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Topic Overview

What is cannabis?

Cannabis (marijuana) is a drug that is made up of the leaves, flowers, and buds of the cannabis plant. People may use cannabis for medical or non-medical reasons.

The two most active chemicals in cannabis are THC and CBD. THC affects how you think, act, and feel. It can make you feel very happy or "high." CBD can help you feel relaxed without the "high."

There are many types, or strains, of cannabis. Each strain has specific THC-to-CBD ratios. Because of this, some strains have different kinds of effects than others. For example, if a strain of cannabis has a higher ratio of THC to CBD, it's more likely to affect your judgment, coordination, and decision making.

How is it used?

There are many ways people can use cannabis. For example, people can:

  • Smoke it as a dried plant.
  • Brew it into tea.
  • Inhale it as a vapour.
  • Spray it under the tongue.
  • Apply it to the skin.
  • Eat it in homemade foods (edibles).

What are the health effects of cannabis?

When you use cannabis, you may be putting your health at risk.

Short-term health effects.

People often use cannabis for the way it makes them feel. Using it may make them:

  • Feel relaxed or very happy ("high").
  • Have less chronic pain or nerve (neuropathic) pain.
  • Feel hungry so they eat more.

But it may also cause unwanted side effects, such as:

  • Impaired short-term memory and ability to concentrate.
  • Poor judgment and coordination.
  • Anxiety or paranoid thoughts.
  • Faster heart rate.
  • Red eyes and dry mouth.
  • Dizziness.
  • Drowsiness.
  • Nausea and vomiting.
  • Changes in blood pressure.

How soon and how long you may feel the effects of cannabis depends on several things, including how it was taken. For example, when cannabis is smoked, the effects can usually be felt within seconds after inhaling. On the other hand, when cannabis is eaten, the full effects may not be felt for up to 90 minutes after you eat it. Since the effects aren't felt right away, people may think they need more and use too much. To avoid this, start with small amounts until you know how edibles affect you.

How much cannabis you've used and how long you've been taking it can also affect how your body responds to it. You may feel the effects of cannabis for hours after you use it.

Long-term health effects.

Long-term use of cannabis may lead to problems such as:

  • Trouble with learning, memory, and concentration. This is most likely if use begins in the teen years.
  • Lung problems if you smoke cannabis. This can lead to coughing or wheezing and lung infections like bronchitis.
  • Mental health problems like anxiety, depression, and psychosis. This is more likely if you have a personal or family history of these disorders or use cannabis products that have high levels of THC.
  • Cannabis use disorder. Some people who use cannabis may find it hard to control their use and keep using cannabis even though it's having harmful effects on their lives.
  • Increased risk for severe nausea and vomiting (cannabinoid hyperemesis syndrome, or CHS). People who have CHS may feel very thirsty and have belly pain and diarrhea. They may vomit more than 20 times a day. Bouts of vomiting may last more than 24 hours.
Health effects of use during pregnancy and breastfeeding.

Using cannabis is not safe for you or your baby. If used during pregnancy, it can harm a developing baby (fetus). It can pass from the mother's blood to the baby's blood. And it can pass from the mother's breast milk to the baby during breastfeeding.

Cannabis can cause problems for you during your pregnancy and when it is time for your baby to be born. It may also affect your baby both before and after he or she is born. This is even more true for people who use a lot of cannabis.

Health Canada says that cannabis use during pregnancy may:footnote 1

  • Cause your baby to be smaller at birth.
  • Be related to problems with brain development, learning, and behaviour.
  • Increase your baby's risk for depression or anxiety later in life.

THC is found in breast milk. It can cause problems with feeding. Health Canada says that babies exposed to THC in breast milk may:footnote 1

  • Be drowsy.
  • Have muscle weakness, which can make it hard for your baby to hold his or her head up.
  • Not suckle well.
Health effects of use in young people.

Using cannabis before the age of 25 can affect a young person's brain development, as well as emotional and social development. Some young people who use cannabis may develop cannabis use disorder. They may find it hard to control their use and keep using cannabis even though it's having harmful effects on their lives.

Cannabis affects the parts of the brain that deal with judgment, decision making, and emotions. This can make it harder for young people to think, learn, reason, remember, solve problems, and make good choices. And they may be less able to control their emotions and actions. For example, they may engage in risky behaviours like driving when "high," having unsafe sex, binge drinking, or using other drugs.

Young people who use cannabis may be more likely to have anxiety and depression than others who don't. And they may have more problems in school, relationships, and work.

Can use lead to cannabis use disorder?

Some people who use cannabis may develop cannabis use disorder. This can range from mild to severe (dependency). They may find it hard to control their use and keep using cannabis even though it's having harmful effects on their lives.

The risk of cannabis use disorder is higher in people who:

  • Start using cannabis when they're young.
  • Use it every day.
  • Have other substance use disorders and mental health disorders.

People who use cannabis often and then quit may have withdrawal symptoms. These include anxiety, trouble sleeping, and intense cravings for the drug.

How can you reduce the risk of harm from cannabis use?

Using cannabis isn't risk-free. But there are things you can do to reduce your risk of getting sick or injured.

Lower-risk use.
  • Limit your cannabis use. Or don't use it at all. Experts recommend limiting use to 1 day a week at most.
  • Don't drive or operate machinery after using cannabis. Using cannabis may affect your judgment, coordination, and decision making. It can also increase your risk of being in a car crash. Make a plan to get home safely. For example, choose a designated driver or take a taxi or bus for a safe ride home.
  • Don't smoke cannabis. The smoke can damage your lungs. If you do smoke it, don't breathe in deeply and don't hold your breath.
  • Don't use cannabis with alcohol, tobacco, or illegal drugs.
  • Reduce the risk of medicine interactions. Cannabis can be dangerous if you use it with medicines that make you sleepy or control your mood. These include sedatives, anxiety drugs, antidepressants, and opioids. They may increase the effects of each. Cannabis can affect blood pressure, so use caution if you take medicine for this condition. It also increases the chance of bleeding if you're on blood thinners.
Know what you're using.
  • Choose products that have low levels of THC. If possible, read the label of any products you're thinking about trying. The type, or strain, strength, and effects of cannabis can vary greatly.
  • Understand how soon you may feel the effects of the product you use, and how long those effects may last. The product label may have this information.
  • Don't use synthetic cannabis, such as K2 and Spice. These drugs are stronger than cannabis and can have very bad side effects.
Keep others safe.
  • Store cannabis in a safe and secure place. This is especially important with edible cannabis, which can be easily mistaken for treats or snacks. Make sure that children, friends, family, and pets can't get to them.
  • Protect others from second-hand smoke. Smoke it outside or choose a room where you can open a window or use a fan to get the smoke outside. If you're around someone who is smoking cannabis, you may feel some effects of the drug.
Know when to call for help.

Don't be afraid to call if you or someone you know needs medical care. The reason for the call won't be reported to the police.



  1. Government of Canada (2018). Is cannabis safe during preconception, pregnancy, and breastfeeding?: Cannabis evidence brief. Government of Canada. https://www.canada.ca/en/health-canada/services/publications/drugs-health-products/is-cannabis-safe-during-preconception-pregnancy-breastfeeding.html. Accessed February 27, 2019.

Other Works Consulted

  • Holitzki H, et al. (2017). Health effects of exposure to second- and third-hand marijuana smoke: A systematic review. Canadian Medical Association Journal Open, 5(4): E814–E822. DOI: 10.9778/cmajo.20170112. Accessed July 19, 2018.
  • Fischer B, et al. (2017). Lower-risk cannabis use guidelines: A comprehensive update of evidence and recommendations. American Journal of Public Health, 107(8): e1–e12. DOI: 10.2105/AJPH.2017.303818. Accessed July 18, 2018.
  • Government of Canada (2018). About cannabis. Government of Canada. https://www.canada.ca/en/health-canada/services/drugs-medication/cannabis/about.html. Accessed July 18, 2018.
  • Government of Canada (2018). Cannabis and mental health. Government of Canada. https://www.canada.ca/en/health-canada/services/drugs-medication/cannabis/health-effects/mental-health.html. Accessed July 18, 2018.
  • Government of Canada (2018). Cannabis health effects. Government of Canada. https://www.canada.ca/en/services/health/campaigns/cannabis/health-effects.html. Accessed July 18, 2018.
  • Government of Canada (2018). Health effects of cannabis. Government of Canada. https://www.canada.ca/en/health-canada/services/drugs-medication/cannabis/health-effects/effects.html. Accessed July 18, 2018.
  • Porath-Waller AJ (2015). Clearing the smoke on cannabis: Maternal cannabis use during pregnancy—An update. Canadian Centre for Substance Abuse. http://www.ccsa.ca/Resource%20Library/CCSA-Cannabis-Maternal-Use-Pregnancy-Report-2015-en.pdf. Accessed July 18, 2018.


Current as of: February 11, 2021

Author: Healthwise Staff
Medical Review: Patrice Burgess MD - Family Medicine
Adam Husney MD - Family Medicine
Martin J. Gabica MD - Family Medicine
Kathleen Romito MD - Family Medicine
Michael F. Bierer MD - Internal Medicine, Addiction Medicine

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