A new study has found that high caffeine use, combined with stress, can cause people to exhibit psychotic symptoms such as hallucinations and delusions.

The research suggests that around five coffees, or the equivalent of 200mg of caffeine, may be enough to tip people over the edge and cause psychotic-like symptoms.

A team at Melbourne's La Trobe University were researching mechanisms between the onset of schizophrenia and stressful life situations.

They were trying to discover what caused stressed individuals who did not have a diagnosis of schizophrenia to show symptoms of the disease.

Lead researcher Simon Crowe says a sample of 92 undergraduate students were played White Christmas by Bing Crosby before being played static white noise for several minutes.

The students were then asked to press a buzzer to indicate when, if at all, they could hear White Christmas playing in the background of the white noise.

Lead researcher Simon Crowe says, however, White Christmas was never played during the white noise.

He says people who were both highly stressed and had a high intake of caffeine were three times more likely to report hearing the song.

"What we found was that people who are highly stressed and who are prone to high-level caffeine use are more likely to report these sorts of phenomena - hearing things that aren't necessarily there as a result of the interaction of those two effects," Professor Crowe said.

He says people who have a genetic predisposition to schizophrenia do not always show the disease, but it can be triggered by a number of things, including highly stressful situations.

"What caffeine does is increase the responsiveness of the system, which is made yet more powerful by [the presence of stress]," he said.

"It creates a situation where people tip further into the spectrum of 'highly stressed', and as a consequence they report these types of clinical phenomena."

Caffeine is the most commonly used psychoactive drug in the world and acts in the same way as other stimulant medications such as amphetamines.

Professor Crowe says as a result of our pressured lifestyles, Australia has become a stimulant-reliant society.

He says the situation needs to be looked at in greater detail.

"Caffeine is a drug that you can buy off the shelves, so it is an unregulated substance," he said.

"We are increasingly focused on how much things like alcohol and tobacco are available freely in the community, and we here we have a stimulant - perhaps not as powerful as cocaine or amphetamine which you can actually buy off the shelf at the supermarket."