Know the Health Effects


Cannabis is also known as: Marijuana, kush, pot, weed, Mary J/MJ, grass, ganja. Cannabis is used today for medicinal and/or non-medical purposes. Non-medical use of cannabis will be legalized in summer 2018. It comes in the form of dried plant buds, hashish (dried resin from plant leaves) or oil (boiled resin). There are many ways that cannabis can be consumed. These include: 

  • Smoking, such as a “joint”, in a pipe or bong;
  • Ingested as an edible or drink;
  • Consumed through vaping.


Not all chemicals in cannabis occur naturally. There are more than 400 chemicals in cannabis, some of which can cause cancer, damage your lungs, and affect the brain. When smoked, chemicals such a carbon monoxide, cyanide, benzopyrene, and tar are produced. Additionally, illegally sourced cannabis may contain pesticides, other drugs, heavy metals, moulds or fungi, or other unknown substances. If present, these are inhaled along with THC.

Delta-9-tetrahydrocannabionl (THC) is the component contained by the cannabis plant, which causes the ‘high’ experienced by users. This varies depending on:

  • The part of the plant that is used;
  • How the plant is bred;
  • How the product is processed.

In some cases, cannabis may be laced with other drugs such as fentanyl, cocaine or tobacco.

Higher potency of THC can result in more harmful effects for those who use it. When consumed, cannabis causes short and long term health effects;

Short term effects:

  • Increased heart rate
  • Decreased attention span
  • Slowed reaction time
  • Lack of a sense of time
  • Nervousness or paranoia

Long term effects:

  • Physical and mental health problems
  • Decreased cognitive functioning (memory, attention, problem solving)
  • Impaired ability to drive a vehicle
  • Reproductive health issues
  • Increased risk of addiction
  • Lung health issues


Health Effects of Cannabis

Adolescence is an important time for the developing brain since most brain development happens before the mid-20’s. The THC in cannabis disrupts the same biological system in the brain that guides brain development. This means that youth are particularly at risk of the negative consequences that can result from cannabis use.

Effects of Cannabis Use During Adolescence

Fact and Fiction about Marijuana

Cannabis Talk Kit - Know how to talk with your teen

What’s with Weed?

Kids Helpline Phone: 1-800-668-6868 - 24 hours a day, seven days a week (Canada only)

The Drug & Alcohol Helpline: 1-800-565-8603

Just like with tobacco, a pregnant woman or new mother's use of cannabis can affect her fetus or newborn child, which can lead to health problems.  There is no known safe amount of cannabis use in pregnancy and when breastfeeding.  The toxins in cannabis are carried through the mother's blood to her fetus during pregnancy and in the breast milk.

Heavy cannabis use during pregnancy can lead to lower birth weight of the baby. It has also been associated with longer-term developmental effects in children and adolescents, such as decreases in:

  • memory function
  • the ability to pay attention
  • reasoning and problem-solving skills
  • hyperactive behavior
  • increased risk for future substance use


If a woman is breastfeeding, the THC in cannabis and other chemicals will get into the breast milk and may affect the baby.  The THC can move into the baby’s fat tissue, as well as into the brain.  Cannabis is not recommended for anyone while pregnant or breastfeeding. 



Risks of Cannabis on Fertility, Pregnancy, Breastfeeding and Parenting

Maternal Cannabis Use during Pregnancy

Cannabis use impairs the key functions required for driving safely, including:

  • cognition
  • attention
  • decision-making
  • motor skills and coordination
  • reaction time


Research has shown that cannabis-impaired driving doubles the risk of being involved in a motor vehicle accident, including fatal collisions. This risk even further increases when cannabis use is combined with alcohol. Impairment can last 3 to 6 hours after use. It is recommended that users do not drive (or operate machinery) for at least 6 hours after use of cannabis.

For young drivers, driving after using cannabis is more common than driving after drinking alcohol. Youth aged 18-19 are most likely to drive following cannabis consumption, followed by those aged 15-17 years old.

Drug Impared Driving



Canadian Drug Policy Coalition Evidence Review – Cannabis Use and Driving

Canadian Centre on Substance Use and Addiction - Cannabis Use and Driving Update

In order to reduce the potential negative health effects of cannabis, it is safest not to use.

However, if you choose to use cannabis:

  • Delay cannabis use (brain continues to develop into mid-20s)
  • Know what you’re using and choose lower-risk, less potent (lower THC), cannabis products
  • Don’t use synthetic, or man-made, versions of cannabis (e.g. K2 or Spice)
  • Avoid smoking cannabis – choose something safer like vaporizers or edibles
  • Avoid ‘deep inhalation’ or ‘breath-holding’
  • Limit and reduce how often you use cannabis
  • Don’t use cannabis and drive, or operate other machinery (wait at least 6 hours after using cannabis to drive)
  • Avoid combining risks listed above
  • Don’t use cannabis if you are at risk for mental health problems or are pregnant


Find more on how to lower your risk of harm:

Lower Risk Cannabis Use Guidelines

Lower Risk Cannabis Use Guidelines (evidence summary for health professionals)

It is illegal to use non-medical cannabis in the workplace, or to be under the influence of cannabis while at the workplace.

Below are a resource and toolkit that employers and employees can look to for information on addressing substance use in the workplace.

Bill C-45, known as the Cannabis Act, is intended to come into effect in 2018. In 2017, the provincial government released a framework outlining how they plan to manage the federal legalization in Ontario. Please note the following information pertains to recreational cannabis use, not cannabis that is prescribed for medical reasons.


Legal Age

No person under 19 years of age can possess or consume cannabis.


Where you can Use Cannabis

The use of cannabis will only be legal in private residences. Cannabis consumption, in any form, will not be allowed in public spaces, workplaces or motor vehicles. 


Cannabis Sales

The Liquor Control Board of Ontario (LCBO) will be the only legal retailer and distributor of recreational cannabis in Ontario. Cannabis will also be available through an online order service operated by the LCBO, which is also expected to be available July 2018. 


Controlling Access

Adults who are 19 years old or older would be legally able to:

  • possess up to 30 grams of legal dried cannabis
  • share up to 30 grams of legal cannabis with other adults
  • purchase dried or fresh cannabis or cannabis oil from a licensed retailer
  • grow up to 4 cannabis plants per residence to a maximum height of 100 cm


Legalizing and Strictly Regulating Cannabis: The Facts

Cannabis Consultation Paper Response Grey Bruce Health Unit

Youth smoking

Cannabis use prevalence rates are highest among youth and young adults (MLHU, 2016).

Results from the Ontario Student Drug Use and Health Survey (Boak, et al., 2015) represent 10,426 students from 220 schools in grades 7-12, and their cannabis use.

  • 21.3% of students report using cannabis at least once during the 12 months before the survey, representing about 203,900
  • Males (22.0%) and females (20.5%) are equally likely to use cannabis;
  • Use significantly increases with grade, to a high of 37.2% among 12th graders;
  • There are no significant regional differences.


Daily Cannabis Use

About 2.1% use cannabis on a daily basis, representing about 20,000 students. There is no significant difference between males (2.3%) and females (1.9%).


Cannabis Dependence (Grade 9-12)

About 2.2% report symptoms of cannabis dependence, representing about 14,900 high school students in Ontario.


Review the Ontario Student Drug Use and Health Survey for further information.



Boak, A., Hamilton, H. A., Adlaf, E. M., & Mann,R. E., (2015). Drug use among Ontario students,1977-2015: Detailed OSDUHS findings (CAMH Research Document Series No. 41). Toronto,ON: Centre for Addiction and Mental Health.

Middlesex-London Health Unit (2016). Cannabis: A Public Health Approach. London, Ontario: Author.


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