There is a little-known secret to quitting smoking that completely made the difference for me, and it involves how caffiene and nicotine interact in the body. Bottom line: watch your caffiene intake when you are quitting, because in the absence of nicotine caffiene has almost twice the effect it normally does- which can cause HUGE amounts of anxiety and lead you right back to smoking.

Againa and again I would stop smoking and experience this unbearable anxiety. I always figured it was just nicotine withdrawal- but I learned that caffiene plays a huge role. Nicotine interferes with caffiene's stimulant effects in the body- allowing smokers to take in much more caffiene than non-smokers. But when the nicotine is gone, and the person is taking the same amount of caffiene- it's full effects take place, causing a minor overdose. Caffiene overdose is a very unpleasant experience of anxiety, dysphoria and insomnia.

Here is a study on the interactions of caffiene and nicotine: http://www.ncbi.nlm.nih.gov/pubmed/9022872

"Continuous caffeine consumption with smoking cessation has been associated with more than doubled caffeine plasma levels."