Antidepressants Have Immediate Effect: Study

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Antidepressants begin to work on improving mood within hours after a patient starts taking them, according to a U.K. study that challenges current thinking.

The Oxford University study of 33 depressed patients found that those who took an antidepressant showed positive improvements within three hours of taking the drugs. For example, they were more likely to have positive thoughts about themselves and others, BBC News reported.

But patients may not notice any obvious effects until they've been taking antidepressants for a few months, said lead researcher Dr. Catherine Harmer and colleagues.

"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," Harmer told the BBC News.

The study appears in the American Journal of Psychiatry.