Why is tobacco so addictive?

Tobacco is an incredibly addictive substance. Studies have found that smoking tobacco can be as addictive as heroin and cocaine, but what makes people crave a cigarette? And why do many people struggle to stop smoking despite being aware of the dangers?

The answer, it turns out, has to do with tobacco changing the way our brains work, making us want more of it, according to the U.S. Food and Drug Administration (FDA).

"Addiction is primarily defined as a loss of control on the use of a substance and continued use despite the consequences," Bernard Le Foll, Chair of Addiction Psychiatry within the Department of Psychiatry at the University of Toronto, told Live Science in an email. 

"Once an addiction to a substance is developed, people will experience cravings and/or withdrawal when not using it for a certain period of time. Tobacco is addictive because it contains nicotine, a psychoactive substance with high addictive potential," Le Foll said.

Related: How does cannabis get you high?

A psychoactive substance is one that impacts how the brain works and, according to the National Cancer Institute (NCI), "causes changes in mood, awareness, thoughts, feelings, or behavior." Other examples of psychoactive substances include LSD, alcohol and caffeine.

Nicotine is especially addictive when smoked or otherwise taken into the lungs because "the onset of the stimulant-like effects occurs very rapidly through this route of administration," David Ledgerwood, a clinical psychologist in the Substance Abuse Research Division at Wayne State University in Detroit, Michigan, told Live Science in an email.

While the initial "hit" of enjoyment from smoking a cigarette is felt almost immediately, it also disappears rapidly, which, according to Ledgerwood, leads smokers to consume tobacco products frequently in a bid to achieve "the same stimulant experience."

When tobacco is consumed, nicotine levels in the bloodstream spike and enter the brain. Once in the cerebrum, nicotine attaches to and activates receptors that release the "happy" brain chemical dopamine, which makes people feel good, according to the Mayo Clinic. As a result, smokers' brains quickly come to consider nicotine as a "feel-good" substance, and will crave it in the gaps between cigarettes.

Chronic smoking increases the number of nicotine receptors in the brain, which explains why addicted smokers have "billions more of these receptors than nonsmokers do," the Mayo Clinic reported.

If someone smokes regularly for months and years, their brain will become used to having nicotine to the point where, eventually, "they need nicotine to function well," Ledgerwood said. During periods when the addicted individual does not smoke, they may experience physical withdrawal symptoms until their brain can adjust to the nicotine's absence. Such symptoms include an inability to concentrate, insomnia, depression and lack of appetite, according to the NCI. 

This, among other factors, explains why so many smokers struggle to give up the habit, Ledgerwood said.

"Add to this physiological effect that cigarettes are legal, available at any gas station or corner store, and can still be smoked in many different locations, it becomes incredibly difficult for someone who wishes to stop smoking to do so," Ledgerwood added.

People who begin using tobacco products as children or teenagers may find it especially difficult to quit, as nicotine exposure can disrupt brain development, according to the FDA. And it's easy for young smokers to get hooked; brain imaging studies have shown that while reward systems in the brain mature early, the control center in the prefrontal cortex matures slowly, according to a 2012 report published originally in Cold Spring Harbor Perspectives in Medicine. "Compared with adults, adolescents are generally more motivated by rewards, are less averse to risks, and are more easily influenced by peers," according to the report.

Are some people addiction-prone?

But are some people more prone to addiction than others? Does everyone face similar difficulties when it comes to giving up smoking, or do some find it relatively easy to go cold turkey?

"I don't believe people are immune to addiction," Ledgerwood said. "Some people may be more prone to developing addictions than others, and it certainly appears to be the case that being exposed to addictive substances at an earlier age puts one at greater risk of developing an addiction."

The Fagerström Test for Nicotine Dependence, developed in 1978 by Swedish psychologist Karl-Olov Fagerström, is a questionnaire used to determine an individual's level of nicotine dependence related to cigarette smoking. The test has gone through several iterations since its introduction, but it remains in use to this day, and is still one of the main ways of assessing addiction. Questions in the test include asking when an individual smokes their first cigarette of the day, how many cigarettes they smoke each day, and whether they would smoke even if they were so ill as to be confined to their bed.

When someone scores particularly highly on this test, Ledgerwood explained, it is likely due to more than simply the body desiring frequent hits of nicotine. "For many people who smoke, there are powerful factors that contribute to their smoking," Ledgerwood said. "These individuals often grow up in homes where parents smoke, and the behavior has been modeled for them. 

"Cigarettes are still easily available in many places, and although there are restrictions on where people can smoke, there are still many opportunities for people to smoke in public. There are also still many depictions of smoking in popular culture (movies, TV shows) that may contribute to a sense that smoking is a normal, and possibly even glamorous, behavior," he added.

Moreover, studies have revealed that genetic factors play a role in nicotine dependence, meaning that addiction can run in families, according to a 2010 review in the journal Current Cardiovascular Risk Reports. The Mayo Clinic states that genetics "​​may influence how receptors on the surface of your brain's nerve cells respond to high doses of nicotine delivered by cigarettes," which could mean that, due to genetic inheritance, once people start smoking, some are more likely than others to subsequently continue the habit. According to 2008 research carried out by the American Psychological Association, "at least half of a person's susceptibility to drug addiction can be linked to genetic factors."

Despite the many risks associated with smoking, and though it is thought to contribute to the deaths of 8 million people worldwide every year — including 1.2 million who die from exposure to second-hand smoke — tobacco remains widely available and easily accessible.

However, while addiction occurs quickly, so do the health benefits once a person quits. According to the Mayo Clinic, within 20 minutes of smoking a cigarette, heart rate decreases; within 12 hours, levels of the toxic gas carbon monoxide return to normal in the blood; within three months, lung function and circulation improves; and after a year, the risk of a heart attack falls by half.

Originally published on Live Science.