Search Knowledgebase

Topic Contents


Topic Overview

Marijuana is a commonly abused drug. It is often called pot, grass, weed, reefer, Mary Jane, or buds. Marijuana cigarettes are commonly referred to as joints or spliffs, and the butt of a marijuana cigarette is called a roach. Marijuana can be eaten, brewed in tea, or smoked. In Canada, it is most often smoked in rolled joints, in pipes, or in hollowed-out cigars (blunts).

Immediate effects of marijuana depend on the strength and potency of its main active chemical, delta-9-tetrahydrocannabinol (THC).

Some people feel relaxed or high after smoking marijuana. Other people feel no change. The effects can be felt in seconds after inhaling and within 30 to 60 minutes after eating it. Effects last about 2 to 3 hours.

A person who has just had marijuana usually has dilated pupils and may have red, bloodshot eyes. He or she may behave differently, such as seeming to be dizzy and to have trouble walking or being silly and giggly for no reason. After smoking marijuana, some people may feel hungry (commonly referred to as having "the munchies") or sleepy. Sometimes people have negative effects from marijuana, such as sudden feelings of anxiety or paranoid thoughts.

Many people do not believe that using marijuana is a problem. But marijuana is not a "safe" drug. It causes physical and psychological problems. Marijuana use is harmful because:1

  • Regular use of marijuana can cause problems with memory and affect problem-solving and learning. It can cause mood swings, anxiety, and depression.
  • It can damage the lungs, which may lead to breathing problems (such as wheezing and bronchitis).
  • It contains many cancer-causing chemicals.
  • When under the influence of marijuana, a person may have reduced inhibitions and impaired judgment. He or she may take risks or have a car crash.
  • It can cause lower sperm counts and increased breast size in males (gynecomastia). In females, it can cause menstrual problems.

Frequent, heavy use of marijuana can lead to needing increasing amounts of it to get the same feelings (tolerance) and possibly to dependence (addiction). If daily use is stopped, flu-like withdrawal symptoms and craving for the drug can develop within 24 to 48 hours and last about 2 weeks.

Long-term, regular use of marijuana may contribute to developing some kinds of cancer, breathing problems similar to smoking (cough and wheezing), and a weakened immune system. Long-term heavy use can lead to reduced motivation and to difficulty maintaining attention to what's going on around you.

Marijuana is strongly absorbed by fatty tissues in various organs of the body. The chemical THC usually can be detected in urine several days after marijuana has been smoked. If marijuana is used heavily, traces of THC may be detected in urine for weeks after use has stopped.

Signs of use

Changes in a person's behaviour that may indicate marijuana use include:

  • Withdrawal, fatigue, and depression.
  • Carelessness with grooming.
  • Hostility and relationship problems.
  • Changes in academic performance and increased absenteeism or truancy. Regular use of marijuana affects short-term memory, learning, and attention span.
  • Loss of interest in favourite activities, including sports.
  • Changes in eating or sleeping habits.

When a teen is using marijuana, the parent may find evidence of the drug and drug paraphernalia, including pipes and rolling papers, in the teen's bedroom. There may be a noticeable sweet odour on the teen's clothing. The teen may burn incense or use room deodorizers to get rid of the marijuana smell. The teen may also use eyedrops to get rid of bloodshot eyes.



  1. Levy S, Woolf AD (2008). Marijuana section of psychoactive substances of abuse used by adolescents. In LS Neinstein, et al., eds., Adolescent Health Care: A Practical Guide, 5th ed., pp. 908–911. Philadelphia: Lippincott Williams and Wilkins.


By Healthwise Staff
Primary Medical Reviewer Kathleen Romito, MD - Family Medicine
Primary Medical Reviewer Andrew Swan, MD, CCFP, FCFP - Family Medicine
Specialist Medical Reviewer Peter Monti, PhD - Alcohol and Addiction
Last Revised October 13, 2010

This information does not replace the advice of a doctor. Healthwise, Incorporated disclaims any warranty or liability for your use of this information.