The use of non-legalised or illicit drugs is common among people with schizophrenia and can be problematic as it can make the course the disease worse.¹ In this article, the link between schizophrenia and illicit drug use is examined and it will highlight why people with schizophrenia may be more prone to drug use than the general population and how to help those that may have a problem.

Whether it is drugs, alcohol or smoking tobacco,  it is estimated that one in four people with schizophrenia will have at least one problem with substance use at some point in their lifetime.¹ This is at least three times higher than in the general population.^1,2
Of note, rates of drug and other substance use are generally twice as high in men than in women with schizophrenia,¹ with particularly high rates seen in young men around the time of experiencing the first episode of psychosis.^3,4

The most commonly used drugs by people with schizophrenia are cannabis (as found in marijuana and hashish) and cocaine.^1,2  
·       The number of people with schizophrenia who will have a problem with cannabis use ranges from 17% to 83%.¹
·       Lifetime problem usage rates for cocaine are lower, starting at around 15% but can reach as high as 50%.¹
 
Importantly, drug use often occurs alongside other substance use disorders.¹ In one large epidemiological study, for instance, it was shown that 47% of people with schizophrenia had a problem with both drugs and alcohol compared to less than one-fifth of people who did not have a mental illness.¹  

The use of illicit drugs is important as it can affect how well someone with schizophrenia can function in their everyday life.¹ Drug use, especially if co-occurring with other substance use disorders, has been linked not only to not taking medications correctly or complying with prescribed treatment programmes, but also to worse clinical outcomes, high rates of relapse, and the need for repeated hospital treatment.¹ The co-existence of substance use disorders has also been linked to high rates of violence – which could lead to arrest and imprisonment – and to an increased risk for suicide.¹ Then there is also the financial cost of a drug habit to consider, which can have a significant impact on someone with schizophrenia who may already have money management difficulties and debt.⁵

So, can drugs cause schizophrenia? There are a variety of substances that can induce a schizophrenia-like psychosis⁸, including cannabis which is the most commonly used drug among people with schizophrenia.¹ There is further evidence that drug use can precede the development of schizophrenia in some cases.¹ It is feasible that drug use, especially if it starts at an early age when the brain is still developing, may produce changes in the brain structures and their functioning that could develop into a mental illness in susceptible people.^{6, 7}
 
Furthermore, data show that people who experience psychosis as a result of drug use are more likely to develop schizophrenia than those who do not use drugs.⁸ In a large evaluation of multiple studies, around one-third of people who used cannabis, one-quarter of those who took hallucinogenic drugs, and one-fifth of those who had taken amphetamines and had psychotic episodes were later diagnosed with schizophrenia.⁸
 
Conversely, there is also evidence that people with schizophrenia may be more vulnerable to developing a drug addiction than others.⁹ Indeed, data show that compared to no mental illness, schizophrenia is associated with a three and a half times higher risk of having a substance use disorder overall, and more than twice as likely to have cannabis use disorder.⁹Importantly, the risk for developing a substance use disorder appears to remain some 10–15 years after being diagnosed with schizophrenia.⁹

How can you tell if someone with schizophrenia could have a problem with drug use? It may be difficult to spot as some signs could be confused with symptoms of schizophrenia, such as social withdrawal, self-neglect or changes in behaviour or concentration.^7.11
 
Some things to look out for include:
 
·       Acting out of character or changes to normal behaviour, such as a lack of interest in favourite activities, or quickly switching mood
·       Becoming secretive or being reluctant to talk about things
·       Altered sleeping or eating patterns, particularly if at odd hours
·       Missing appointments and important events
·       Having trouble at school or work
·       Sudden money problems and requests for loans
·       Having trouble with family and friends
 

If you are worried about your own or your loved one’s drug use, please talk to a healthcare professional about your concerns. While there may be anonymous and confidential services to help people with drug addiction generally, people with schizophrenia may require more specialist advice which only a healthcare professional who specialises in mental health can provide.¹¹
 
Remember that healthcare professionals are duty bound to treat all the information provided in the utmost confidence and will be able to refer you to the right services for your situation.¹¹
 
Some recommended approaches to addressing drug use in people with schizophrenia include:¹¹
 
·       Encouraging the individual to discuss their drug use in a non-judgmental way
·       Reassuring the person that there will be no repercussions of disclosing information about their drug use
·       Making sure that schizophrenia is being appropriately treated (e.g., with antipsychotic medication)
·       Offering counselling alongside schizophrenia medications
·       Helping the individual address the use of other substances, helping them to reduce or stop drinking alcohol or quit smoking.