Side effects and interactions: what to know

Every treatment has benefits and risks, and medical cannabis is no exception. Being plant-based does not mean it is free of side effects. The good news is that, with the right dose and proper follow-up, most effects are mild, predictable and manageable.

The most common side effects

Effects vary with the product, the dose and each person's sensitivity. The most commonly reported are:

• drowsiness or tiredness, especially at first;
• dry mouth;
• changes in appetite (increased or decreased);
• diarrhea or an upset stomach;
• dizziness, especially when standing up quickly.

With THC, there may also be a faster heart rate, red eyes and, at higher doses, anxiety, confusion or changes in perception. Many of these effects ease within the first few weeks or improve with adjustments to the dose and timing.

Start low, go slow

The most important principle in medical cannabis is titration: starting with a low dose and increasing it gradually while watching the response. This approach, known as “start low, go slow,” reduces the risk of unwanted effects and helps find the lowest effective dose for you.

Interactions with other medications

This is one of the points that needs the most attention. CBD inhibits liver enzymes that break down many medications, especially those in the cytochrome P450 system, such as CYP3A4 and CYP2C19. In practice, this can raise the levels of other medications in the blood.

Some examples that call for caution:

• clobazam: drowsiness may increase;
• valproate: combining it with CBD can affect liver function tests;
• warfarin and other anticoagulants (blood thinners): blood clotting may need to be checked more often;
• sedatives, anti-anxiety medications and alcohol: their drowsiness effects can add up;
• some antidepressants, antipsychotics and immunosuppressants.

That's why it's essential to tell your doctor about all the medications, supplements and herbal remedies you take, including occasional ones.

Driving and operating machinery

Products containing THC can affect attention, reflexes and coordination, and even CBD can cause drowsiness. Do not drive or operate machinery if you feel drowsy, dizzy or in any way not yourself. Avoid these activities at the start of treatment and after dose increases, until you know how you respond, and follow the specific advice given for your case.

Pregnancy and breastfeeding

Medical cannabis is not recommended during pregnancy or breastfeeding, because there aren't enough studies to show that it is safe for the baby. If you are pregnant, breastfeeding or planning a pregnancy, bring it up during your consultation.

When to seek help

Contact the team or seek medical care if you notice:

• severe drowsiness or difficulty staying awake;
• confusion, agitation, severe anxiety or changes in perception;
• palpitations, fainting or shortness of breath;
• seizures getting worse, in patients with epilepsy;
• yellowing of the skin or eyes, dark urine or persistent abdominal pain.

In summary

• The most common side effects are usually mild and can be managed by adjusting the dose.
• CBD can interact with many medications: tell your doctor about everything you take.
• Avoid driving until you know how you respond to treatment.
• Pregnancy and breastfeeding call for a specific conversation with your doctor.