With cannabis legalization in Ontario, trusted sources of drug information are more important than ever. As one of the most accessible healthcare professionals, pharmacists can support you by ensuring the safe use of cannabis, and help you understand the clinical impacts of recreational marijuana use.

Tetrahydrocannabinol (THC)

  • Psychoactive
  • May elicit both depressant and stimulant effects (i.e. increased appetite, mood elevation or depression, increased heart rate and/or blood pressure, paranoia, relaxation)
  • May be used medicinally for appetite stimulation, pain relief, nausea and vomiting, and as an antispasmodic

Cannabidiol (CBD)

  • Minimally psychoactive (in combination with THC may reduce psychoactive effects or ‘high’ that is often seen with THC alone)
  • May be used medicinally for pain relief, inflammation, nausea and vomiting, anxiety, and epilepsy


Medicinal cannabis and starting materials can only be legally accessed through Health Canada’s licensed producers (LPs) with a medical document completed by a physician and faxed directly to the LP of choice. The LP then mails the product directly to the patient. Some private insurance plans are beginning to provide coverage for medicinal cannabis.

Under the provincial government, recreational marijuana will be available for online ordering initially through the Ontario Cannabis Stores (OCS) as of October 17, 2018, and later on (by April 1, 2019) through private retailers. Until then, marijuana dispensaries and “pop-ups” are unregulated storefronts that sell marijuana of uncertain quality or standards and are currently illegal.

Dosage and Administration

  1. Smoking (dry flower): Begins working within minutes, with effects lasting 2-4 hours, and may need to be used several times per day to achieve desired effects. Pharmacists and other health care professionals discourage smoking of all plant substances. Smoking cannabis is not safer than smoking cigarettes, and the same concerns related to lung health and cancer risk apply to the smoking of cannabis
  2. Vaporization (dry flower/oil/distillate): Begins working within 5 minutes, with effects lasting 3-6 hours. Vaporizing results in a smaller quantity of toxic by-products, and may be a safer alternative to smoking
  3. Oral ingestion (oils/capsules): Begins working within 1-3 hours, with effects lasting up to 10 hours. It is important to note that because of the delayed onset of action with oral dosing, patients should be cautious to wait at least 3 hours before re-dosing to avoid overdosing
  4. Alternative routes (i.e. suppositories, topical, transdermal, wafers, sprays etc.) may be available in the future; however, only dried marijuana, fresh marijuana, cannabis oil, and starting material (i.e. seeds and plants) will be legally available when the federal legislation changes

In general, new users of marijuana should take a “Start Low-Go Slow” approach. This is so you are able to understand how your body responds to it and minimize side effects. The ideal amount is different for each person. Most patients using cannabis for medicinal purposes start with a dose of 0.5-1mg of THC and use between 1-3g of dried cannabis per day.

Potential Drug Interactions

Some antidepressants, antibiotics, antifungals, blood pressure and HIV medications can interact with cannabis. Caution is advised when cannabis is combined with other sedating drugs, including alcohol, due to additive side effects. This combination should be avoided where possible.

When to Avoid

  • History of OR current psychiatric or drug/alcohol addiction disorders or suicidal ideation
  • Age < 25yrs (recreational marijuana legally accessible to persons 19+yrs)
  • Certain cardiovascular diseases/arrhythmias/high blood pressure
  • Respiratory disorders (for smoking cannabis)
  • Pregnancy/Lactation

Potential Side Effects

Immediate (acute) side effects include, but are not limited to:

  • Neurological effects (i.e. fatigue, dizziness, drowsiness, decreased attention span, and/or impaired motor coordination/performance)
  • Eye, nose and chest irritation (for smoking cannabis)
  • Gastrointestinal effects (i.e. diarrhea, nausea, dry mouth)
  • Short-term memory impairment and altered judgment
  • Heart effects (i.e. increased heart rate, arrhythmias, changes in blood pressure, cases of stroke and other cardiac effects have been reported)
  • Increased appetite
  • Paranoia, anxiety, acute panic reactions and/or hallucination

    Note: To date, there have been no reported fatalities from cannabis overdose.

Concerns in long-term (chronic) users include:

  • Worsening of pre-existing psychiatric conditions
  • Loss of motivation
  • Withdrawal symptoms upon stopping the drug abruptly (i.e. trouble sleeping, restlessness, irritability, sweating, diarrhea, hiccups and loss of appetite)
Note: Cannabis has a lower potential for dependence compared to alcohol, tobacco, and some prescription medications such as benzodiazepines and opioids.

Cannabis use in youth and/or starting at an early age, is particularly concerning due to evidence of possible earlier onset of schizophrenia in those at risk, and lower IQ scores in adolescents who are frequent users.

Cannabis can have negative effects on babies born to mothers who use it while pregnant. Babies exposed in utero may be born early, have low birth weight, and/or difficulties with normal neurological development. There is also an increased risk of fetal loss.