In the Dark Ages, doctors would tell depressed patients "it's all in your head."
That was before depression was recognized as a medical disorder. Nowadays, antidepressants are the most common prescription drugs for adult Americans under 45.
But, as it turns out, a patient's beliefs may matter more than the treatment itself, Reuters reports.
Researchers revisited a 2002 clinical trial that compared the antidepressant Zoloft, the herbal depression treatment St. John's wort and a placebo.
At the time, scientists concluded that neither treatment was more effective than the placebo at improving symptoms in 340 people with moderate depression.
But the current study asked a different question: Were patients who assumed they were getting Zoloft or St. John's wort more likely to improve than those who thought they were on a placebo?
The answer was yes.
In fact, more than 50 per cent of patients who thought they were on Zoloft showed significant improvement after eight weeks, compared with 24 per cent of patients who thought they were on a placebo.
The results for St. John's wort were even more impressive: 68 per cent of patients who thought they were using the herb became less depressed. And of those who guessed correctly that they were taking St. John's wort, 80 per cent responded to treatment.
Nevertheless, the findings don't prove that recovering from depression is a question of mind over matter, researchers said.
"I do not think the placebo effect is the whole story," said lead researcher Justin Chen, a resident in psychiatry at Massachusetts General Hospital.
It's possible that patients who felt better at the eight-week mark were more likely to guess correctly that they were on a real treatment, Dr. Chen said.
He added that antidepressants may be more effective for people with severe depression.
A 2010 review of six clinical trials, published in the Journal of the American Medical Association, found that antidepressants showed a "substantial" benefit over placebos for people with severe depression. But the advantage was minimal to "non-existent" for patients with mild to moderate depression.
Antidepressants are the most common prescription drugs taken by Americans aged 18-44 years, according to a 2011 report by the U.S. Centers for Disease Control and Prevention.
"There is a lot of debate right now about antidepressant efficacy," Dr. Chen said.
Would you take medication for mild to moderate depression? What do you make of the mind's role in depression treatment?