Cannabis has been legal for adults in Canada since 2018, which means most people who use it buy a regulated, labelled product. Legal does not mean risk-free. This article covers what THC does, who is most at risk, and the lower-risk use guidance public-health researchers have published.

What THC does

THC (delta-9-tetrahydrocannabinol) is the main intoxicating compound in cannabis. It acts on the body’s cannabinoid receptors, which are involved in mood, memory, appetite, pain, and coordination. Typical effects include relaxation, altered perception of time, increased appetite, and mild euphoria. Unwanted effects include anxiety, paranoia, rapid heartbeat, dry mouth, dizziness, and impaired memory and coordination.

How strong the experience is depends on the product’s THC content, how it is taken, and the person’s tolerance, body, mood, and setting. Modern cannabis products, especially concentrates, can be much more potent than the plant products that older research was based on.

Smoking, vaping, and edibles behave differently

• Inhaled cannabis acts within minutes and fades over a few hours.
• Edibles are absorbed through digestion, so effects begin much later and last much longer. The most common mistake is taking more because nothing seems to be happening. See the dedicated article on edibles later in this series.
• Concentrates can be very potent and are associated with a higher risk of dependence and intense reactions.

Who is most at risk

Young people. The brain continues developing into the mid-20s. Early, frequent use is associated with higher risks of dependence, poorer school outcomes, and mental health problems. This is why public-health guidance recommends delaying use.

People with a personal or family history of psychosis. Cannabis, especially high-THC products used frequently, is associated with an increased risk of psychotic disorders in vulnerable people. The association is well documented, although the causal details are still debated.

People who are pregnant or breastfeeding. Guidance advises against use.

People with heart or lung conditions. THC raises heart rate, and smoking harms the lungs.

Daily or near-daily users. Risk of cannabis use disorder rises with frequency and potency.

Cannabis use disorder

Not everyone who uses cannabis becomes dependent, but some do. Commonly cited estimates are that roughly one in ten people who ever use cannabis develop dependence, with higher rates among those who start in adolescence and use daily. Signs include using more than intended, failed attempts to cut down, strong cravings, withdrawal (irritability, poor sleep, low appetite) when stopping, and continuing despite problems. Treatment exists, including counselling and support programs.

Cannabis hyperemesis syndrome

Long-term heavy users can develop cycles of severe nausea and vomiting that are relieved temporarily by hot showers. The only reliable fix is stopping cannabis. If this sounds familiar, see a clinician; repeated vomiting can cause dehydration and kidney problems.

Driving

Cannabis impairs reaction time, attention, and coordination. Canada has legal limits for THC in blood, and driving while impaired is a criminal offence. Public-health guidance generally recommends waiting at least six hours after inhaling, and longer after edibles, and many people need more time than that. Combining cannabis with alcohol makes impairment worse. The safest rule is not to drive if you used and feel anything at all.

Lower-risk use guidance

Canadian researchers published Lower-Risk Cannabis Use Guidelines that summarize the evidence. Their recommendations include:

• Not using, or delaying use, until later in life, since risks are higher for young people.
• Choosing lower-THC products, or products with CBD, and avoiding synthetic cannabinoids.
• Avoiding harmful inhalation practices such as deep inhalation and breath-holding, and preferring methods other than smoking.
• Limiting frequency, since occasional use carries lower risk than daily use.
• Not driving or operating machinery after use.
• Avoiding use if you are at high risk of mental health problems, or pregnant.
• Avoiding combining with other substances, including alcohol and tobacco.

For new users, the standard advice is to start low, go slow, and wait to see the full effect before using more. Product labels in the regulated market show THC content, which makes this easier.