there are no physical reasons to start smoking. The body doesn't need tobacco the way it needs food, water, sleep, and exercise. In fact, many of the chemicals in cigarettes, like nicotine and cyanide, are actually poisons that can kill in high enough doses.

The body is smart. It goes on the defense when it's being poisoned. For this reason, many people find it takes several tries to get started smoking: First-time smokers often feel pain or burning in the throat and lungs, and some people feel sick or even throw up the first few times they try tobacco.

The consequences of this poisoning happen gradually. Over the long term, smoking leads people to develop health problems like heart disease, stroke, emphysema (breakdown of lung tissue), and many types of cancer — including lung, throat, stomach, and bladder cancer. People who smoke also have an increased risk of infections like bronchitis and pneumonia.

These diseases limit a person's ability to be normally active, and they can be fatal. Each time a smoker lights up, that single cigarette takes about 5 to 20 minutes off the person's life.

Smokers not only develop wrinkles and yellow teeth, they also lose bone density, which increases their risk of osteoporosis (pronounced: ahs-tee-o-puh-row-sus), a condition that causes older people to become bent over and their bones to break more easily. Smokers also tend to be less active than nonsmokers because smoking affects lung power.

Smoking can also cause fertility problems and can impact sexual health in both men and women. Girls who are on the pill or other hormone-based methods of birth control (like the patch or the ring) increase their risk of serious health problems, such as heart attacks, if they smoke.

The consequences of smoking may seem very far off, but long-term health problems aren't the only hazard of smoking. Nicotine and the other toxins in cigarettes, cigars, and pipes can affect a person's body quickly, which means that teen smokers experience many of these problems:

Bad skin. Because smoking restricts blood vessels, it can prevent oxygen and nutrients from getting to the skin — which is why smokers often appear pale and unhealthy. An Italian study also linked smoking to an increased risk of getting a type of skin rash called psoriasis.
Bad breath. Cigarettes leave smokers with a condition called halitosis, or persistent bad breath.
Bad-smelling clothes and hair. The smell of stale smoke tends to linger — not just on people's clothing, but on their hair, furniture, and cars. And it's often hard to get the smell of smoke out.
Reduced athletic performance. People who smoke usually can't compete with nonsmoking peers because the physical effects of smoking (like rapid heartbeat, decreased circulation, and shortness of breath) impair sports performance.
Greater risk of injury and slower healing time. Smoking affects the body's ability to produce collagen, so common sports injuries, such as damage to tendons and ligaments, will heal more slowly in smokers than nonsmokers.
Increased risk of illness. Studies show that smokers get more colds, flu, bronchitis, and pneumonia than nonsmokers. And people with certain health conditions, like asthma, become more sick if they smoke (and often if they're just around people who smoke). Because teens who smoke as a way to manage weight often light up instead of eating, their bodies lack the nutrients they need to grow, develop, and fight off illness properly.

source:kidshealth.org