Professor Murray Cairns, who will lead the trial, received a $2.9 million grant for the project through the National Health and Medical Research Council.

"It's using genetics and precision medicine to identify people likely to respond to better treatments," he said.

"We urgently need new treatments for schizophrenia."

The trial followed Professor Cairns overseeing the largest genetic study of vitamin A in the world, with more than 22,000 participants.

The study, published in Nature Communications last month, screened for "the effect of the vitamin on nearly 20,000 different disorders or traits, schizophrenia among them".

Vitamin A is a fat-soluble nutrient that is abundant in meat, as well as green and orange plants.

The plant form starts out as beta-carotene and the liver turns it into vitamin A, also known as retinol.

There were two genes known to be associated with the vitamin from previous studies.

"We found another six," he said.

He said the research could help "better understand vitamin A's role in a range of complex health conditions".

The study, co-led by postdoctoral fellow Dr William Reay, was an international collaboration coordinated through Hunter Medical Research Institute and University of Newcastle.

It examined thousands of individual genomes to determine what genetic factors regulate vitamin A levels in the blood.

Vitamin A is, in a sense, a "Goldilocks vitamin".

"Like salt and sugar, we need enough of this vitamin. But if we have too much or too little, it can cause all sorts of problems," Professor Cairns said.

"We do know it is very important and potent. In pregnant women it's very dangerous if you have too much because it can damage the foetus."

Vitamin A is particularly important for the brain, vision, skin and immune system.

Professor Cairns said neuron connections in the brain were "thought to be altered in people with schizophrenia and other psychiatric conditions".

"If someone has a tendency for low levels of vitamin A, that may put them at risk for different conditions," he said.

"Vitamin A is even used therapeutically in some forms of cancer."

People have different levels of vitamin A. But due to the way genes regulate the vitamin, diet alone may not be enough to boost levels.

"If we know people are likely to have low levels because of their genetics, we can try to compensate by giving more of the vitamin or synthetic drugs."

Professor Cairns and his team developed a score based on genetic variation in multiple genes.

"This enables us to specifically measure an individual's likelihood of benefiting from a synthetic drug - pharmacological grade vitamin A," he said.

His interest in vitamin A emerged from his research into its potential links to schizophrenia risk.

There was some evidence that poor diet and low levels of vitamin A had an association with schizophrenia.