Because this drug has the ability to release neurotransmitters, it should not be taken lightly. The effects of this chemical are very well documented and are influenced by several factors. Genetics plays a certain role in how it affects the body. Metabolism (length of time it stays in your body) is very important when understanding how it will affect you. Diet, age, sex, race/ethnicity, presence of liver or kidney disease, pregnancy, use of medications and smoking will all influence the time it stays in your body.
It's important to note here that it accumulates elsewhere in the body. High concentrations are found in stomach acid and saliva. Perhaps more importantly, breast milk, fetal serum and amniotic fluid are places where nicotine collects. So you have to be aware the effects of this drug go beyond your own body.
In short-term, low concentrations, as seen with smoking, nicotine just kind of speeds everything up. The effects include tremor, increased heart rate, blood pressure, respiratory rate, and level of alertness. The effects of nicotine themselves can be seemingly harmless or disastrous, depending upon your current health situation.
In addition, it's fortunate that only a small fraction (around one-tenth) of the available drug in tobacco is absorbed during smoking. Imagine what would happen if just another ten or fifteen percent were absorbed. On a side note, chain smokers suffer more of these side effects because they are delivering more drug to their bloodstream.
Millions of people suffer from the ill effects of addiction and think nothing about it. That is, until they get a dreadful medical diagnosis like heart, cardiovascular or pulmonary disease. But it should now be clear why we should not underestimate the effects of this drug.