Scientists have discovered that antidepressants work subconsciously as soon as they are taken to help improve the patient's mood.
Researchers from Oxford University found such drugs began working immediately and helped to change negative thoughts just hours after they were taken.
The study challenges traditional beliefs in the UK, but recognised the effects may not be noticed by patients until months later.
The American Journal of Psychiatry said small, positive changes build up over time to ease the depression and may explain why sufferers can be helped by talking therapies. Such therapies help break negative thought cycles.
The study compared the reactions of patients who were given either an antidepressant or a dummy drug. A total of 33 depressed patients took part along with 31 healthy controls.
Dr Catherine Harmer and her team found the depressed patients who were given the active drug were increasingly likely to view themselves in a positive light.
They showed positive improvements within three hours of taking the drug in three specific measures. And they were less likely to think about their bad points and those of others.
Dr Harmer said: "We found the antidepressants target the negative thoughts before the patient is aware of any change in feeling subjectively."
This improvement was despite experiencing no change in anxiety or mood.
"I totally agree with the above article. Several years ago I was depressed and was prescribed an antidepressant. When I told my doctor that I felt a positive effect from the tablets almost instantly, she told me that this was not possible and wrote "query abuse" in my notes!" - Fiona Mcintosh, Scotland