When marijuana is smoked, its effects begin immediately after the drug enters

the brain and last from 1 to 3 hours. If marijuana is consumed in food or drink,

the short-term effects begin more slowly, usually in 1/2 to 1 hour, and last longer,

for as long as 4 hours. Smoking marijuana deposits several times more THC into

the blood than does eating or drinking the drug.

Within a few minutes after inhaling marijuana smoke, an individual's heart begins

beating more rapidly, the bronchial passages relax and become enlarged, and blood

vessels in the eyes expand, making the eyes look red. The heart rate, normally 70

to 80 beats per minute, may increase by 20 to 50 beats per minute or, in some

cases, even double.  This effect can be greater if other drugs are taken with

marijuana.

 As THC enters the brain, it causes a user to feel euphoric - or "high" - by acting

in the brain's reward system, areas of the brain that respond to stimuli such as

food and drink as well as most drugs of abuse. THC activates the reward system

in the same way that nearly all drugs of abuse do, by stimulating brain cells to release

the chemical dopamine.

 A marijuana user may experience pleasant sensations, colors and sounds may seem

more intense, and time appears to pass very slowly. The user's mouth feels dry, and

he or she may suddenly become very hungry and thirsty. His or her hands may tremble

and grow cold. The euphoria passes after awhile, and then the user may feel

sleepy or depressed. Occasionally, marijuana use produces anxiety, fear, distrust, or panic.

Heavy marijuana use impairs a person's ability to form memories, recall events and shift attention from one thing to another.  THC also disrupts coordination and balance by

binding to receptors in the cerebellum and basal ganglia, parts of the brain that regulate

balance, posture, coordination of movement, and reaction time.  Through its effects

on the brain and body, marijuana intoxication can cause accidents. Studies show that approximately 6 to 11 percent of fatal accident victims test positive for THC.

In many of these cases, alcohol is detected as well. In a study conducted by the

National Highway Traffic Safety Administration, a moderate dose of marijuana

alone was shown to impair driving performance; however, the effects of even a

low dose of marijuana combined with alcohol were markedly greater than for either

drug alone.  Driving indices measured included reaction time, visual search frequency

(driver checking side streets), and the ability to perceive and/or respond to changes

in the relative velocity of other vehicles.

 Marijuana users who have taken high doses of the drug may experience acute

toxic psychosis, which includes hallucinations, delusions, and depersonalization -

a loss of the sense of personal identity, or self-recognition.  Although the

specific causes of these symptoms remain unknown, they appear to occur more

frequently when a high dose of cannabis is consumed in food or drink rather

than smoked.

 How does marijuana use affect physical health?

Marijuana use has been shown to increase users' difficulty in trying to quit smoking

tobacco.  This was reported in a study comparing smoking cessation in adults who

smoked both marijuana and tobacco with those who smoked only tobacco. The

relationship between marijuana use and continued smoking was particularly strong

in those who smoked marijuana daily at the time of the initial interview, 13 years

prior to the followup interview.

A study of 450 individuals found that people who smoke marijuana frequently but

do not smoke tobacco have more health problems and miss more days of work

than nonsmokers do.  Many of the extra sick days used by the marijuana smokers

in the study were for respiratory illnesses.

 Even infrequent marijuana use can cause burning and stinging of the mouth and

throat, often accompanied by a heavy cough. Someone who smokes marijuana

regularly may have many of the same respiratory problems that tobacco smokers

do, such as daily cough and phlegm production, more frequent acute chest illnesses,

a heightened risk of lung infections, and a greater tendency toward obstructed

airways.

Cancer of the respiratory tract and lungs may also be promoted by marijuana

smoke. A study comparing 173 cancer patients and 176 healthy individuals produced

strong evidence that smoking marijuana increases the likelihood of developing cancer

of the head or neck, and that the more marijuana smoked, the greater the increase.

A statistical analysis of the data suggested that marijuana smoking doubled or tripled

the risk of these cancers.

 Marijuana has the potential to promote cancer of the lungs and other parts of the

respiratory tract because it contains irritants and carcinogens.  In fact, marijuana

smoke contains 50 percent to 70 percent more carcinogenic hydrocarbons than does

tobacco smoke.  It also produces high levels of an enzyme that converts certain

hydrocarbons into their carcinogenic form, levels that may accelerate the changes

that ultimately produce malignant cells.  Marijuana users usually inhale more deeply

and hold their breath longer than tobacco smokers do, which increases the lungs'

exposure to carcinogenic smoke. These facts suggest that, puff for puff, smoking

marijuana may increase the risk of cancer more than smoking tobacco does.

Some adverse health effects caused by marijuana may occur because THC impairs

the immune system's ability to fight off infectious diseases and cancer. In laboratory

experiments that exposed animal and human cells to THC or other marijuana

ingredients, the normal disease-preventing reactions of many of the key types of

immune cells were inhibited. In other studies, mice exposed to THC or related

substances were more likely than unexposed mice to develop bacterial infections

and tumors.