The causes of psychosis have three main classifications:

• psychosis caused by psychological conditions,
• psychosis caused by general medical conditions, and
• psychosis caused by substances, such as alcohol or drugs.

Psychological causes

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

• schizophrenia,
• bipolar disorder,
• severe stress or anxiety,
• severe depression, and
• lack of sleep.

The underlying psychological cause will often have influence the type of psychotic episode experienced. For example, someone with bipolar disorder is more likely to have delusions of grandeur, whereas somebody 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/AIDS,
• malaria,
• syphilis,
• Alzheimer's disease,
• Parkinson's disease,
• hypoglycaemia (abnormally low blood sugar levels),
• lupus (a condition where your immune system attacks healthy tissue),
• Lyme disease (a bacterial infection),
• multiple sclerosis (a neurological disorder), and
• brain tumour.

Substances

Drug abuse can trigger a psychotic episode. A psychotic episode can also be triggered if you suddenly stop taking a drug after using it for a long time. This is known as drug withdrawal. Drugs that are known to trigger psychotic episodes include:

• alcohol,
• cocaine,
• amphetamine (speed),
• methamphetamine (crystal meth),
• MDMA (ecstasy),
• cannabis,
• LSD (acid),
• psilocybins (magic mushrooms), and
• ketamine.

Psychosis may also rarely occur as a side-effect of some types of medication, or as a result of an overdose of that medication. These medications include:

• tranquillisers, such as barbiturates or benzodiazepines,
• anti-epileptic medication,
• antidepressants,
• anticholinergics medications (medicines that are often used to help open up the airways), and
• L-dopa (a medication used to treat Parkinson's disease).

Grey matter

Research has revealed that during a psychotic episode, several physical and biological changes occur in the brain.

MRI scans carried out on people with a history of psychosis have shown that they tend to have less grey matter than normal. Grey matter is the part of the brain responsible for processing thoughts. This has lead to some researchers suggesting that repeated episodes of psychosis may actually cause physical damage to the brain. However, further research is required to confirm this.

Alternatively, both the reduction of grey matter and a history of psychosis could both be symptoms of a yet-unidentified underlying condition.

Dopamine

Researchers also believe that dopamine plays an important role in psychosis. Dopamine is a neurotransmitter, one of many chemicals that are used by our brain to transmit information from one brain cell to another. Dopamine is associated with feelings of pleasure and reward. For example, when you experience an enjoyable or pleasurable event, such as sex, the levels of dopamine in your brain are increased.

It is thought that in people with psychosis, the levels of dopamine in their brain become too high. The excess dopamine interrupts the specific pathways of the brain that are responsible for some of its most important functions, such as memory, emotion, social behaviour and self-awareness. The disruption to these important brain functions may explain the symptoms of psychosis.

The evidence for the role of dopamine in psychosis is that medications that are known to reduce the effects of dopamine in the brain also reduce the symptoms of psychosis. Whereas, illegal drugs that are known to increase the levels of dopamine in the brain, such as cannabis, cocaine and amphetamines, can trigger psychosis.