There are several medical implications for smoking cessation especially when long-term high-volume consumption is ceased. Today, in 2015, we tend to think of a 1PAD (one pack a day) habit as significant. Average consumption is currently (2015) reported as around 12 - 15 cigarettes per day by smokers in the UK.
What appears to be, currently, the most comprehensive listing of symptoms associated with smoking cessation can be found here :
[disclaimer: this is my work]
The list is provided within the context of vaping simply because many people have, in the past, ascribed such symptoms to vaping; we now know that presenting with these conditions is related to smoking cessation rather than anything that occurs subsequently.
A quick note on harm
It is reasonably obvious that smoking is associated with harm. On average, smoking is harmful; and the longer it is carried out for, the higher the risk. Therefore, although it is absolutely incorrect to state that smoking has no benefits - this is obviously wrong when the issues are researched, as can be seen clearly below - there are likely to be better options in the 21st century.
It is equally wrong to state that smoking cessation has no significant implications - it can and does lead to both minor and serious morbidity, as outlined below. However, since we are indeed in the 21st century now, perhaps it is time to explore other avenues of prophylaxis and treatment for vulnerable sub-populations.
From the above list, we see that there are numerous common minor symptoms of smoking cessation, almost all of which pass with time; and much less commonly, symptoms related to some serious medical conditions associated with smoking cessation for various reasons.
As an example of the minor symptoms, the Quit Ulcers and the Quit Zits appear fairly common. The 'quit ulcers' are mouth ulcers experienced by some, which disappear reasonably quickly. The 'quit zits' are an interesting but less common phenomenon: facial boils and spots resembling acne that appear after smoking cessation even in persons who never had acne in youth. These facial aberrations can be distressing and inexplicable for those who have never experienced acne; and can be persistent. That is to say, they can last for months; and the sufferers are unlikely to report they consider the problem to be insignificant or of no importance. Indeed, some report this effect of smoking cessation as significantly disruptive to their lives.
The more serious implications are luckily rare, as they can and do relate to serious disease. Thyroid conditions and auto-immune diseases are implicated. For completeness, we must also consider the possible implications for serious neurodegenerative diseases associated with non-smoking status.
The thyroid issues are currently under investigation. It appears that smoking, or smoking cessation, are associated in some individuals with thyroid conditions such as Hashimoto's Thyroidosis. That is to say, we do not know if smoking causes it and cessation triggers it or increases its effects; if smoking masks it; or if smoking cessation triggers it in those without any previous sign of the condition.
The ECF forum, although strictly speaking a vaping forum, is also by its very nature the world's largest smoking cessation forum, and groundbreaking research has been carried out by the professors of medicine, doctors, researchers, chemists and pharmacists who are members - not to mention the vast membership. The 16 million posts on the forum address many of these issues, and if you wish to research thyroid issues and smoking, the Health section on ECF is as good a place as any to start . Incidentally, there are some alarming symptoms associated with incorrect thyroid medicine dosages reported there, in case that is of interest to anyone.
Neurodegenerative and auto-immune disease are strongly associated with non-smoking status. Smoking cessation has serious implications for those with a genetic vulnerability to one auto-immune disease (ulcerative colitis). As an example, it is reported that more than 40 separate clinical studies show a strong correlation between non-smoking status and Parkinson's disease. Dr Maryka Quik, a senior researcher, states: “A huge literature says that smoking protects against Parkinson's." 
Perhaps smoking protects against Parkinson's due to the nicotine inclusion, since research shows nicotine is effective in treatment, and some genetic subgroups may require a significant supplementation of this normal dietary component; but after a certain time period, the harmful effects of smoking will tend to override any benefits, and therefore nicotine might best be supplemented in some other way, if it is indeed the sole agent in prophylaxis.
One auto-immune disease, ulcerative colitis, is so strongly associated with non-smoking status that it has been termed 'the non-smoker's disease'. It is also apparent but not confirmed by research that presentation can be associated with smoking cessation: a peak in presentations appears to occur shortly after cessation. It is clearly a genetic condition since it tends to run in families; and non-smokers are at more risk than smokers.
These serious conditions are uncommon. Nevertheless, they are life-changing and in some cases eventually fatal. Therefore it would seem to indicate that some research of family history might be beneficial: if such diseases can be found in the history then perhaps some preventive measures could be put in place before or immediately subsequent to smoking cessation.
Currently, we have no idea what works best (or at all) in this situation, but some form of dietary nicotine supplementation seems advisable . The problem is essentially this: we think nicotine is the dietary nutrient that prevents nerodegenerative and auto-immune conditions for the vulnerable sub-population (but are not absolutely sure, especially about its sole activity); but we have no idea what dosages are appropriate or even if nicotine is the sole prophylactic or treatment agent - anatabine for example is another alkaloid found in tobacco, and has a proven medical role as a powerful anti-inflammatory agent - it is now used in treatments for rheumatism. This may be relevant since auto-immune diseases can also be categorised as inflammatory diseases.
In the past it is said that doctors have occasionally prescribed two cigarettes a day in such situations, given the serious nature of such diseases, in the case of a person with a family history indicating vulnerability, and because the elevation of risk for morbidity with 2 cigarettes per day is infinitessimal compared with significant risk for a serious disease. Today, it seems likely the same doctor would advise some other form of nicotine supplementation, given the availability of numerous ultra-low-risk nicotine products both with and without pharmaceutical licenses.
As yet we do not have enough information on nicotine-based prophylaxis to make any definitive statements; Dr Newhouse of Vanderbilt leads the treatment investigation currently . It is clear there are benefits and it is also clear that nicotine has all the indications necessary for classification as a B vitamin, in a similar way to its sister compound nicotinic acid . We simply don't know enough about its effects in isolation, or the supplementary dosages (i.e. the RDA, and its supplementation for vulnerable populations) to give advice that is likely to be accurate or widely agreed.
For clarity, it is worth pointing out that if you have a family history of neurodegenerative disease, or auto-immune disease, and are a smoker seriously considering cessation, then perhaps it may be wise to look at the issues along with your doctor. At this time, considering the prevalence of propaganda, and how it is proven to affect even experts and may result in medical negligence , you might consider finding better advice if your current doctor knows nothing about these issues or offers advice that is clearly wrong by the latest standards.
- This listing is regularly updated as new information is presented.
 ECF: https://www.e-cigarette-forum.com/forum/
- In 1966, Harold Kahn, an epidemiologist working for the NIH, National Institutes of Health, uncovered a curious fact: non-smokers were about three times more likely to die of Parkinson’s Disease as smokers. This effect was later confirmed in numerous studies.
 Nicotine is present in the normal diet, in numerous vegetables and foodstuffs. Everyone consumes nicotine and everyone tests positive for nicotine, at a background level in the blood stated by Benowitz to be on average a plasma level of 3ng/ml (which can be viewed as a historical statement, in light of the reducing consumption of vegetables due to diets generally worsening in the urbanised western world). In the several large-scale clinical trials that examined this, including one of 800 subjects by the CDC, no person has ever tested negative for nicotine.
 See: http://www.ecigarette-politics.com/vaping-quotes.html
- scroll down to the first quote listed in the main body of the text, for the reference.
 Nicotinic acid is a metabolyte of nicotine in some organisms, so clearly there is a relationship apart from the molecular similarity. It is also known as vitamin B3; a new name, 'niacin', had to be invented for it in order to ensure acceptance by the classification committee responsible. It shares many functions in common with nicotine in the human organism; each has its own additional functions. For example: B3 protects against pellagra in vulnerable sub-populations; nicotine apparently protects against neurodegenerative disease, again perhaps in vulnerable sub-populations.
 See: http://www.ecigarette-politics.com/references-nicotine.html
>> Nicotine, #5.
- Doctors are apparently just as vulnerable to propaganda as anyone else, as can be seen by opinions directly opposite to their latest official guidance.