Over 4% of people have experienced auditory and/or visual hallucinations, which is far more than had previously been thought, according to research by a psychiatrist at the Royal College of Surgeons in Ireland.

The research found that contrary to general belief, hallucinations occur less among those suffering from schizophrenia than among those who do not.

In fact, the study found hallucinations more commonly occur across a range of mental illnesses and even among people who do not suffer from mental illness at all.

The research was carried out by Dr Ian Kelleher, RCSI Research Lecturer in Neurological and Psychiatric Disorders, and is published in the British Journal of Psychiatry.

It was based on data from the Adult Psychiatric Morbidity Survey in England.

The study found 17% of those who have obsessive compulsive disorder had experienced hallucinations, as had 14% of those with depression.

One of four of those with agoraphobia have also had hallucinations.

"People who experience these symptoms will often research them online and, based on what they read, worry that they may be developing schizophrenia," said Professor Kelleher.

"It's important for people to realise that hallucinations are common in a whole range of illnesses, not just schizophrenia and at times even happen in people with no illness at all."

"So it's important not to jump to conclusions based on online self-diagnosis."

Instead, Dr Kelleher recommends anyone experiencing hallucinations talk to their doctor.