Substance abuse – such as alcohol abuse – increases the risk of schizophreniaReuters

Substance abuse can increase the risk of developing schizophrenia in the long-term, scientists have shown. The most significant correlation was found for cannabis and alcohol abuse.

It is not the first time that a study focuses on the impact that abusing alcohol and illicit drugs has on mental health, Many research papers conclude that the risk of developing schizophrenia is heightened for people who are important consumers of alcohol and other drugs. However, the evidence is most of the time too limited to come up with clear-cut results.

The results of this latest study, presented this week at the Early Psychosis Association (IEPA) meeting in Milan (Italy), are particularly robusts because they are based on a great amount of data, collected from more than 3 million individuals.

The research confirms the clear association between substance abuse and schizophrenia risk and enables the researchers to address the limitations from previous findings.

Large population study

The scientists conducted a nationwide prospective study. It included data from all residents born in Denmark from 1955 till 1999, collected from a national register, effectively making this one of the largest studies conducted on the topic to date.

In total, they examined the medical data of 3,133,968 people – 204,505 of which were diagnosed with substance abuse while and 21,305 were diagnosed with schizophrenia. Performing a statistical analysis, the scientists were able to identify which person suffering from substance abuse also suffered from schizophrenia. They thus measured the probability of someone developing schizophrenia for each type of substance abuse.

Cannabis was the substance most associated with a risk of developing schizophreniaPedro Pardo/ AFP

A diagnosis of any substance abuse increased the risk of developing schizophrenia by 6 times, for cannabis the increased risk was 5.2 times and for alcohol 3.4 times, hallucinogenic drugs 1.9 times, sedatives 1.7 times, amphetamines 1.24 times and other substances 2.8 times.

Another problem highlighted by the study is that schizophrenia could emerge long after the substance abuse diagnosis. "The increased risk was found to be significant even 10 to 15 years after a diagnosis of substance abuse. Our results illustrate robust associations between almost any type of substance abuse and an increased risk of developing schizophrenia later in life", the scientists point out.

This study does not prove that substance abuse causes schizophrenia, but thanks to the huge amount of data, it underscores a strong correlation. One possibility is that people who are likely to develop schizophrenia also have a higher probability of abusing substances. They may have a susceptibility to both diseases.