How does marijuana affect health?

Marijuana is the most widely used illegal drug in Canada. About 14% of Canadians over the age of 15 report using it in 2005. It is a mood-altering drug that comes from the Cannabis sativa plant, and it has many street names, including weed, pot, reefer, hash, grass and dope. The effect or high of marijuana is caused by the chemical THC (delta-9-tetrahydro-cannabinol). Possessing, producing and trafficking in marijuana can result in fines, prison sentences, and criminal records.

Short-term effects

The effects of marijuana depend on the person, the setting, and the amount used. The concentration of THC in marijuana can vary widely, as can the effects on users. Some people who use marijuana may feel relaxed and happy. Others may become quiet and withdrawn.

If marijuana is smoked, the effects are felt quite quickly and last for two to four hours. If marijuana is eaten, its effects occur more slowly and might last for a longer time. Some of the immediate effects of marijuana are:

  • forgetfulness
  • trouble concentrating
  • slower reactions

Some users may feel:

  • severe anxiety
  • panic attacks
  • fearful and suspicious (paranoia)

These effects usually disappear within hours. Driving or operating machinery after using marijuana is not safe, especially if combined with other drugs, including alcohol.

Long-term effects

Using marijuana heavily for a long period of time can have serious side effects. Marijuana smoke contains chemicals that damage the lungs and can lead to chronic coughing, lung infections, and in some cases, cancer. People who smoke both marijuana and tobacco may develop lung, neck and head cancers at a younger age than those who smoke only tobacco.

Heavy marijuana use at a young age may affect brain development, especially in the areas of the brain that control the ability to focus attention. Many long-term users have problems with:

  • concentration
  • abstract thinking
  • short-term memory

Most of these problems disappear after a few weeks without marijuana, but some may last longer. Heavy marijuana users may also experience mental health problems, such as anxiety and depression. In some cases, it may trigger schizophrenia in people who have a family history of this disease.

Marijuana and pregnancy

Women who use marijuana during pregnancy are more likely to have premature or underweight babies.  As the children grow up, they may have some learning and behavioural problems.


Regular users of marijuana develop a tolerance to it, which means that they need to use more of the drug to produce the same effect. Heavy, long-term use of marijuana can cause dependence. Dependent people crave the drug and may have difficulty reducing or stopping their use of it.

Dependent users who quit may feel mild withdrawal symptoms including:

  • troubled sleep
  • irritability
  • anxiety
  • nausea
  • sweating
  • loss of appetite

These symptoms usually last less than a week, but cravings can last much longer.

Medical marijuana

Marijuana is an illegal drug in Canada. Possessing, producing, buying and selling it can result in fines, a prison sentence and a criminal record. There is an exception, however, for some people who use marijuana for medical reasons, such as to decrease the nausea caused by anti-cancer drugs or, in the case of HIV/AIDS patients, to increase appetite.

Although research hasn’t yet shown a clear and unique medical benefit of marijuana, Health Canada brought in regulations in 2001 to allow some patients and caregivers to possess and grow marijuana for medical purposes.

For help in your province, search the Treatment Services in Canada database produced by the Canadian Centre on Substance Abuse.

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