I was just reading an interesting paper on the interaction between antipsychotic drugs, caffeine and smoking and I found this interesting snippet on how smokers need to take in three to four times more caffeine than non-smokers to get the same effect, owing to the fact that by products of increases enzymes in the liver which break-down caffeine.
Byproducts of tobacco smoking, particularly the polycyclic aromatic hydrocarbons, are metabolic inducers. These byproducts are inducers of the [liver enzyme] cytochrome P450 isoenzyme 1A2 (CYP1A2) and of the less understood UDP-glucuronosyltransferases (UGTs).The metabolic inductive effects are not specific to tobacco smoking; they can also be expected from marijuana smoking.
Because inducers require the synthesis of new enzymes, several weeks are usually needed before the maximum effects of inducers are seen. Inducers’ effects may take a few weeks to disappear as well….
Additional pharmacologic support of the relevance of smoking’s inductive effects comes from caffeine intake studies. Caffeine, a drug that is more than 90 percent dependent on CYP1A2 for its metabolism and that is widely used in the United States, can exemplify smoking’s effects on drug metabolism.
The C/D [concentration-dose ratio] of caffeine appears to be threefold to fourfold as high among nonsmokers compared with smokers. This higher ratio means that smokers need three to four times the caffeine “dosage” as nonsmokers on average to get the same plasma caffeine levels.
It turns out that two antipsychotic drugs, olanzapine and clozapine, are also broken down by the same enzyme, so smoking will reduce the effect of these drugs.
Hence smokers need larger doses to have the same effect, and patients on these drugs who give up smoking might find a sudden increase in side effects if the dose isn’t dropped.
We tend to think of the effect of psychotropic drugs as happening in the brain but drug metabolism happens all over the body with the liver and kidneys being particularly important and having a profound impact on the effect of the compound.
Link to ‘Atypical Antipsychotic Dosing: The Effect of Smoking and Caffeine’.
Mind Hacks 
I found the same thing when I was being treated with Tegretol for my Trigeminal Neuralgia. The larger the dose of Tegretol I was on, the more I smoked. I was told it was down to the nicotine (or whatever) in cigarettes inducing the same enzyme in the liver as the Tegretol. (I’m no longer on it, it only has a limited usefulness for TN, and once you’re past a certain dose it just gets ridiculous. I’m down to less than ten cigarettes a day now …)
As a much larger proportion of people with schizophrenia are smokers compared to the normal population, the interaction between cigarette smoking and antipsychotic medication is an important one.
http://ajp.psychiatryonline.org/cgi/content/abstract/156/11/1751
The suggestion is that nicotine helps to alleviate the negative symptoms by binding to nicotinic receptors.
In fact though, if nicotine were an illegal drug, there might well be many researchers or clinicians arguing that it was a cause of the condition and not just an association. What if Nicotine actually does break down endogenous antipsychotic compounds in the smokers body?
Amazing, so in some way caffeine in tea and coffee and smoking go together, I always like a fag with a cuppa, and now I know why. What effect then does caffeine have on nicotine? I suppose beer and fags have a similar effect, the alcohol dampening the nicotine, another one of my pass times.