Merck Manual

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Judith J. Prochaska

, PhD, MPH, Stanford Prevention Research Center, Stanford University

Reviewed/Revised Dec 2020 | Modified Sep 2022
Topic Resources

Smoking tobacco is harmful to almost every organ in the body.

  • Smoking increases the risk of heart attack, lung cancer, chronic obstructive pulmonary disease (COPD), and other disorders.

  • Nicotine is the addictive substance present in tobacco.

  • People who stop using nicotine may become irritable, anxious, sad, and restless during the period of withdrawal.

Nicotine is the substance in tobacco (present in cigarettes, cigars, and pipe and chewing tobacco as well as e-cigarettes) that users become dependent on. In addition to nicotine, smoked cigarettes contain tar and carbon monoxide, along with almost 4,000 other ingredients, many of which are toxic. Nicotine is also the active ingredient in some drug products used to help people quit smoking. When delivered by smoking cigarettes, nicotine reaches the brain rapidly (within 10 seconds), and is thus highly addictive. In contrast, delivery of nicotine by a transdermal patch is slow and steady and does not cause addiction.

Most nicotine exposure is from smoking tobacco, although children may accidentally eat it (usually cigarettes or butts left in ashtrays or sometimes nicotine gum, patches, or e-liquid), and some people use smokeless tobacco. Nearly all smokers smoke cigarettes. A small percentage of smokers smoke cigars or pipes.

Cigarette smoking remains the leading cause of preventable disease and death in the United States and globally. About two in three long-term smokers will die prematurely of a disorder caused by smoking. Over half a million Americans die each year from a tobacco-related disease: that is, 1 in 5 U.S. deaths are related to smoking. Smoking is deadly because smokers inhale hundreds of substances, many of which can cause cancer Overview of Cancer , heart disease Acute Coronary Syndromes (Heart Attack; Myocardial Infarction; Unstable Angina) Acute coronary syndromes result from a sudden blockage in a coronary artery. This blockage causes unstable angina or a heart attack (myocardial infarction), depending on the location and amount... read more Acute Coronary Syndromes (Heart Attack; Myocardial Infarction; Unstable Angina) , and lung disease Chronic Obstructive Pulmonary Disease (COPD) . Smokeless tobacco products are not safe alternatives to smoking because they too contain toxins.

Overview of Tobacco Dependence

Smoking also presents another danger in that it is the most common cause of unintentional home fires in the United States. The Federal Emergency Management Agency (FEMA) estimates that about 7,600 smoking-related fires occur in residential buildings each year, causing about 365 deaths, 925 injuries, and $326 million in property loss.

Currently, about 14% of U.S. adults smoke cigarettes, compared to the mid-1960s when 1 in 2 men and 1 in 3 women were smokers. Yet, due to population growth, the absolute number of smokers in the United States has remained about the same at nearly 35 million adults. Cigarettes are marketed heavily, to adolescents as well as adults, primarily at the point of sale. Every day, about 1,600 youth under age 18 smoke their first cigarette, and nearly 200 youth become daily cigarette smokers.

Pregnant women and children

Smoking during pregnancy robs the developing fetus of oxygen and can cause low birth weight, preterm birth, and fetal death. Smoking during pregnancy also increases risk of sudden infant death syndrome (SIDS) Sudden Infant Death Syndrome (SIDS) Sudden infant death syndrome is the sudden, unexpected death, usually during sleep, of a seemingly healthy infant between 1 month and 1 year of age. The cause of sudden infant death syndrome... read more . Smoking is a pediatric disease: 9 in 10 smokers start before the age of 18, which is prime time for brain development.


Smoking is harmful to almost every organ in the body.

Immediate effects

Nicotine is a stimulant that activates the pleasure center in the brain. When obtained through smoking, nicotine can increase energy and concentration and decrease appetite. Once a person is addicted, smoking will reduce symptoms of nicotine withdrawal and may feel relaxing. People not used to nicotine may have nausea, flushing, or both.

People who handle large amounts of tobacco leaves may absorb nicotine through their skin and develop nausea, vomiting, diarrhea, sweating, and weakness. This illness has been termed green tobacco sickness.

Children who eat tobacco products or nicotine gum or ingest e-liquid can develop nausea, vomiting, diarrhea, sweating, and weakness, along with agitation and confusion, sometimes from as little as one cigarette. However, serious or fatal toxicity in children is uncommon, in part because the vomiting empties the stomach.

Long-term effects

The leading smoking-related health problems are the following:

Smoking also increases the risk of stroke Overview of Stroke A stroke occurs when an artery to the brain becomes blocked or ruptures, resulting in death of an area of brain tissue due to loss of its blood supply (cerebral infarction). Symptoms occur suddenly... read more , other cancers (such as bladder Bladder Cancer Most bladder cancers are of a type called transitional cell because they affect the same kinds of cells (transitional cells) that are usually the cancerous cells responsible for cancers of the... read more , cervical Cervical Cancer Cervical cancer develops in the cervix (the lower part of the uterus). Most cervical cancers are caused by human papillomavirus (HPV) infection. Cervical cancer usually results from infection... read more Cervical Cancer , colorectal Colorectal Cancer Family history and some dietary factors (low fiber, high fat) increase a person’s risk of colorectal cancer. Typical symptoms include bleeding during a bowel movement, fatigue, and weakness... read more Colorectal Cancer , esophageal Esophageal Cancer Esophageal cancers develop in the cells that line the wall of the esophagus (the tube that connects the throat to the stomach). Tobacco and alcohol use, human papillomavirus infections, and... read more Esophageal Cancer , kidney Kidney Cancer Most solid kidney tumors are cancerous, but purely fluid-filled tumors (cysts) generally are not. Almost all kidney cancer is renal cell carcinoma. Another kind of kidney cancer, Wilms tumor... read more , liver Hepatocellular Carcinoma Hepatocellular carcinoma is a cancer that begins in the liver cells and is the most common of the primary liver cancers. Having hepatitis B or hepatitis C or fatty liver disease, or drinking... read more , pancreatic Pancreatic Cancer Smoking, chronic pancreatitis, male sex, being Black, and possibly long-standing diabetes are risk factors for pancreatic cancer. Abdominal pain, weight loss, jaundice, and vomiting are some... read more , throat Mouth and Throat Cancer Mouth and throat cancers are cancers that originate on the lips, the roof, sides, or floor of the mouth, tongue, tonsils, or back of the throat. Mouth and throat cancers may look like open sores... read more Mouth and Throat Cancer , and stomach Stomach Cancer A Helicobacter pylori infection is a risk factor for stomach cancer. Vague abdominal discomfort, weight loss, and weakness are some typical symptoms. Diagnosis includes endoscopy and biopsy... read more Stomach Cancer ), pneumonia Overview of Pneumonia Pneumonia is an infection of the small air sacs of the lungs (alveoli) and the tissues around them. Pneumonia is one of the most common causes of death worldwide. Often, pneumonia is the final... read more Overview of Pneumonia and other respiratory infections, asthma Asthma Asthma is a condition in which the airways narrow—usually reversibly—in response to certain stimuli. Coughing, wheezing, and shortness of breath that occur in response to specific triggers are... read more Asthma , osteoporosis Osteoporosis Osteoporosis is a condition in which a decrease in the density of bones weakens the bones, making breaks (fractures) likely. Aging, estrogen deficiency, low vitamin D or calcium intake, and... read more Osteoporosis , periodontitis (gum disease) Periodontitis Periodontitis is a severe form of gingivitis, in which the inflammation of the gums extends to the supporting structures of the tooth. Plaque and tartar build up between the teeth and gums and... read more Periodontitis , peptic ulcer disease Peptic Ulcer Disease A peptic ulcer is a round or oval sore where the lining of the stomach or duodenum has been eaten away by stomach acid and digestive juices. Peptic ulcers can result from infection with Helicobacter... read more Peptic Ulcer Disease , cataracts Cataract A cataract is a clouding (opacity) of the lens of the eye that causes a progressive, painless loss of vision. Vision may be blurred, contrast may be lost, and halos may be visible around lights... read more Cataract , erectile dysfunction Erectile Dysfunction (ED) Erectile dysfunction (ED) is the inability to attain or sustain an erection satisfactory for sexual intercourse. (See also Overview of Sexual Dysfunction in Men.) Every man occasionally has... read more , and fertility problems Overview of Infertility Infertility is usually defined as the inability to achieve a pregnancy after 1 year of regular sexual intercourse without birth control. Frequent intercourse without birth control usually results... read more Overview of Infertility .

Secondhand smoke

People who do not smoke but who are exposed to smoke from a burning cigarette or the smoke exhaled by a nearby smoker (passive, or secondhand smoking) can develop many of the same disorders as smokers, particularly with repeated and sustained exposure. The Surgeon General concluded there is no safe level of secondhand smoke exposure.

Children exposed to cigarette smoke lose more school days because of illness than nonexposed children.

Smokeless tobacco


Other effects

Smoking can interact with other drugs. The effects are largely due to tars in the liver as a by-product of smoking and not due to the nicotine; hence, most effects are not seen with nicotine replacement therapy (NRT). Smoking dries and wrinkles a person's skin, thins the hair, and turns the teeth and fingers yellow. Smokers tend to weigh about 10 pounds less than they would if they did not smoke, but not everyone gains weight when they quit smoking. Also, the harmful effects of smoking far outweigh the risks of weight gain. Employees who smoke cost employers on average over $5,000 more per year than nonsmoking employees due to greater health care costs and more missed days of work. Smoking increases the risk of unemployment and makes it harder to find re-employment.

Withdrawal symptoms

Nicotine withdrawal Nicotine Withdrawal While often very challenging, quitting smoking is one of the most important things smokers can do for their health. Quitting smoking brings immediate health benefits that increase over time... read more may result in many unpleasant symptoms, including a craving for nicotine, irritability, anxiety, poor concentration, restlessness, trembling (tremor), depressed mood, weight gain, headaches, drowsiness, and stomach upset. Withdrawal is most troublesome in severely dependent people. The symptoms of nicotine withdrawal peak in the first 3 days and subside over 2 to 4 weeks, but some symptoms, such as craving, may continue longer.


  • Interview of smoker

It is recommended that doctors ask everyone about tobacco use. For many people, smoking is an addiction needing medical treatment. Assessing a person's quantity of use (the number of cigarettes smoked per day [presently and in the past]) and how soon they smoke upon wakening (within 30 minutes is a useful measure) can provide an indication of the severity of tobacco dependence and nicotine addiction. Responses also can help guide the choice of cessation medication and its dosing.

Nicotine poisoning can be overlooked. For example, children may swallow cigarettes or nicotine gum without being seen. Even when children are observed with tobacco in their mouth, it can be difficult to tell how much they have actually swallowed. People with green tobacco sickness may not connect their symptoms with handling tobacco.


  • Treatment of symptoms

  • Quitting smoking

Emergency treatment is rarely required except for children who have eaten products that contain nicotine. Doctors usually give activated charcoal by mouth to absorb any drug remaining in the gastrointestinal tract. Children who are very agitated may be given a sedative such as lorazepam.

Stopping smoking can be very difficult and relapse is common. Quitting successfully usually requires many attempts. Evidence-based treatments more than double the chances of long-term success.

More Information about Smoking

The following English-language resources may be useful. Please note that The Manual is not responsible for the content of these resources.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Commit, Habitrol, Nicoderm CQ, NICOrelief , Nicorette, Nicotrol, Nicotrol NS
Chantix, Tyrvaya
Ativan, Loreev XR
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