Some studies done in animals or in the lab suggest that substances from medical cannabis might affect how cancer cells grow. However, there is no strong evidence from studies in people that cannabis or cannabinoids can treat cancer or slow its growth.

Because of this, cannabis is currently used in cancer care only to help manage symptoms, not to treat the cancer itself.​

The cannabis plant contains many active ingredients, but the two main ones are tetrahydrocannabinol (THC) and cannabidiol (CBD).

Medical cannabis products generally fall into three groups:
• Mostly THC
• Mostly CBD
• A balanced mix of THC and CBD

THC can cause a “high" feeling in large doses. These are known as psychoactive effects. It can also help with some symptoms and is usually well-tolerated at low doses.

CBD does not cause a high and can lessen the effects of THC while helping with symptoms on its own.

A BC Cancer-led study found that different people benefit from different products: some prefer THC, some prefer CBD, and others prefer an equal mix of both. That's why it's important to know exactly how much THC and CBD are in any product you use.​

Medical cannabis can be accessed through:

• Licensed medical producers with an authorization from a doctor or nurse practitioner
• Health Canada–approved prescription cannabinoids from a pharmacy with a prescription from a doctor or nurse practitioner
• Unlicensed dispensaries
• Home growing by an authorized person or grower
  

Recommended ways to take medical cannabis

The best form of medical cannabis is an oil extract. Oil extract comes in small bottles with child resistant caps, do not look like candy and have little smell. The extract is taken as drops under the tongue and is absorbed fairly quickly. Effects usually last about 6 to 8 hours.

The label should clearly show how strong the extract is (how many milligrams are in the bottle). Because oils are very concentrated, the starting dose is usually a very small amount measured with a small syringe. The licensed producer should tell you how much to take. If you switch products, always re‑check the strength — the same dose in milligrams may be a different volume. Strength can also vary between batches.

Other ways to take medical cannabis

Smoking or vaping cannabis is not recommended because it can harm the lungs, expose others to second‑hand smoke and has a strong smell.

Edible products like gummies or brownies are not recommended. They are a common cause of accidental overdoses in children and pets, take a long time to work, and last a long time, which makes dosing harder. ​

Suppositories may be an option if taking oil under the tongue is not possible (for example, mouth sores or severe nausea). Otherwise, they offer no clear benefit and absorption can be unpredictable. Taking cannabis this way can be risky for people with low immunity or bleeding problems as even mild trauma to the anus or rectum can be dangerous.

Creams or ointments may help some people, but there is little research so clear guidance is not available.​

A BC Cancer-led study found that it's not possible to know ahead of time which type of cannabis oil extract will work best for any one person. Because of this, it can be helpful to try THC, CBD and an equal mix for a few days each and see how you feel.

Some people find it useful to keep a simple daily note of their symptoms and any side effects. This can help you compare the oils and decide whether any are helpful for you and which one works best. If you feel anxious, you may be more likely to prefer a balanced 1:1 oil, but it's still worth trying all three if possible.​

Starting dose

A reasonable starting dose is 2.5 mg of THC, CBD or the two combined. Because oil extracts are very concentrated, this is usually a very small amount and needs to be measured carefully with a syringe, not guessed. Check the amount you should take with the licensed producer that you buy the extract from or ask your pharmacist.

Start low and increase slowly if needed. Some people may need up to 5 mg, but lower doses may work and can still cause side effects, especially if you are new to cannabis.​

For symptoms that last all day, many people use the oil up to three times a day. For sleep, a dose taken about one hour before bedtime often works best, sometimes increasing gradually over the evening.​​

Costs can vary depending on where you get your medical cannabis. No cannabinoid medicines are covered by provincial health plans, and extended health insurance rarely covers them.

Receipts from lcensed medical cannabis producers can be claimed as medical expenses on your income tax return at the end of the year.​

Receipts from walk‑in cannabis dispensaries cannot be claimed as medical expenses for tax purposes.

​​Like all medicines, cannabis products can cause side effects. These are common but usually mild and improve quickly if you lower the dose or stop taking the product.

Common side effects include:

• Dry mouth
• Dizziness or light‑headedness
• Stomach rumbling
• Increased appetite

Cannabis does not usually cause constipation.

If you take too much during the day, you may feel sleepy. If this happens, reduce your dose.​

Driving and safety

Cannabis can cause drowsiness. Do not drive or use heavy machinery right after taking it. Once you know how it affects you, you may be able to drive normally at least 4 hours after a dose, as long as you feel alert and safe to do so.

Heart and blood pressure effects

Cannabis can lower blood pressure and increase heart rate. People with unstable heart conditions should not use cannabis products.

Mental health considerations

THC can worsen confusion or psychosis in some people. If you have had recent confusion (delirium) or a history of psychosis (such as schizophrenia), a CBD‑only product is recommended if cannabis is considered at all.

Severe vomiting with THC (rare)

Some people who previously used a lot of cannabis have had a condition called cannabis‑induced hyperemesis syndrome, which causes repeated vomiting. If this has happened to you, you should avoid THC entirely.​

Pregnancy, breastfeeding and fertility

Cannabis should not be used during pregnancy or breastfeeding, as it can harm the baby. Cannabis can also affect sperm and should be avoided by people trying to conceive.

Medication interactions

Cannabis can interact with some medicines, including certain blood thinners. Ask your pharmacist, family doctor or cancer care team to check for interactions before you start, and again if you start any new medication.

​Addiction

Having a substance use disorder should not prevent someone from using medical cannabis during cancer care. Cannabis has been shown to reduce cravings for some substances and, in places where it is legal, has been linked to fewer opioid overdose deaths.

Tolerance and dependence can occur with high doses, however, cannabis misuse disorder (addiction) does not happen when cannabis-based medicine is used in low doses for symptom management.

For some patients, especially those who have both cancer and a substance use disorder, cannabis can make it easier to manage symptoms and stay on cancer treatment. It may work better for pain than opioids like morphine, hydromorphone or oxycodone, particularly if those medicines no longer work well for them.​