Taking antidepressants for longer than the usual six months could significantly reduce the risk of people becoming ill with depression again, researchers suggest.
Many patients remain at risk
They say extending the period people took medication for by at least a year would be beneficial.
Researchers from the University of Oxford looked at 31 studies previously carried out across the world, covering around 4,400 people.
They had all already been treated for acute depression, and were either given antidepressants or a dummy pill.
Most studies were carried out amongst patients being treated by hospital consultants, rather than by GP, meaning they were likely to have experienced recurring episodes of depression.
The University of Oxford researchers found the studies concluded that taking antidepressants for one or two years "substantially reduced" the risk of a relapse into depressive illness.
What works for one person may not work for the next
People taking antidepressants had an 18% risk of relapse, compared to a 41% risk for those taking the dummy pills.
The difference between the groups remained even when other factors such as the underlying risk of relapse, or how long the patients had been treated for prior to the study was taken into account.
Antidepressants cost the NHS £395m in 1999, over twice what the drugs cost the health service in 1995.
Professor Guy Goodwin from the Department of Psychiatry at the University of Oxford told BBC News Online: "The recommendations for use of antidepressants are relatively short-term.
"And there is also a belief that they stop working.
"This research is saying this is not necessarily the case.
"If people have responded to the treatment and they are at risk of further episodes, going on taking them looks to be a useful thing to do."
He added: "Few other interventions in psychiatry are supported by such robust findings.
"We know that many patients remain at appreciable risk of recurrence after four to six months of treatment with antidepressants, and another one or two years of continuation treatment will approximately halve their risk of another episode.
"In other words, the positive benefits of antidepressants do not wear off over time.
Rowena Daw, head of policy development for the mental health charity Mind, said: "Mind welcomes any research which will help people avoid experiencing a recurrence of their symptoms."
But she added: "We believe very strongly that a 'one size fits all' approach should not be employed when dealing with mental health treatments.
"People's experience of mental health is as individual as they are themselves, and issues around side-effects, preferences and choices must be taken into account.
"Antidepressants vary widely in their effects; what works for one person may not work for the next, and pills are not always the answer.
"We want to see doctors involving patients in their treatment decisions much more closely to ensure that treatment is effective."
Guidance from the Royal College of Psychiatrists states: "Antidepressants don't necessarily treat the cause of depression or take it away completely.
"Without any treatment, most depressions will get better after about eight months.
"If you stop the medication before eight or nine months is up, the symptoms of depression are more likely to come back.
"The current recommendation is for most people to continue taking antidepressants for six months after they start to feel better."
The guidance says some people with severe depression need to take antidepressants for several years to stop their depression coming back.
The research is published in The Lancet.