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Cannabis and your health

Cannabis contains substances that affect the brain and body, including delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD). THC causes the intoxicating effects of cannabis. CBD is not intoxicating but can still have effects on the brain.

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The short-term effects of cannabis use

Everyone’s response to cannabis differs and can vary from one time to the next.

When cannabis is used, it can:

  • Impair your ability to drive safely or operate equipment
    • Cannabis can cause drowsiness, slow reaction times, lower your ability to pay attention and impair coordination Footnote 1 Using cannabis and then driving or operating equipment can result in an accident, serious injuries or death.
    • Cannabis can impair your thinking, concentration, memory and decision-making, and can impact your ability to perform well on the job or at school. Footnote 2
    • Though cannabis can cause euphoria (a high) it can also cause anxiety or panic. Footnote 2
    • In rare cases, cannabis can trigger a psychotic episode (not knowing what is real, experiencing paranoia, having disorganized thoughts and, in some cases, hallucinating). Footnote 2

    The long-term risks of cannabis use

    Using cannabis frequently (daily or almost daily) and over a long time (several months or years) can:

    • Hurt your lungs and make it harder to breathe, if smoked
      • Cannabis smoke contains many of the same harmful chemicals found in tobacco smoke. Footnote 3
      • Frequent use of THC over a long time increases the risk of cannabis dependence, also called:
        • addiction
        • cannabis use disorder
        • problematic cannabis use

        The effects of cannabis on young people’s health

        Cannabis affects the same biological system in the brain that is responsible for brain development. Footnote 11

        Youth and young adults are more likely to experience harms from cannabis because their brains develop until about age 25. The earlier you start consuming cannabis, the more harm it can do. Footnote 11

        Starting as a teen, consuming frequently (daily or near daily) and over a long time (several months or years) increases the risk of mental health problems. These problems include dependence and disorders related to anxiety and depression. Footnote 11

        Frequent use of cannabis over a long time can also harm important aspects of your thinking, like learning and memory. Stopping use can help improve these deficits. However, some of these harms may persist for months or years, or may not be fully reversible. Footnote 12 Footnote 13 Footnote 14

        Lowering your risks when consuming cannabis

        There are risks associated with cannabis use. The best way to protect your health is to avoid using cannabis or cannabis products completely.

        Cannabis can be consumed in different ways. Two common ways are:

        • inhalation (smoking or vaping)
        • ingestion (eating or drinking)

        Each way carries different health and safety risks.

        Everyone’s response to cannabis is different, depending on:

        • sex
        • age
        • THC and CBD content
        • any pre-existing medical conditions
        • experience with cannabis, frequency of use
        • consumption of food, alcohol, other drugs or health products

        Everyone’s response to cannabis can also differ from one time to the next.

        Research suggests that there are ways to reduce the risks:

        • Use it in a safe and familiar environment and with people you trust, especially if you are inexperienced or a new user.
        • Delay cannabis use until the brain is fully developed. This occurs around the age of 25.
          • The earlier you begin using cannabis, the higher your risk of serious health issues, including dependence and other mental health problems.
          • The higher the THC content of a product, the more likely you are to experience adverse effects and greater levels of impairment. CBD is known to reduce some of the effects of THC.
          • Frequent use of cannabis over a long time can contribute to mental health problems. These include dependence, anxiety and depression.
          • Using cannabis at the same time as drinking alcohol and/or using other drugs can cause more severe levels of impairment and adverse effects. Other drugs include pain medications (opioids) and tranquillizers (benzodiazepines).
          • After alcohol, cannabis is the drug most often linked to car accidents. Cannabis can affect your concentration, attention and coordination, and slow your reaction time. Using it and driving or operating machinery increases the risk of having an accident, which can result in serious injuries or death.
          • Products known as synthetic cannabis (K2, spice) are not cannabis at all. These products are very different, have much stronger effects and are more dangerous. Using synthetic cannabis can lead to severe health problems, such as seizures, irregular heartbeat, hallucinations and, in rare cases, death.
          • psychosis
          • schizophrenia
          • problematic substance use

          For more information, please see the lower-risk cannabis use guidelines developed by Canadian experts in mental health and addiction.

          The effects of cannabis on pregnancy and breastfeeding

          Avoid cannabis completely if you are pregnant or breastfeeding. Substances in cannabis are transferred from the mother to child and can harm your unborn or newborn baby.

          If you have questions, visit your health care provider.

          Cannabis for medical purposes

          Some people use cannabis for their health problems. Deciding if cannabis is appropriate to treat your symptoms is best done in discussion with a health care provider.

          To help you and your health care provider make informed decisions about the potential benefits and risks, we have published information on using cannabis for medical purposes.

          Cannabis addiction

          Cannabis addiction is real, although the risk of addiction is lower than it is for other substances such as:

          • tobacco
          • alcohol
          • opioids

          Using cannabis frequently can lead to a pattern of problematic use or use disorder. This can result in dependence or addiction.

          Experiencing a cannabis addiction can cause serious harm to your:

          • health
          • social life
          • school work
          • work and financial future
          • Adolescents who use cannabis frequently can increase their chance of becoming addicted. Footnote 16
          • Close to 1 in 10 adults who use cannabis will become addicted. This statistic rises to about 1 in 6 for people who started using cannabis as a teen. Footnote 16
          • Between 1/4 and 1/2 of all those who smoke cannabis daily will become addicted. Footnote 16

          Quitting is not always easy. If you are struggling with your cannabis use, you can:

          • discuss your cannabis use with your physician or other trusted health care provider or counsellor
          • reach out to organizations or groups that deal with addiction

          Know the signs of cannabis addiction and where to get help.

          Cannabis poisoning

          Accidentally consuming or consuming too much cannabis at a time can lead to temporary adverse effects, also known as cannabis poisoning. Cannabis poisoning is not generally known to be fatal. It can, however, be very unpleasant and potentially dangerous, sometimes requiring emergency medical attention and, in some cases, hospitalization. Children and pets are at greater risk of cannabis poisoning.

          • chest pain
          • rapid heartbeat
          • nausea/vomiting
          • psychotic episode
          • respiratory depression
          • severe anxiety and/or panic attack

          The higher the THC content in a product, the higher the likelihood of experiencing adverse effects/poisoning, especially if you are a first-time or inexperienced user.

          It is also easier to be poisoned when ingesting (eating or drinking) cannabis compared to inhaling cannabis (smoking or vaping). This is because some of these products may be confused with similar non-cannabis products. It can also take much longer to feel the effects. The result is that people consume more before they feel the full effects.

          • 2 hours for you to start to feel the effects
          • 4 hours for you to feel the full effects
          • 12 hours for acute effects to subside

          Store all cannabis products safely, keeping them out of reach of children, youth and pets. This is particularly important for edible cannabis, which may be mistaken for regular food or drinks.

          If you’ve consumed cannabis and are experiencing particularly unpleasant or harmful effects:

          • stop using it
          • seek immediate medical attention or call your local poison control centre

          Note: If you have consumed cannabis, don’t drive. There is no guidance to drivers about:

          Cannabis: the facts – Healthy body

          The effects of cannabis can vary a lot from person to person. It can also vary depending on how much or how often it’s taken and what it contains.

          Some examples include:

          • feeling chilled out, relaxed and happy
          • laughing more or become more talkative
          • feeling hunger pangs (“the munchies”)
          • feeling drowsy, tired or lethargic
          • feeling faint or sick
          • having problems with memory or concentrating
          • experiencing mild hallucinations
          • feeling confused, anxious or paranoid

          Can you get addicted to cannabis?

          It’s possible to get addicted to cannabis, especially people who are considered regular or heavy users.

          If regular users stop taking cannabis, they may get withdrawal symptoms, such as feeling moody and irritable, feeling sick, difficulty sleeping, difficulty eating, sweating, shaking and diarrhoea.

          Regularly smoking cannabis with tobacco also increases the risk of becoming addicted to nicotine and experiencing withdrawal symptoms from nicotine as well as cannabis if you cut down or give up.

          Regularly using tobacco also increases the risk of tobacco-related diseases such as cancer and coronary heart disease.

          Trying to give up cannabis?

          If you need support with giving up cannabis:

          • see a GP
          • visit Frank’s Find support page
          • call Frank’s free drugs helpline on 0300 123 6600
          • see Drug addiction: where to get help is a free self-help group. Its “12 step” programme involves stopping using marijuana with the help of regular face-to-face and online support groups. You can call them on 0300 124 0373 (callback service).

          Cannabis and mental health

          Regular cannabis use increases the risk of developing a psychotic illness, such as schizophrenia.

          A psychotic illness is one where you have hallucinations (seeing things that are not really there) and delusions (believing things that are not really true).

          The risk of developing a psychotic illness is higher in people who:

          • start using cannabis at a young age
          • smoke stronger types, such as skunk
          • smoke it regularly
          • use it for a long time
          • smoke cannabis and also have other risk factors for schizophrenia, such as a family history of the illness

          Cannabis also increases the risk of a relapse in people who already have schizophrenia, and it can make psychotic symptoms worse.

          Other risks of cannabis

          Other risks of regularly using cannabis can include:

          • feeling wheezy or out of breath
          • developing an uncomfortable or painful cough
          • making symptoms of asthma worse in people with asthma
          • reduced ability to drive or operate machinery safely

          If you drive while under the influence of cannabis, you’re more likely to be involved in an accident. This is one reason why drug driving, like drink driving, is illegal.

          Cannabis and pregnancy

          Cannabis use may affect fertility. Regular or heavy cannabis use has been linked to changes in the female menstrual cycle and lower sperm count, or lower sperm quality in men.

          Using cannabis while pregnant may harm the unborn baby. Cannabis smoke contains many of the same harmful chemicals found in cigarette smoke.

          Regularly smoking cannabis with tobacco increases the risk of a baby being born small or premature.

          Cannabis has not been linked to birth defects, but research suggests that using cannabis regularly during pregnancy could affect a baby’s brain development as they get older.

          Does cannabis have medicinal benefits?

          Cannabis contains active ingredients called cannabinoids. 2 of these – tetrahydrocannabinol (THC) and cannabidiol (CBD) – are the active ingredients of a prescription drug called Sativex. This is used to relieve the pain of muscle spasms in multiple sclerosis.

          Another cannabinoid drug, called Nabilone, is sometimes used to relieve sickness in people having chemotherapy for cancer.

          Clinical trials are under way to test cannabis-based drugs for other conditions including cancer pain, the eye disease glaucoma, appetite loss in people with HIV or AIDS, and epilepsy in children.

          Read the latest updates on cannabis, cannabinoids and cancer – the evidence so far on the Cancer Research UK website.

          Page last reviewed: 3 December 2020
          Next review due: 3 December 2023