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Medical Cannabis

Topic Overview

What is medical cannabis?

Cannabis (marijuana) is a drug that is made up of the leaves, flowers, and buds of the cannabis plant. Medical cannabis can help treat symptoms like pain, nausea, and lack of appetite. It may be used by people who have conditions like cancer, AIDS, or multiple sclerosis.

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." And it may lessen pain and other symptoms.

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. Your doctor may be able to tell you about the different strains you can try for your health problem and their possible effects.

What is it used for?

Medical cannabis may be helpful for some health conditions. These may include:

  • Nausea and vomiting from chemotherapy (chemo) for cancer.
  • Low appetite and weight loss for people who have AIDS.
  • Muscle stiffness for some multiple sclerosis or spinal cord injury patients.
  • Chronic nerve pain or pain at the end of life.

Cannabis is available as a dried plant, oil extract, and products you put on your skin. It can also be added to homemade foods such as baked goods or candy (edibles). Cannabis products usually contain both THC and CBD.

Medicines that contain THC are also available. These include:

  • THC and CBD (Sativex). This is a combination medicine that can relieve pain in people with advanced cancer and relieve muscle stiffness in people with multiple sclerosis. This drug has naturally occurring THC.
  • Nabilone (Cesamet). This medicine is used to relieve nausea and vomiting caused by chemo. It may also improve the appetite of people who have AIDS. Nabilone contains man-made THC.

If you think you might want to try medical cannabis, talk to your doctor about what type of cannabis might help relieve your symptoms. You can also visit the Health Canada website at for more information.

How is it used?

There are many ways people can use medical 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).

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 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. Or follow your doctor's instructions on how much to use.

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.

What are the risks of medical cannabis?

Cannabis can interact with many other medicines. It 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. It can also be dangerous to use cannabis with alcohol, tobacco, and illegal drugs.

Cannabis can affect your 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.

Before you try cannabis, talk to your doctor about other medicines you use. Also talk to your doctor about any personal or family history of substance use disorder or mental health disorders. Using cannabis may make these problems worse.

Cannabis may affect your judgment, memory, concentration, coordination, and decision making. Don't drive or operate machinery after using cannabis. Talk with your doctor about when it is safe to drive.

Cannabis can affect different people in different ways. Side effects may include:

  • Dry mouth.
  • Red eyes.
  • Anxiety or paranoid thoughts.
  • Faster heart rate.
  • Dizziness.
  • Drowsiness.
  • Nausea and vomiting.

Long-term regular use of cannabis may increase your 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.

If you smoke cannabis, the smoke could damage your lungs. It may make you cough or wheeze, and cause lung infections like bronchitis.

If you are using medical cannabis and are pregnant or think you might be or are breastfeeding, talk to your doctor. It can affect your baby's development.

Can regular use lead to cannabis use disorder?

Some people who regularly 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.
  • 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. Talk to your doctor about when it is safe to drive.
  • 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, control your mood, affect your blood pressure, or thin your blood.
Know what you're using.
  • Talk to your doctor about using products that have low levels of THC. The type, or strain, strength, and effects of cannabis can vary greatly. Follow your doctor's instructions on what type of cannabis to try, how to take it, how much to use, and how often to take it.
  • Understand how soon you may feel the effects of the product you use, and how long those effects may last. Ask your doctor what you can expect. The product label may also have this information.
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.

Contact your doctor if you have unwanted side effects or you think you have a problem with cannabis use.


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. Accessed July 18, 2018.
  • Government of Canada (2018). Cannabis and mental health. Government of Canada. Accessed July 18, 2018.
  • Government of Canada (2018). Cannabis health effects. Government of Canada. Accessed July 18, 2018.
  • Government of Canada (2018). Consumer information: Cannabis. Government of Canada. Accessed July 19, 2018.
  • Government of Canada (2018). Health effects of cannabis. Government of Canada. 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. Accessed July 18, 2018.


Current as of: December 20, 2019

Author: Healthwise Staff
Medical Review: E. Gregory Thompson MD - Internal Medicine
Adam Husney MD - Family Medicine

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