The question of whether drug abuse increases the risk of developing schizophrenia and other mental illnesses has been a hotly debated topic for decades. New research from Denmark that includes data from more than 3 million individuals takes an in-depth look at the conundrum.

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The relationship between drug abuse and schizophrenia is complex.

There has been a wealth of research on the impact that alcohol, cannabis, and other drugs might have on the risk of developing schizophrenia and other psychiatric disorders.

However, it is a difficult area to study, and previous research has been controversial and often contradictory.

As one example, many earlier studies could not take into account co-abuse; in other words, people who abuse a number of compounds.

Dr. Stine Mai Nielsen and Prof. Merete Nordentoft, from Copenhagen University Hospital, Mental Health Center in Denmark, recently embarked on one of the largest studies of its type.

Their findings, presented at this year's International Early Psychosis Association (IEPA) meeting in Milan, Italy, add another piece to the puzzle.

Schizophrenia and drug abuse

To dive into this question, the team of investigators used data from 3,133,968 individuals born between 1955-1999 from nationwide Danish registers. In all, they identified more than 200,000 cases of substance abuse and over 21,000 schizophrenia diagnoses.

Data was analyzed using a range of statistical measures; they also controlled for a number of factors including gender, urbanity, other psychiatric diagnoses, co-abuse, parents' immigration to Denmark, parents' economic status, and psychiatric history.

The team found that abuse of any substance increased the risk of developing schizophrenia. The increased risks were as follows:

  • Cannabis: 5.2 times
  • Alcohol: 3.4 times
  • Hallucinogenic drugs: 1.9 times
  • Sedatives: 1.7 times
  • Amphetamines: 1.24 times
  • Other substances: 2.8 times.

The authors concluded:

"We present a large scaled population-based cohort study analyzing a wide variety of substances. Our results illustrate a robust association between almost any type of substance abuse and an increased risk of developing schizophrenia later in life."

Although the results are clear, an age-old problem with the research remains: it is impossible to prove whether the abuse caused the schizophrenia or vice versa. It is a possibility that someone who is predisposed to schizophrenia is more likely to abuse drugs; similarly, individuals could be susceptible to both developing schizophrenia and substance abuse.

The authors note that the relationship between mental illness and drug abuse is likely to be incredibly complex.

Parental drug-taking and schizophrenia

In a second leg of the study, the same group of researchers at Copenhagen University Hospital, led by Dr. Carsten Hjorthøj, opened the question of parental role. They wanted to identify whether parental substance abuse influenced the risk of schizophrenia.

Parental drug abuse was split into two categories - diagnosed before birth and after. Schizophrenia diagnoses were taken from Denmark's Psychiatric Central Research Register.

Both maternal and parental cannabis, whether diagnosed before or after birth, increased the risk of schizophrenia in the offspring. For mothers, it was associated with a sixfold increase and for the father a 5.5-fold increase.

For alcohol, maternal abuse diagnosed before the birth of the infant was associated with a 5.6 times increase in schizophrenia risk, but if diagnosed after the birth, this dropped by roughly 50 percent. Similarly, in fathers, pre- and post-birth risk were 4.4 times and 1.8 times, respectively.

The authors explain the potential reasons for the difference between cannabis and alcohol use:

"Secondhand exposure to cannabis is apparently linked to schizophrenia. While it is easy to be exposed to secondhand smoke, with other substances, such as alcohol, there is no secondhand exposure, which could explain the much lower associations observed after birth for these substances."

Although, as mentioned earlier, these studies can not definitively tease apart cause and effect, they are sure to add fuel to the fiery debate. Whether drugs cause schizophrenia or whether someone who is susceptible to schizophrenia is more likely to abuse drugs, unpicking the relationship and gaining insight into who may be most at risk is vital for early intervention and more successful treatment.

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