A commonly-prescribed antidepressant reduces anxiety first and has a smaller effect on depressive symptoms weeks later, a study suggests.
Researchers at University College London said it made people feel better but worked in unexpected ways.
Their trial involved 653 UK patients, half of whom were given sertraline and the other half a placebo (dummy pill).
Psychiatrists say the findings are reassuring for doctors and patients, confirming the benefits of treatment.
Antidepressants are one of the most commonly-prescribed medications in the UK and concerns have often been raised that too many are being given to patients.
Sertraline, a selective serotonin reuptake inhibitor (SSRI), is one of the most common drugs used to treat symptoms of depression and anxiety.
But scientists still have little idea of how these kind of drugs work.
In this study, published in The Lancet Psychiatry journal, patients with mild to severe depressive symptoms, or anxiety, or a mix of both, were selected from 179 GP surgeries in the UK and enrolled on the trial.
In all cases, GPs were not sure whether to prescribe an antidepressant.
After six weeks, the patients taking sertraline reported a 21% greater improvement in anxiety symptoms – such as feeling worried, nervous and irritable – compared to the control group taking a dummy pill. After 12 weeks, the gap was 23%.
But there was little evidence of the drug reducing depressive symptoms, such as poor concentration, low mood and lack of enjoyment after six weeks – and only marginal improvements (13%) after 12 weeks.
Nonetheless, the group taking antidepressants were twice as likely as the other trial participants to say their mental health felt better overall.