Once nicotine enters the brain, it starts to mimic the brain’s most powerful chemical messengers. The result is a temporary improvement in brain chemistry that is experienced by the smoker as enhanced pleasure, decreased anxiety, and a state of alert relaxation.

As a result of this positive reinforcement many dozens of times per day, smoking becomes thoroughly a part of every aspect of the smoker’s life. These positive effects of smoking explain why the smoking habit holds its victims in such a tenacious grip.

Most smokers will say that smoking helps them concentrate, keeps them from being bored, and helps reduce the perceived level of tension in their lives. As well, smoking helps them cope with an over-stimulating environment, gives them positive pleasure, helps them relax, reduces their feeling of distress, helplessness, and loneliness, helps them keep weight down, and makes them feel more at ease in social situations. Smoking can even provide a burst of energy when feeling tired, and can even help a smoker concentrate more effectively. In fact, smoking helps a smoker control his moods. Understandably, these substantial benefits would be difficult to give up.

These “rewards” of smoking go a long way toward minimizing the negative consequences, and an even longer way toward ensuring that the act of smoking will be repeated again and again, until it becomes a habit so well ingrained that you do it without even thinking about it.

But smoking is not just a habit – it is also an addiction. The nicotine in cigarettes is a powerful addictive drug that makes smokers feel good. Each time you smoke, the positive biological effects of nicotine add to all the other positive rewards of smoking, which makes the smoking habit even stronger.

The Effects of Nicotine
Nicotine affects almost every system in the body. When you take a puff, your heart beats faster, your pulse quickens, your veins constrict, your blood pressure increases. Your adrenal glands pump out adrenaline that increases your heart rate, relaxes many of your smooth muscles, and raises your metabolic rate.  Even the electrical activity in your brain changes.

These are powerful biological effects. Indeed, nicotine is a very powerful drug. In fact, it is one of the most toxic of all drugs, comparable to cyanide. Take enough nicotine and it can kill you. But the amount of nicotine in a single cigarette is only 8 to 9 milligrams on average.

The amount of nicotine that smokers inhale from each cigarette is even smaller. Most popular brands of cigarettes deliver less than 1.5 milligrams per cigarette. This amount may be somewhat higher or lower for each smoker, depending on how deeply you puff and how many puffs you take from each cigarette.

But nicotine is so potent that even this small dose causes significant changes in the functioning of numerous organs and systems in your body. When people first take up smoking, these physiological changes seem extremely unpleasant. Beginning smokers usually experience nausea, dizziness, headache, stomach upset, coughing and other uncomfortable symptoms. But people who continue to smoke soon develop a tolerance to these symptoms, until they become unnoticeable.

Tolerance is a term used to describe an important feature of addiction. Tolerance has developed when, after the repeated administration of a drug (in this case, nicotine) produces a decreased effect. As well, tolerance has developed when increasingly larger doses must be administered to obtain the effects observed with the original dose.

What does this mean for the smoker? The small dose of nicotine delivered by several puffs on a cigarette may make people feel ill the first few times they try smoking. But after they’ve been smoking for a week or so (repeated “self-administration” of nicotine), several puffs and even an entire cigarette no longer have that effect. Now they’ll feel ill only if they smoke several cigarettes one after another (a larger dose).

Psychologically, tolerance to the unpleasant effects of nicotine allows the smoker to focus on nicotine’s pleasurable physiological effects. Many smokers don’t realize that nicotine’s effects on the heart, the nervous system, and the endrocine system are significant contributors to the relaxation, alertness, stress relief, and other good feelings they experience.

This combination of physiological and psychological effects provides so many positive reinforcements that smoking quickly becomes an established habit.

As the term tolerance implies, a smoker actually becomes accustomed to having a certain level of nicotine in his or her body. In fact, research studies have shown that (without realizing it) smokers regulate the number of cigarettes they smoke in order to maintain their own personally preferred level of nicotine. For example, smokers who are given a very high nicotine cigarette will puff less often than usual, so they don’t take in more nicotine than their preferred amount. Likewise, with a low nicotine cigarette, the smoker will take more puffs than usual, in order to get that preferred amount of nicotine.

When no cigarettes are smoked for a while (when someone is trying to quit), the smoker doesn’t get any nicotine. And it is the lack of nicotine that produces unpleasant physiological symptoms in the body. Medically, these symptoms are called “withdrawal effects.”

To relieve these withdrawal effects, many smokers must continue to take in their usual amount of nicotine. This is a sign of “physical dependence” on nicotine. Doctors define physical dependence as a change in the body’s functioning that is produced by repeated administration of a drug, such that continued doses of the drug are needed to prevent withdrawal symptoms.

But that’s not all. Smokers also become accustomed to the psychological effects of smoking. After the smoking habit is established, the smoker needs to smoke in to feel “normal.” In other words, the effects produced by nicotine, and the behaviors associated with smoking, become necessary to maintain the person’s optimal state of well-being. This condition is referred to as “psychological dependence.”

At the extreme, many smokers who run out of cigarettes or are unable to smoke, become totally preoccupied by thoughts of having a cigarette. This behavior is often referred to as “compulsive drug use.”