Psychiatrists have been aware for years that low serotonin levels may not cause depression, despite continuing to prescribe the pills, a chair of psychology has said.
Dr Jonathan Raskin, from State University of New York, told DailyMail.com he'd been concerned that the theory that depression was caused by low serotonin levels was 'incomplete' for 'a while'.
But he said many medics continued to prescribe the medication, even while they were unsure if they were effective, because it was 'easier' than offering more time-intensive care.
The pills could still help some patients, he added, but they are not a 'cure-all' for those suffering from depression.
This week a landmark UK study called into question society's ever-growing reliance on antidepressants like Prozac.
The $15billion-a-year industry — set to grow to $21billion in the next decade — sees patients prescribed pills like Prozac on the promise they will cure people's depression by raising levels of serotonin, a neurotransmitter in the brain.
But after reviewing 17 major studies scientists at University College London said they found no convincing evidence that a lack of this chemical sparked the condition.
About 13 percent of American adults take antidepressants every year, figures show.
But rates are much higher among women, with up to 18 percent prescribed the medications annually.
Asked whether low serotonin levels caused depression, Raskin — who is also a psychotherapist — said: 'I think most mental health professionals familiar with the research have known for a while that the serotonin theory of depression is incomplete and has mixed research support.
'Depression is a complicated issue, and the idea we would be able to reduce it simply to serotonin is not right.
'When we give antidepressants, we don't do this based on biological tests showing they don't have enough serotonin — but if we think it could help them.'
Asked whether people should keep taking the pills, he said: 'I think that this is a conversation worth having."
One academic involved in the UK study described the findings as 'eye-opening', and that 'everything I thought I knew has been flipped upside down'.
Lead author Professor Joanna Moncrieff, a psychiatrist, said: 'The popularity of the "chemical imbalance" theory has coincided with a huge increase in the use of antidepressants.
'Thousands suffer from side effects of antidepressants, including severe withdrawal effects that can occur when people try to stop them, yet prescription rates continue to rise.