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.
READ MORE @ BBC NEWS