There are some signs you might be able to see. If someone is high on marijuana, he or she might:

• seem dizzy and have trouble walking;
• seem silly and giggly for no reason;
• have very red, bloodshot eyes; and
• have a hard time remembering things that just happened.

When the early effects fade, the user can become very sleepy.

Parents should be aware of changes in their child’s behavior, although this may be difficult with teens. Parents should look for withdrawal, depression, fatigue, carelessness with grooming, hostility, and deteriorating relationships with family members and friends. In addition, changes in academic performance, increased absenteeism or truancy, lost interest in sports or other favorite activities, and changes in eating or sleeping habits could be related to drug use. However, these signs may also indicate problems other than use of drugs.

In addition, parents should be aware of:

• signs of drugs and drug paraphernalia, including pipes
  and rolling papers;
• odor on clothes and in the bedroom;
• use of incense and other deodorizers;
• use of eye drops; and
• clothing, posters, jewelry, etc., promoting drug use.

Within a few minutes of inhaling marijuana smoke, the user will likely feel, along with intoxication, a dry mouth, rapid heartbeat, some loss of coordination and balance, and a slower than normal reaction time. Blood vessels in the eye expand, so the user’s eyes look red.

For some people, marijuana raises blood pressure slightly and can double the normal heart rate. This effect can be greater when other drugs are mixed with marijuana, but users do not always know when that happens.

As the immediate effects fade, usually after 2 to 3 hours, the user may become sleepy.

THC in marijuana is readily absorbed by fatty tissues in various organs. Generally, traces (metabolites) of THC can be detected by standard urine testing methods several days after a smoking session. In heavy, chronic users, however, traces can sometimes be detected for weeks after they have stopped using marijuana.

Can there be a bad reaction to marijuana smoking?

Some users, especially those who are new to the drug or in a strange setting, may suffer acute anxiety and have paranoid thoughts. This is more likely to happen with high doses of THC. These scary feelings will fade as the drug’s effects wear off.

In rare cases, a user who has taken a very high dose of the drug can have severe psychotic symptoms and need emergency medical treatment.

Other kinds of bad reactions can occur when marijuana is mixed with other drugs, such as PCP or cocaine.

Marijuana can be harmful in a number of ways, through immediate effects and through 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 behaviors, which could lead to 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.

Although all of the long-term effects of marijuana use are not yet known, there are studies showing serious health concerns. For example, a group of scientists in California examined the health status of 450 daily smokers of marijuana, but not tobacco. They found that the marijuana smokers had more sick days and more doctor visits for respiratory problems and other types of illness than did a similar group who did not smoke either substance.

Findings so far show that the regular use of marijuana may play a role in cancer and problems of the immune and respiratory systems.

Marijuana smoke contains some of the same cancer-causing compounds as tobacco, sometimes in higher concentrations. It is not certain whether marijuana damages the immune system of people. But both animal and human studies have shown that marijuana impairs the ability of T-cells in the lungs’ immune system to fight off some infections.

People who smoke marijuana regularly may develop many of the same breathing problems that tobacco smokers have, such as daily cough and phlegm production, more frequent chest colds, a heightened risk of lung infections, and a greater tendency toward obstructed airways.

source: NIDA