Cannabis and Psychosis

​What is cannabis?

Cannabis is the general name for marijuana 'grass' 'pot', 'weed', 'dope', 'hooch' and 'hashish'. Cannabis comes from a plant and is smoked or eaten. It is a 'depressant' drug (that is, one that slows down thinking and the nervous system) and can also cause mild hallucinogenic (mind altering) effects.

How many people use cannabis?

Cannabis is the most widely used illicit drug. A study conducted in Christchurch (Christchurch Health and Development Study) shows that by age 21 years, 69% of the people interviewed had used cannabis.

Why do people use cannabis?

Some people use cannabis, like alcohol, to help them to relax. Some use cannabis because they feel it helps them with conversation and in social situations, while others use cannabis in an attempt to cope with life's problems. Some people use cannabis in an attempt to treat the symptoms of psychosis.

Are there dangers in using cannabis?

Most people who use cannabis don't experience any obvious harmful effects, but regular use may produce a number of short term effects including paranoia, confusion, increased anxiety, and even hallucinations, which can last up to several hours.

The Christchurch Health and Development Study found that cannabis use, and particularly regular or heavy use, was associated with increased rates of a range of adjustment problems in adolescence/ young adulthood including other illicit drug use, crime, depression and suicidal behaviours. These adverse effects were most evident for school-aged regular users.

In addition, longer term risks may include asthma and bronchitis, cancers of the mouth, throat, and lungs, poor concentration and memory, learning difficulties, and occasionally, psychosis.

A psychosis is a condition where a person experiences some loss of contact with reality. This is discussed more fully on the page:  What is psychosis.

Does cannabis cause psychosis?

It is believed that cannabis use may cause a condition known as a drug-induced psychosis, which can last up to a few days and is often characterised by hallucinations, delusions, memory loss and confusion.  However, in some cases, cannabis use may contribute to the development of a serious mental illness, such as schizophrenia.

A link between cannabis use early in life and the development of adult schizophrenia has been reported by researchers at the University of Otago's Health & Development Research Unit.

The study, published in the Bristish Medical Journal, shows that early use of cannabis can significantly increase the risk of schizophrenic symptoms in adulthood. The research used data collected from 759 young New Zealanders who were studied from their birth in 1972 until age 26 in 1998. The results showed that adolescents who began using cannabis by age 15, were three-to-four times more likely to be diagnosed with schizophreniform disorder in adulthood, than their counterparts. Among individuals who used cannabis before age 15, 10 per cent developed a schizophreniform disorder by the age of 26, compared to 3 per cent of the remaining people.

How does cannabis affect someone who is already experiencing a psychosis?

Cannabis use can prolong the duration of symptoms of mental illness and can lower a person's chances of recovering fully from a psychotic episode.

Who is most at risk from cannabis use?

People most at risk are those with a family history of psychotic illness or those who have already experienced a psychotic episode. So, people with a family or personal history of psychotic illness should avoid drugs like cannabis completely.

What about other drugs?

Although this page focuses on cannabis, other legal and illicit drugs should not be ignored. For example, some evidence suggests that substances such as alcohol, speed, hallucinogens (eg. acid, magic mushrooms, datura etc.) have a greater effect than cannabis in the development of a psychosis. Research is not yet available on the influence that party drugs such as ecstacy, have on psychosis, but these drugs have certainly been reported as being a significant factor in the emergence of psychosis.  For information on the consequences of using various drugs, visit the New Zealand Drug Foundation Consequences of Drugs website.

Where can I turn to for help?

Help can be obtained from a variety of sources. These range from family and friends to your local general practitioner and mental health services. In addition an Alcohol Drug Helpline is available from 10am - 10pm every day. Phone 0800 787 797. Further information is also available on


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Page last reviewed: 05 September 2013