The capsules are rich in omega-3
Taking a daily fish oil capsule can stave off mental illness in those at highest risk, trial findings suggest.
A three-month course of the supplement appeared to be as effective as drugs, cutting the rate of psychotic illness like schizophrenia by a quarter.
The researchers believe it is the omega-3 in fish oil - already hailed for promoting healthy hearts - that has beneficial effects in the brain.
A "natural" remedy would be welcomed, Archives of General Psychiatry says.
"The finding that treatment with a natural substance may prevent, or at least delay, the onset of psychotic disorder gives hope that there may be alternatives to antipsychotic drugs," the study authors said.
Antipsychotic drugs are potent and can have serious side effects, which puts some people off taking them.
Fish oil supplements, on the other hand, are generally well tolerated and easy to take, say the scientists.
The international team from Austria, Australia and Switzerland tested the treatment in 81 people deemed to be at particularly high risk of developing psychosis.
Their high risk was down to a strong family history of schizophrenia, or similar disorders, or them already showing mild symptoms of these conditions themselves.
For the test, half of the individuals took fish oil supplements (1.2 grams of omega-3 fatty acids) for 12 weeks, while the other half took only a dummy pill. Neither group knew which treatment they were receiving.
Dr Paul Amminger and his team followed the groups for a year to see how many, if any, went on to develop illness.
Two in the fish oil group developed a psychotic disorder compared to 11 in the placebo group.
Based on the results, the investigators estimate that one high-risk adult could be protected from developing psychosis for every four treated over a year.
They believe the omega-3 fatty acids found in the supplements may alter brain signalling in the brain with beneficial effects.
Alison Cobb, of the mental health charity Mind, said: "If young people can be treated successfully with fish oils, this is hugely preferable to treating them with antipsychotics, which come with a range of problems from weight gain to sexual dysfunction, whereas omega-3s are actually beneficial to their general state of health.
"These are promising results and more research is needed to show if omega-3s could be an alternative to antipsychotics in the long term."