Medical cannabis side effects: what you need to know

Possible side effects of THC

THC is the main psychoactive compound in cannabis. Effects listed by Health Canada include:

• Drowsiness, dizziness, loss of balance or coordination
• Dry mouth, red or irritated eyes, increased appetite
• Trouble concentrating or remembering
• Fast heart rate, blurred vision
• Severe anxiety, paranoia, hallucinations, psychosis
• Nausea and vomiting, including cannabinoid hyperemesis syndrome (repeated episodes of severe nausea and vomiting)
• Dependence

Possible side effects of CBD

CBD is not psychoactive, but it is not free of effects. Health Canada notably lists:

• Drowsiness, weakness, decreased appetite
• Nausea, vomiting, diarrhea or constipation
• Skin or allergic reactions
• Liver problems (fatigue, dark urine, abdominal pain, yellowing of the skin or eyes)

Some products sold as "CBD" also contain enough THC to cause effects: always read the label.

Depending on how it is used

Smoking or vaping cannabis can irritate the throat and lungs and cause coughing, shortness of breath or chest pain. Oils, capsules and topicals avoid these respiratory effects, but ingested products act more slowly: waiting before taking another dose lowers the risk of overconsumption.

Long-term use

Prolonged use may be linked to tolerance, problematic use, effects on the lungs or liver and hormonal changes (menstrual cycle, fertility). Stopping abruptly can cause withdrawal symptoms: anxiety, insomnia, irritability, tremors. Cannabis may also increase the risk of psychiatric disorders in vulnerable people. That is why regular follow-up appointments are recommended.

Situations that call for extra caution

• Pregnancy and breastfeeding — cannabinoids can pass from mother to child; talk to your healthcare professional.
• Medications — cannabis can interact with other medications or substances (sedatives, alcohol, some blood thinners, etc.). Always provide your up-to-date medication list.
• Psychiatric history — psychosis, schizophrenia, bipolar disorder: careful assessment is needed.
• Driving — in Quebec, the zero-tolerance principle prohibits driving with any detectable presence of cannabis in your body. Never drive under the influence of cannabis.

Lowering the risks: good habits

• Start with a low dose and increase slowly, as recommended by your healthcare professional
• Favour low-THC or CBD-rich products at first, if appropriate for you
• Store products locked up, out of reach of children and pets
• Do not combine with alcohol or other sedating substances
• Keep track of your doses and effects to discuss at follow-up

When to seek emergency care

Call 9-1-1 for seizures, trouble breathing, chest pain, loss of consciousness, suicidal thoughts or a severe allergic reaction, and say that you used cannabis. For accidental or excessive use, the Quebec Poison Control Centre can be reached at 1-800-463-5060.