@ Framey
Nicotine is almost completely harmless in most people, and in many ways it is very much like caffeine.
It does not cause cancer but it has a few mildly negative effects in some people. It has some good effects, some bad effects, and some very bad effects, very much like caffeine, but overall, it's pretty safe in that it won't kill you. People might try to scare you by saying that nicotine is a natural pesticide, but so is caffeine. In the amounts they are taken, these drugs don't affect the human nervous system in such a way as to cause death, like they do in insects. The cardinal rule of toxicology is that the dose makes the poison. If you drink a glass of water in an hour, you will have no ill effects. If you drink a hundred glasses of water in an hour, you will probably die. One caffeine tablet might keep you awake. Twenty might land you in hospital. Fifty might kill you. Same applies for Tylenol, Pepsi and nicotine. 1 mg of nicotine is mostly harmless. 10 mg will make a person very sick. 100 mg will almost certainly kill.
THE BAD
Nicotine increases your heart rate by a few beats per minute, which does little more than give a slight energy boost, but in a small number of people with weak hearts it might be harmful, but on the same level as a cup of coffee might be harmful for them. There is some evidence that nicotine might also be slightly harmful to the circulatory system in other ways. It isn't very good for undeveloped brains, so it's particularly bad to smoke during pregnancy. In addition, short term effects of nicotine are unpredictable in people who are not used it and vary from person to person. These include a racing heart, dizziness, feeling faint, heaviness in the limbs and powerful tingling sensations and nausea. These can be both pleasant or quite unpleasant but they do not cause harm and subside as a person becomes tolerant. Some people experience pleasure instead of nausea and it is for them that nicotine is most addictive.
THE VERY BAD
Unlike caffeine, which is only slightly addictive, nicotine is extremely addictive when inhaled via smoking, and somewhat addictive, though much less so, when taken orally or via patches. The reason for this is that when it is inhaled, it gets to the brain very quickly and in high quantities, causing it to release significant amounts of dopamine, which the brain registers as pleasure, even as the novice smoker dizzily stumbles about feeling like they're going to throw up. When taken orally or via patches, the dose is usually much smaller and it reaches the brain far more gradually so the pleasure response is minimal, reducing addiction potential. That's why you don't see many people hooked on nicotine lozenges. Both caffeine and nicotine have very unpleasant withdrawal symptoms. For nicotine, the withdrawal can involve impulsiveness and inability to control one's temper and emotions, blurred vision, headache, crawling sensations, extreme appetite, depression, tiredness and, of course, severe craving. Suddenly stopping using large amounts of caffeine can cause tiredness and give you very severe headaches that last for days, but because caffeine does not stimulate the brain's reward circuity and effect a release of dopamine, it does not cause severe cravings like nicotine does and it makes it much easier to taper it gently and avoid withdrawal altogether.
THE GOOD
Nicotine has a few mild effects some people might consider to be positive. We all know smokers put on weight when they quit, and that's because nicotine and a few other chemicals in cigarette smoke kept them thinner than they otherwise would be. Nicotine forces the body to release stored glucose into the bloodstream, boosting energy and preventing the sugar from being converted into fat. This also cheats the body into thinking you've eaten something sweet, which reduces appetite and craving for sugary foods, though all the reasons why appetite is reduced are not fully understood. It also acts directly on fat cells, increasing lipolysis (the breakdown of fat), and it increases metabolism by as much as 6000 calories a month. Caffeine has some, but not all, of the same effects, and to a much lesser degree, so people don't generally put on much weight if they give up coffee, or they may even lose some, given that many coffees nowadays have as many calories as a hamburger. Caffeine is just a stimulant, but nicotine is different. Nicotine has both stimulant and depressant effects, so it can both stimulate and calm, depending on the situation. Much like caffeine, nicotine also improves mood, memory and concentration, and just like caffeine, it also protects the brain from certain types damage (reduction in Alzheimer's and Parkinson's risk), but again, nicotine's effects are significantly more powerful than that of caffeine.
So, nicotine is a lot like caffeine, with a few other effects, though it doesn't keep you awake quite as much. Both drugs are mostly harmless, but nicotine is far, far more addictive.