By Michelle Roberts
Health reporter, BBC News
The drugs appear to promote positive thinking
Antidepressants get to work immediately to lift mood, contrary to current belief, UK researchers say.
Although patients may not notice the effects until months into the therapy, the team say they work subconsciously.
The action is rapid, beginning within hours of taking the drugs, and changes negative thoughts, according to the Oxford University researchers.
These subtle, positive cues may add up over time to lift the depression, the American Journal of Psychiatry reports.
It may also explain why talking therapies designed to break negative thought cycles can also help.
Psychiatrist Dr Catherine Harmer and her team at Oxford University closely studied the reactions of 33 depressed patients and 31 healthy controls given either an antidepressant or a dummy drug.
The depressed patients who took the active drug showed positive improvements in three specific measures within three hours of taking them.
These patients were more likely to think about themselves in a positive light, rather than dwelling on their bad points, the researchers said.
They were also more likely to see the positive in others.
For example, if they saw a grumpy person they no longer internalised this to think that they must have done something wrong to upset the person.
This was despite feeling no improvement in mood or anxiety.
Dr Harmer said: "We found the antidepressants target the negative thoughts before the patient is aware of any change in feeling subjectively.
"Over time, this will affect our mood and how we feel because we are receiving more positive information."
She said the findings could help scientists looking for new drugs to treat depression.
Dr Michael Thase, a psychiatrist from the University of Pennsylvania, said the findings challenged conventional wisdoms and were potentially "paradigm-changing".
But he said much more research was needed.
"The highest research priority is to confirm that the rapid effects observed in this study are predictive of eventual clinical benefit."
He said it was possible that switching off the negative thoughts was a crucial part of the therapy.
Alternatively, it might merely be a sign that the drug was beginning to work at the cell level in the brain.
Paul Farmer, chief executive of Mind, said: "This research may contribute to our understanding of how our bodies respond to antidepressants, but the changes recorded can't always be felt by patients and it can be some weeks before they begin to feel the symptoms of depression easing.
"We must also remember that the side-effects of medication can often be felt straight away long before the benefits really kick in, and this will always affect people's experiences in the initial stages of treatment."