SummaryRead the full fact sheet
- Cannabis has many harmful effects on health.
- Cannabis is an illegal drug.
- Cannabis impairs the user’s driving ability.
- People with a psychotic illness should not use cannabis.
- Help is available 24 hours a day, seven days a week.
On this page
Cannabis, or marijuana, is the most commonly used illegal drug. It comes from the Cannabis sativa plant. It can be smoked or eaten, and comes in a variety of forms, such as dried plant leaves and flowers (‘heads’), a crumbly, light-brown or dark-brown resinous material called ‘hash’, or a very potent oil called ‘hash oil’.
The chemical in cannabis that makes users feel ‘high’ is called THC (delta-9 tetrahydrocannabinol). THC is a psychoactive substance, which means that it travels in a person’s bloodstream to the brain. It disrupts their brain’s normal functioning and causes certain intoxicating effects.
Cannabis use can cause drug-induced psychosis, trigger the first episode of a psychotic illness or make a pre-existing psychotic illness worse. People who have, or may be at risk of developing, a psychotic illness should not use cannabis.
Types of cannabis
There are several forms of cannabis, including:
- marijuana – the most common and least concentrated form. It is made from dried plant leaves and flowers. The flowers, or ‘heads’, are the most potent forms of the plant. Marijuana looks like chopped grass and ranges in colour from grey-green to greenish-brown. It is usually smoked in a water pipe (bong), pipe or hand-rolled cigarette (joint)
- hashish – small blocks of dried cannabis resin. Blocks range in colour from light brown to nearly black. The concentration of THC in hashish is higher than in marijuana
- hash oil – a thick, oily liquid extracted from hashish, ranging in colour from golden-brown to black. It is usually spread on the tip or paper of cigarettes and then smoked. Hash oil is more potent than the other forms of cannabis.
In recent years, a wide range of synthetic products, claiming to have similar effects to cannabis, have also been available in Australia. Synthetic cannabis is made up of chemicals that are designed to activate the same chemical systems in the brain as THC. These drugs are marketed as having similar physical and psychological effects as cannabis, but can have more unpredictable effects and are potentially more harmful than cannabis.
Medicinal cannabis refers to approved, quality assured cannabis products prescribed by your doctor and taken to treat the symptoms of a medical condition or the side effects of treatment. Read more about medicinal cannabis.
Effects of cannabis use
Cannabis is a depressant drug. Depressant drugs do not necessarily make you feel depressed. Rather, they slow down your central nervous system and the messages going between your brain and your body.
Cannabis has different effects on different people. The effect can also vary according to the mood or atmosphere in which the person uses it. The immediate effects of cannabis can last approximately two to three hours and may include:
- a feeling of relaxation and wellbeing
- loss of inhibition
- reduced concentration and memory
- distorted perceptions of time, space and distance
- increased heart rate
- increased appetite
- increased talkativeness
- reddened eyes
- reduced coordination – this makes it dangerous to drive or operate machinery while under the influence of the drug
- anxiety and paranoia.
Larger doses or stronger forms of cannabis can increase these effects, and can also cause:
- feelings of excitement
- anxiety or panic
- detachment from reality
- psychosis (severe mental illness where the person loses connection with external reality).
Cannabis and psychosis
Research suggests that cannabis use can make a person’s existing psychotic symptoms worse. It may even bring on psychotic symptoms in people who are predisposed to psychosis if they have a personal or family history of psychosis.
Cannabis use can cause a condition known as drug-induced psychosis. Symptoms usually appear quickly and last a few days until the effects of the cannabis wear off. Disorientation, memory problems and hallucinations (seeing or hearing things that are not really there) are the most common symptoms.
Cannabis and psychotic illness
The effects of cannabis begin within minutes and can last several hours. However, for people with a psychotic illness (such as schizophrenia), the effects can be longer lasting and more intense.
Cannabis and first episode of psychosis
If someone has a predisposition to a psychotic illness such as schizophrenia, use of drugs such as cannabis may trigger the first episode in what can be a lifelong, disabling condition. There is increasing evidence that regular cannabis use precedes and causes higher rates of psychotic illness.
Psychotic illnesses are characterised by:
- delusions – for example, the person believes they have special powers
- hallucinations – for example, the person hears voices or sees things that aren’t really there
- thought disorder – for example, the person has difficulty organising their thoughts.
When people experience psychotic symptoms, they are unable to distinguish what is real. They lose contact with reality.
Worsening of psychotic symptoms through cannabis use
Cannabis use generally makes psychotic symptoms worse and lowers the person’s chances of recovery from a psychotic episode. People with a psychotic illness who use drugs experience more delusions, hallucinations and other symptoms. They have a higher rate of hospitalisation for psychosis, and treatment is generally less effective for them. People with a psychotic illness should not use cannabis.
Risks from long-term cannabis use
Cannabis may have additional physical or mental effects on long-term users, including:
- the risk of asthma, emphysema, shortness of breath, chest infections, and cancers of the throat, mouth and lungs
- poor concentration, memory loss and learning difficulties
- depression of the immune system, which increases the risk of developing infections.
Cancer-causing (carcinogenic) substances in cannabis
Cannabis smoke has a higher concentration of certain cancer-causing (carcinogenic) agents than the smoke from tobacco. Evidence suggests that cannabis may cause cancers of the lung and the aerodigestive tract (which includes the respiratory tract and the upper digestive tract).
Young people and cannabis
Generally speaking, people who start smoking cannabis earlier (early adolescence) and smoke heavily are more likely to have problems as a result. These may be mental health problems, or more general life problems, like conflict at home, school or work, financial problems and memory problems.
Cannabis tolerance and dependence
A person who uses cannabis regularly can develop a tolerance to the drug. This means the person has to take larger and larger quantities to get the same effect. Some people can become psychologically or physically dependent (or both) on cannabis.
Preventing the negative health effects of cannabis
The best way to avoid the harmful effects of cannabis is not to use it, especially for people who have experienced a psychotic episode in the past or who have a family history of psychosis.
Where to get help
- Your GP (doctor)
- Hospital emergency department
- DirectLine Tel. 1800 888 236 – for 24-hour confidential drug and alcohol telephone counselling, information and referral
- DrugInfo Tel. 1300 85 85 84
- SANE Australia Helpline Tel. 1800 187 263
- Family Drug Help Tel. 1300 660 068 – for information and support for people concerned about a relative or friend using drugs
- Cannabis Information and Support
- Drug and alcohol treatment centre
- Cannabis and psychotic illness, SANE Australia.
- Cannabis, 2013, DrugInfo, Australian Drug Foundation.
- Cannabis and psychosis – information for healthcare workers, Department of Health Victoria.
- What is psychosis?, SANE Australia.
- Room R, Fischer B, Hall W, et al. 2008, Cannabis policy: moving beyond the stalemate (The Global Cannabis Commission report), The Beckley Foundation, Oxford University Press. More information here.
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