London - Millions are taking antidepressants which do little to lift their mood, a major study has revealed.
Sertraline – the most common antidepressant - was found to have few benefits when tackling symptoms such as suicidal feelings, trouble concentrating and poor sleep.
However, a clinical trial said the drug does help relieve feelings of anxiety, including worry and restlessness – which in turn improves overall mental health.
The authors said they were shocked by the "unexpected" findings, but added: "There is still a benefit. Antidepressants work, but in a different way than we thought."
The trial by University College London, published in The Lancet, is the largest of its kind to examine the effectiveness of antidepressants. It found no evidence of a reduction in patients’ symptoms of depression after six weeks on sertraline, and only a "modest" improvement after 12.
It belongs to a group of drugs called selective serotonin reuptake inhibitors (SSRIs).
More than half of antidepressant prescriptions are for SSRIs, which are thought to work by increasing serotonin levels in the brain. The study authors said their findings were applicable to all forms of SSRIs, taken by approximately four million people in England.
They were split into two groups – the first was given a placebo drug, while the second was given sertraline. After six weeks treatment on sertraline, there was no evidence of a reduction in depression. After 12 weeks, there was a 13 percent reduction in the symptoms – a finding the experts described as "weak".
But the drug did offer benefits on reducing anxiety, with a 21 percent reduction in symptoms at six weeks and 23 percent at 12 weeks.
Patients who took the drug for 12 weeks were twice as likely to say their mental health had improved compared to those taking the placebo. Professor Glyn Lewis, from UCL, said the team were "shocked and surprised" by the results but stressed that "no one is considering throwing antidepressants away".
He added: "The clinical implications are that antidepressants work but in a different way to how we thought. They work on the anxiety symptoms first before any smaller and later possible effects on depression."