There are two reasons which are understudied and under-researched. Possibly because doctors have such a limited repertoire of tests and drugs and are so poor at diagnosing and dealing with these problems. Partly because the MD has to rely on the "unscientific' reports of the patient. (ie Doc has to listen to
and believe what his patient is saying. Arrogance and intellectual snobbery prevents many a good diagnosis)
1. Pain
Possibly the human condition 'most badly' or untreated by the medical fraternity--especially long term cronic pain.
Nicotine is a pain killer. See:
Nicotine to Snuff out Pain :The Scientist [28/3/2005]
Many people suffer from chronic untreated pain. Also a surprising few/(many?) suffer from silent or "painless" pain. I include in this 'mental suffering'-depression, anxiety, loss. It seems more than a coincidence that 90% of people hospitalised for 'psychiatric disorders' smoke (I.M. oberservations). Note also that nicotine is a more effective painkiller with women- who are now the sex most likely to smoke and to be now taking up smoking. Note also that nicotine is synergistic with opioids--perhaps why many with "addictive personalities" (!?) smoke.
I suspect nicotine will be found to be synergistic with other pain killers like caffeine, marijuana and perhaps even alcohol.
2. Allergies food/chemical intolerances
Nicotine jags the adrenals into action. Food allergies and chemical intolerances (combined with modern day stress?) 'exhaust' the adrenals (Not a term you will find in any conventional medical text). So it is likely that nicotine masks allergies and makes symptoms temporarily more tolerable.
I suspect there is a racial/genetic component/factor in all this yet to be discovered.