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Marijuana has many dangers; through both immediate effects and damage to health over time.

Marijuana hinders the user's short-term memory (memory for recent events), and he or she may have trouble handling complex tasks. With the use of more potent varieties of marijuana, even simple tasks can be difficult.

Because of the drug's effects on perceptions and reaction time, users could be involved in auto crashes. Drug users also may become involved in risky sexual behavior. There is a strong link between drug use and unsafe sex and the spread of HIV, the virus that causes AIDS.

Under the influence of marijuana, students may find it hard to study and learn. Young athletes could find their performance is off; timing, movements, and coordination are all affected by THC.

Marijuana affects many skills required for safe driving: alertness, the ability to concentrate, coordination, and reaction time. These effects can last up to 24 hours after smoking marijuana. Marijuana use can make it difficult to judge distances and react to signals and sounds on the road.

There is data showing that marijuana can play a role in crashes. When users combine marijuana with alcohol, as they often do, the hazards of driving can be more severe than with either drug alone.

A study of patients in a shock-trauma unit who had been in traffic accidents revealed that 15 percent of those who had been driving a car or motorcycle had been smoking marijuana, and another 17 percent had both THC and alcohol in their blood.

In one study conducted in Memphis, TN, researchers found that, of 150 reckless drivers who were tested for drugs at the arrest scene, 33 percent tested positive for marijuana, and 12 percent tested positive for both marijuana and cocaine. Data also show that while smoking marijuana, people show the same lack of coordination on standard "drunk driver" tests as do people who have had too much to drink.

Smoking any drug is unhealthy. Marijuana is no exception. The smoke actually contains higher concentrations of carcinogenic polycyclic aromatic hydrocarbons (PAHs) than tobacco smoke. Marijuana smokers generally inhale more smoke for longer depositing more than 4 times as much tar on their lungs as cigarette smokers.

Worse is if you combine marijuana and tobacco. If you are a heavy smoker of marijuana and tobacco joints (more than 10 a day) you are significantly increasing your risk of contracting lung disease. Recent studies show that the greatest pre-cancerous abnormalities appear in those who smoke the two drugs together.

A common side-effect, usually for first time or early users, is anxiety, panic, paranoia and feelings of impending doom. In a recent study, between 10%-15% of people who smoked marijuana reported "paranoid" or "confused" feelings as a disadvantage of smoking marijuana. And over 27% reported "anxiety" as a regular or occasional effect. Around 30% gave "negative experiences" as their reason for permanently quitting marijuana.

Effects on the Brain

Scientists have learned a great deal about how THC acts in the brain to produce its many effects. When someone smokes marijuana, THC rapidly passes from the lungs into the bloodstream, which carries the chemical to organs throughout the body, including the brain.

In the brain, THC connects to specific sites called cannabinoid receptors on nerve cells and influences the activity of those cells. Some brain areas have many cannabinoid receptors; others have few or none. Many cannabinoid receptors are found in the parts of the brain that influence pleasure, memory, thought, concentration, sensory and time perception, and coordinated movement.

The short-term effects of marijuana use can include problems with memory and learning; distorted perception; difficulty in thinking and problem solving; loss of coordination; and increased heart rate. Research findings for long-term marijuana use indicate some changes in the brain similar to those seen after long-term use of other major drugs of abuse. For example, cannabinoid (THC or synthetic forms of THC) withdrawal in chronically exposed animals leads to an increase in the activation of the stress-response system and changes in the activity of nerve cells containing dopamine. Dopamine neurons are involved in the regulation of motivation and reward, and are directly or indirectly affected by all drugs of abuse.