makes your bullets hurt more bad.

In list form:

• All chems now have blood concentrations and tolerance levels!
• Build tolerance to chems and become better able to get the exact effects you want from them!
• Build tolerance to chems and suffer withdrawal! Withdrawal is worse if you're really really tolerant.
• OD and die horribly, or maybe not!
• Med-X now makes you ignore the effects of crippled limbs instead of being able to take more bullets! If you still walk slow after taking Med-X, try crouching and getting back up. It's a game engine thing, i can't do anything about that.
• Psycho now gives damage reduction, plus temporary health, plus ups your melee and unarmed damage considerably!
• Slasher is really neat; it cuts your Psycho level in half, but doubles the effects! Take lots and lots of Psycho, then take Slasher!
• Oh, right, and everything has weight now. (Except Jet, I still need to do that. Can anyone tell me how much an inhaler weighs?)
• If you have a high tolerance of a chem, there IS a spot where you stop getting withdrawal effects, but don't get the chem's effects. Take more chems!
• The perks still do things!
• Mentats now yummy!
  

But how am I going to keep track of all this stuff, you may ask? Why, with your new Pip-Boy extension, the Pharm-Boy 5200! Find it under the Aid section. The Pharm-Boy 5200 is designed for use by real medical professionals with real degrees and real stethoscopes, so if you're an ignorant tribal with no Medicine skill, you'll probably have no idea what you're looking at, except for the numbers part. (The alternate chem names ARE NOT meant to represent real-world names for the chems. They represent your character's ignorance of medicine. They'll go away when you have a higher skill. Please do not try to correct my spelling or terminology; the names are gibberish.)

Also note that this mod is by no means complete. Features to come include:

• Actual balancing! I want things to be basically in line with the base game, just more interesting. If withdrawal comes on too fast, please tell me!
• New chems, maybe!
• Chem interactions! Apparently Buffout, Psycho and Fixer cure Jet addiction better. I need to add that in.
• Other things! If you guys have good ideas or help me with the balancing, I'll put your names in the credits section and in the section bellow, along with what cool things you did.
  

SPECIAL THANKS:
Imp of the Perverse, as this mod would not exist without his. Nearly everything I learned about the GECK, I learned from looking at his mods.

MOTOSXORPIO and MadCat221 for looking at one of my scripts that was screwing up. It turns out I'd left an AND at the end of an if statement with nothing after it. A programmer friend of mine figures that means the code only would have run if the universe inverted!

BenTheGamingGuy, the first person to download this mod! While I was still looking at it, even!