A new research published at 2016 International Early Psychosis Association meeting in Milan shows that alcohol, cannabis and other illicit drugs can greatly increase the risk of developing schizophrenia in later life.
The study is conducted by Copenhagen University Hospital and Mental Health Center Copenhagen in Denmark.
Previous research has analysed potential links between substance abuse and schizophrenia. However, due to methodological limitations uncertainties remain.
In this new study, the researchers analysed nationwide Danish registers to establish a cohort of 3,133,968 individuals and identified 204,505 cases of substance abuse and 21,305 diagnosed with schizophrenia.
Information on substance abuse was taken from various registers and did not include psychotic symptoms caused by substance abuse in the definition.
The result showed that a diagnosis of any substance abuse increased the risk of developing schizophrenia by 6 times. For marijuana the increased risk was 5.2 times, alcohol 3.4 times, hallucinogenic drugs 1.9 times, sedatives 1.7 times, amphetamines 1.24 times and other substances 2.8 times.
Researchers suggest that the increased risk was found to be significant even 10 to 15 years after a diagnosis of substance abuse.
The result illustrates robust associations between almost any type of substance abuse and an increased risk of developing schizophrenia later in life.
Researchers also mention that as the study is a cohort study and hence it is not possible to determine whether alcohol or substance abuse actually causes the increased risk of schizophrenia.
It is possible that people who are likely to develop schizophrenia are then more likely to abuse substances, and/or that people could be susceptible to both substance abuse and developing schizophrenia.
In a second study by the same group, the researchers studied the potential role of parental substance abuse in the development of schizophrenia.
The abuse was split into two categories depending on whether it was first diagnosed before or after the birth of the offspring. Cases of schizophrenia were identified in Denmark’s Psychiatric Central Research Register.
The researchers found that maternal marijuana abuse was associated with a 6-times increased risk of schizophrenia in the child, if diagnosed before the birth of the child, with a similar increased risk if diagnosed after.
For paternal marijuana abuse, there was a 5.5 times increased risk of schizophrenia in the child whether diagnosed before or after the birth.
Maternal alcohol abuse diagnosed before the birth of the child was associated with a 5.6 times increased risk of schizophrenia in the child, but this fell to twice the risk if the diagnosis of alcohol abuse was after the birth, with a similar relationship for fathers (4.4 times increased risk with a pre-birth diagnosis versus 1.8 times for a diagnosis after the birth).
The finding suggests that second-hand exposure to marijuana is apparently linked to schizophrenia.
News source: International Early Psychosis Association.
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