Nicotine is a naturally occurring odorless, colorless liquid that only acquires the brown color and familiar tobacco odor when burnt and smoked. It was discovered in the early 1800's but the recognition of its addictive qualities happened much later.

Modern scientific advances like neuro imaging have provided clear indications of the complex and sometimes unpredictable effects of the drug. Nicotine is absorbed through the skin and mucosal linings of the mouth and nose as well as the lungs through inhalation. The chemical is swiftly distributed throughout the body but the addictive effects arise out of its particular actions on the pleasure and reward centers of the brain. 

The drug acts on dopamine, an important regulator of our "reward" and pleasure pathways. This neurotransmitter is THE brain chemical associated with the action of addictive substances. The effects of nicotine can be felt within ten seconds of inhalation. This effect, though swift, is short-lived hence the need to take another puff. This is the reason why smoking is so addictive.
 
The NIDA (National Institute on Drug Abuse) has recorded that up to 35 million smokers try to quit each year. Statistics show that less than 7% are successful. There are several easily purchased treatments available over the counter including nicotine patches and nicotine gum. A combination of behavioral therapy and pharmaceutical treatments has been found to be the most effective way to treat this widespread addiction.

Sources: NIDA - National Institute on Drug Abuse (Public Domain)
 

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