T–W

tobacco A plant cultivated as a crop in the
southern United States and regions around the
world with similar climate. In the United States
federal and state laws regulate the sale and posses-
sion of tobacco products, with most states restrict-
ing sales to people age 18 or older (age 19 in a few
states). Further restrictions on smoking affect the
use of tobacco throughout the United States.
Though tobacco releases hundreds of chemicals
when it burns, its primary psychoactive ingredient
is NICOTINE, a powerful stimulant and vasoconstric-
tor (narrows and stiffens the BLOODvessels). Nico-
tine enters the blood circulation within seconds of
the first inhalation of cigarette smoke and remains
active in the body for up to two hours after the
person finishes smoking the cigarette. Its primary
route is via absorption in the LUNGS, though the
mucous membranes of the MOUTH(oral mucosa)
and NOSEalso absorb nicotine from the smoke.
Mucosal absorption is the primary route for nico-
tine and tobacco’s other chemicals to enter the
blood circulation with cigar smoking and forms of
tobacco other than cigarettes, such as chewing
tobacco and snuff.

No form of tobacco or tobacco product

is safe to use. Any use of tobacco,

smoked or oral, can cause numerous

health conditions. Smoking tobacco fur-

ther exposes others to health risks.

Nicotine is one of the most addictive substances
known. It rapidly crosses the BLOOD–BRAIN BARRIER
to act directly on neurotransmitters and neurore-
ceptors in the BRAIN, affecting primarily DOPAMINE
and acetylcholine. Dopamine has numerous func-
tions related to mood, emotion, and sensations of
pleasure; researchers believe dopamine plays a

key role in addiction not only to nicotine but to

most psychoactive drugs. Acetylcholine affects the

release of other neurotransmitters in the brain and

hormones in the body, notably EPINEPHRINEand

NOREPINEPHRINE. These chemicals stimulate brain

activity as well as increase BLOOD PRESSURE and

HEART RATE. Though cigarette smoking provides the

most rapid release of nicotine with tobacco use,

oral forms of tobacco (chewing tobacco and snuff)

deliver significantly higher concentrations of nico-

tine into the blood circulation and thus have high

risk for addiction though many people perceive

them to have fewer health risks.

Dozens of the chemicals in tobacco are carcino-

genic (cause cancer). Many are most potent when

burning, such as smoking a cigarette or cigar,

releases them. Tobacco use, primarily cigarette

smoking, is the leading cause of CARDIOVASCULAR

DISEASE(CVD), LUNG CANCER, laryngeal cancer, and

CHRONIC OBSTRUCTIVE PULMONARY DISEASE (COPD).

Chewing tobacco and snuff cause nearly all oral

cancers (cancers of the mouth); cigar smoking

also causes oral cancers. Smoked forms of tobacco

also expose other people to carcinogenic chemi-

cals.

See also ANTISMOKING EFFORTS; CANCER RISK FAC-

TORS; CARCINOGEN; ENVIRONMENTAL CIGARETTE SMOKE;

SMOKING AND CANCER; SMOKING AND CARDIOVASCULAR

DISEASE(CVD); SMOKING AND HEALTH; SMOKING CESSA-

TION; WITHDRAWAL SYNDROME.

tolerance The need for increasing amounts of a

DRUGto reach the same level of effectiveness. Tol-

erance is an expected physiologic effect with many

drugs; for some, the therapeutic benefits rely on it.

Tolerance accounts for the ability of long-time

users to take doses of drugs that might otherwise

be harmful.

353