YOUNG people at risk of schizophrenia can speed up the condition's onset by one year for every illicit drug they take, new Australian research has revealed.
The study found heavy users of cannabis, cocaine and amphetamines experienced their first symptoms at a younger age than those who abstained or took only one drug.
Cannabis is known to increase the risk of developing psychotic illnesses earlier, but this is the first evidence that taking additional drugs further accelerates the process.
The research, involving 167 men, showed the mean age of onset of schizophrenia for non-drug users was 23.3 compared with 22.5 for those who had smoked cannabis.
Men who had used cannabis and amphetamines developed the illness at a mean age of 20.8, while the onset for those who used cannabis, amphetamines and cocaine was 19.6.
Published in the latest edition of the Australian and New Zealand Journal of Psychiatry, the findings - involving West Australian psychiatric patients - have prompted calls for public health campaigns to warn young people against multiple drug use.
''If you can delay the age of onset of the illness in a person who is predisposed to schizophrenia, you open potential ways by which you can intervene early, and what our study shows is that these major drugs do indeed bring forward the age of onset compared with young people who never took drugs before they got ill,'' said the study's co-author, Professor Nikos Stefanis, from the University of Western Australia.
''Research shows that the longer a person is psychosis-free, the easier their condition is to treat when they start having symptoms.
''This study implies there is one or two years of potential intervention for young people who consume drugs. Public health messages should be focusing on vulnerable young people who are susceptible to psychosis and are taking drugs and we need to be saying, hey, this is dangerous stuff, it really brings forward the illness.''
Professor Jon Currie, head of addiction medicine at Melbourne's St Vincent's Hospital, said early use of drugs was a risk for all young people, not just those predisposed to schizophrenia, because the brain was still developing until the early 20s.
''These drugs can derail that normal development, which also means derailing the normal protective mechanisms, which stop you getting these mental health problems. The drugs can affect the brain's neurochemical systems but, more importantly, they may actually alter the plasticity of the brain and the wiring and connections and have physical effects, which can be long-standing,'' Professor Currie said.
While he welcomed the need for education campaigns warning of the risks of drug use, Professor Currie argued they should be ''reality-based'' to avoid turning young people off.
''We need to point out that it's not that you will definitely get a problem if you use drugs, because most people don't, but the message is that you are increasing your risk. It's a risk message and it needs to be a brain message. The message we give is simple, it's your brain, you need it, later in life you'll really need it, don't damage it now.''