Some kinds of mental illness are preventable. That came as a shock to me when I first realised  that fact.

Mental illness is really a physical illness which manifests itself as mental illness.It is a disorder of some of the 300 or so neurotransmitters in the brain. In other words it is  a manifestation of a chemical imbalance in the brain. Some people describe this as oxidative stress.

Mental illness is to a great extent a genetic condition but not all those with a genetic loading will get it. Let us look at identical twins and the possibility to schizophrenia. Suppose twin A becomes schizophrenic at 18.  What are the chances of twin B developing the condition? 100% ?. No, it is about 50%,although a range of 40-60% is often quoted.Someting in the environment may trigger it off. One of the significant causes of the onset of schizophrenia  in young women in America for example is the breakdown of a romantic relationship. Those who smoke cannabis increase their risk of becoming schizophrenics by about 50%. The normal chance of getting it is in the region of !% but if somebody smokes cannabis it is about 1.5% or 1 in 66. One of my patients had her first psychotic episode 2 hours after she smoked her first and last joint. Other drugs may induce schizophrenia too. 

In the February 2010 issue of Archives of General Psychiatry  there is an article  "Long-chain omega-3 fatty acids indicated for Prevention of Psychotic Disorders". You can read a summary of it on their website. The conclusion is that long-chain omega-3 fatty acids  reduce the risk of progression to psychotic disorder and may offer a safe and efficacious strategy for  indicated prevention in young people with subthreshold psychotic states"