Most of us know the risks we take by using illicit substances. Even when we consume alcohol, the health risks are numerous. We can do irreversible harm to our own bodies just by ingesting a significant amount of alcohol. We may even do harm to others if we get behind the wheel, and a situation can quickly become disastrous. It’s a significant problem: a person is injured in a drunk driving crash every two minutes.

But alcohol also has a big impact on the mind — perhaps more than we ever realized. A new study has revealed that the use of alcohol, marijuana, and other drugs may increase the likelihood of developing schizophrenia later in life.

Though other research has looked at the potential connection between schizophrenia and substance abuse, the findings have been up for debate. This is largely due to the fact that many previous studies have not taken multiple-compound abuse into account. It’s a challenging area to study; definitive answers are hard to come by, and findings are often contradictory and controversial.

This new Danish study analyzed national registers in order to establish a database of over three million individuals. Among these, the study identified 204,505 cases of substance abuse and 21,305 diagnoses of schizophrenia. Researchers then analyzed the data using statistical measures. Unlike other studies in the past, this one accounted for factors like co-abuse, other psychiatric diagnoses and psychiatric history, socioeconomic status, and statistics from parents, in addition to other control factors like gender and age.

Researchers found that substance abuse of any kind definitively increased the risk of schizophrenia development. Those who abused alcohol had more than three times the risk of developing the condition. Hallucinogenic drugs, sedatives, and amphetamines all had between one and two times the normal risk. Surprisingly, marijuana had the highest risk factor, resulting in individuals having more than five times the normal risk for schizophrenia development.

Although the study’s authors concluded that there is a definitive connection between substance abuse and schizophrenia development, it is a bit of a “chicken or the egg” conundrum. It’s nearly impossible to scientifically prove whether the abuse led to schizophrenia, or whether a predisposition to the condition caused individuals to abuse drugs and alcohol. It’s also possible that individuals with other shared traits or risk factors are at a higher risk of using drugs and developing a psychological condition later in life. Though the study proves correlation, it does not prove causation.