Causes of psychosis 

The causes of psychosis have three main classifications.

They are psychosis caused by:

  • mental (psychological) conditions
  • general medical conditions
  • substances, such as alcohol or drugs

The three classifications are described in more detail below.

Psychological causes

The following conditions have been known to trigger psychotic episodes in some people:

  • schizophrenia – a long-term (chronic) mental health condition that causes hallucinations and delusions
  • bipolar disorder – a condition that affects a person's moods, which can swing from one extreme to another (highs and lows)
  • severe stress or anxiety
  • severe depression – feelings of persistent sadness that last for more than six weeks, including postnatal depression, which some women experience after having a baby
  • lack of sleep

The underlying psychological cause will often influence the type of psychotic episode someone experiences.

For example, a person with bipolar disorder is more likely to have delusions of grandeur, whereas someone with depression or schizophrenia is more likely to develop paranoid delusions.

General medical conditions

The following medical conditions have been known to trigger psychotic episodes in some people:

  • HIV and AIDS – a virus that attacks the body's natural defence against illness and infection (the immune system)
  • malaria – a tropical disease spread by infected mosquitoes
  • syphilis – a bacterial infection usually passed on through sexual contact
  • Alzheimer's disease – the most common form of dementia, which causes a decline in mental abilities like memory and reasoning
  • Parkinson's disease – a long-term condition that affects the way the brain co-ordinates body movements, including walking, talking, and writing
  • hypoglycaemia – an abnormally low level of sugar (glucose) in the blood
  • lupus – a condition where the immune system attacks healthy tissue
  • Lyme disease – a bacterial infection spread to humans by infected ticks
  • multiple sclerosis – a condition that affects the nerves in the brain and spinal cord, causing problems with muscle movement, balance, and vision 
  • brain tumour – a growth of cells in the brain that multiply in an abnormal and uncontrollable way


Alcohol misuse and drug misuse can trigger a psychotic episode.

A person can also experience a psychotic episode if they suddenly stop drinking alcohol or taking drugs after using them for a long time. This is known as withdrawal.

It's also possible to experience psychosis after drinking large amounts of alcohol or if you're high on drugs.

Drugs known to trigger psychotic episodes include:

  • cocaine
  • amphetamine (speed)
  • methamphetamine (crystal meth)
  • mephedrone (MCAT or miaow)
  • MDMA (ecstasy)
  • cannabis
  • LSD (acid)
  • psilocybins (magic mushrooms)
  • ketamine

In rare situations, psychosis can also occur as a side effect of some types of medication or as a result of an overdose of that medication.

For example, levodopa, a medication used to treat Parkinson's disease, can sometimes cause psychotic episodes. However, any medicine that acts on the brain can cause psychosis with an overdose.

Never stop taking a prescribed medication unless advised to do so by your GP or another qualified healthcare professional responsible for your care.

See your GP if you're experiencing psychotic side effects caused by medication.

The brain

There's been a great deal of research into how psychosis affects the brain and how changes in the brain can trigger symptoms of psychosis.

Grey matter

Research has revealed several physical and biological changes occur in the brain during a psychotic episode. Grey matter is the part of the brain responsible for processing thoughts.

The results of magnetic resonance imaging (MRI) scans have shown some people with a history of psychosis have less grey matter than most other people. However, it's not yet fully understood why this is.


Researchers also believe dopamine plays an important role in psychosis.

Dopamine is a neurotransmitter, one of many chemicals the brain uses to transmit information from one brain cell to another. It is associated with how we feel whether something is significant, important, or interesting.

It's thought levels of dopamine in the brain become too high in people with psychosis. The excess dopamine interrupts specific pathways in the brain that are responsible for some of its most important functions, such as:

  • memory
  • emotion
  • social behaviour
  • self-awareness

Disruption to these important brain functions may explain the symptoms of psychosis.

Evidence for the role of dopamine in psychosis comes from several sources, including brain scans and the fact medications known to reduce the effects of dopamine in the brain also reduce the symptoms of psychosis.

However, illegal drugs known to increase levels of dopamine in the brain – such as cannabis, cocaine and amphetamines – can trigger psychosis.

Cannabis: real stories

Smoking cannabis can lead to cannabis psychosis, causing you to lose touch with reality. Two men describe how it happened to them.

Media last reviewed: 17/06/2015

Next review due: 17/06/2017

'Therapy changed my life'

Hina, 39, a British Asian, had periods of feeling sad and tired but was too ashamed to talk about it.

Page last reviewed: 31/07/2014

Next review due: 30/11/2016