Have we been treating depression the wrong way for decades?

This is an excerpt from Second Opinion, a weekly analysis of health and medical science news. If you haven't subscribed yet, you can do that by clicking here.

A new analysis of the cause of depression has seemingly upended what we know about this common condition and challenged the use of antidepressants. But it may also leave patients with more questions than answers as the science evolves.

A systematic umbrella review of 17 studies published in Molecular Psychology on July 20 looked at the decades-old theory that depression is caused by low serotonin, and found there was "no consistent evidence" of "an association between serotonin and depression."

The theory that depression is caused by a chemical imbalance in the brain has been around since the 1960s. But for years, many experts have doubted this, feeling it oversimplified a complex condition.

"The serotonin theory is very old and has been very popular since the '90s, when the pharmaceutical industry started promoting it," said Dr. Joanna Moncrieff, a psychiatry professor at University College London and lead author of the study.

"But since about 2005, probably a bit before then, there's been sort of rumours that actually the evidence isn't very strong, or it's inconsistent. Some studies are positive, some studies are negative, but no one's really got that evidence together anywhere."

Moncrieff and her team set out to challenge the serotonin theory in a systematic review of available research. They also went a step further in their conclusion by suggesting that antidepressants are ineffective at treating depression — and have largely worked as a placebo.

"Evidence from placebo-controlled trials show that antidepressants are a little bit better than a sugar tablet," she said. "And if that little difference is not to do with rectifying a chemical imbalance, improving low serotonin levels, what is it to do with?"

The research paints a compelling picture that depression isn't caused by low serotonin alone. Many experts say this is already widely accepted and that it's also true that antidepressants can be extremely beneficial to some patients — even if we don't know exactly why.

So where does this leave patients and physicians, and could the analysis impact the way we treat depression in the future?

Antidepressants are widely believed to affect the behaviour of neurotransmitters, chemical messengers in the brain like serotonin and dopamine, in a way that can alter emotions and mood to help improve the symptoms of depression in some patients.

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People with depression can face a wide range of symptoms, including persistent feelings of sadness and desperation, changes in appetite, sleep deprivation, fatigue, irritability and loss of interest in hobbies and social connections that can impact everyday life.

While it's unclear exactly how antidepressants work at a biological level to alleviate those symptoms, it's clear that they can still be hugely helpful to some patients.