I am lifting this comment from a response to a question at the AskJoel board about quitting cold turkey being too dangerous:
As far as I know there has never been any credible research done that had proved that quitting cold turkey was too dangerous. I actually haven't even had the question posed to me for many years.
There was a time when I used to get the question quite frequently. In my early years of doing programs I would hear it from people who told me that they had personal physicians who would tell them that quitting was just too much of a shock to their system and not worth the risk. It was often advice given to pregnant women by their obstetricians.
What must be understood about this information is was what the level of total misunderstanding there was by the physicians at the time, as well as by the entire medical and scientific community. It was at a time that there was a good chance that if a woman were to ask her physician if smoking was harmful to her baby, that the physician could have reached into his shirt pocket, pulled out a cigarette, took a few puffs while in deep contemplation and came back with the answer that smoking didn't really pose any real risk. The same kind of conversation could have been held between a man and his cardiologist or any person with almost any medical condition talking to his or her doctor. Back in the 1950's over half of the doctors in our country smoked.
What we know now about the dangers of smoking as it relates to many conditions makes it totally obvious to almost any health care professional in any field that smoking is deadly, even though in the past the lack of solid information caused the wrong advice to be standard fare.
We now have decades of experience with millions and millions of people successfully quitting smoking, the vast majority of them doing so by going cold turkey. It should be obvious to almost any one now that the dangers of quitting smoking is not what smokers need to be concerned with, it is the dangers they face if they do not quit smoking.
On a personal note, I have personally run over 4,500 people through cold turkey smoking programs for almost 30 years now. Out of those 4,500 people I only had two people who died during the two week period of the clinic. One was a younger man, probably in his thirties with severe heart disease and diabetes who was forced into the program by his wife and doctor because it was clear to both of them that he was in real danger of dying if he didn't quit smoking. Unfortunately, while the man's wife and doctor were both convinced that he was in immediate danger, the man himself didn't accept the risk for in fact, he did not quit smoking during that clinic. He was cheating throughout the program and his wife was not ever sure he had reduced his smoking at all from the first day of the program. He died on the fifth or sixth day.
The other death was from a man who was also in really bad shape, having just had major cardiac surgery, was still having ongoing problems with chronic heart failure and had a terrible prognosis coming in. His doctor had told him that he was a walking time bomb and he meant it in very literal terms. He died about ten days into the program. He had quit and had eased up in the withdrawal, was in fact very proud of the fact that he had quit and was happy with his decision to do so. I actually went to his funeral. His wife was very happy to see me there, and excitedly introduced me to a number of their family members and friends, explaining how I was the person who helped her husband to quit smoking. They were all very proud of the man and felt that he really was trying to give himself a fighting chance to live. That seemed very important to his loved ones at that time.
Other than these two cases, I have never encountered a person who had died during the quitting process, which is quite remarkable considering the state of health that many people who come to clinics are in.
Again, don't waste your energy on the fear of quitting. It is a baseless fear. If you spend time doing any real research on the effects of not quitting though, the fear that you will feel will be totally warranted for the magnitude of risk posed by smoking is tremendous. The good news is that all of the risks posed by smoking can be minimized by simply making and sticking to a personal commitment to never take another puff.