We all know about the mysterious powers of placebos: pills that can provide health benefits despite the fact that they contain no active medication.

It's long been thought that the reason that they work is because patients think they're getting real medicine. So it would be a little weird if placebos worked even when patients knew exactly what it is they were swallowing, right?

Well, that's exactly what a new study has found.

A paper published in the journal Pain shows that patients who knowingly took placebos along with their traditional treatment for lower back pain saw more improvement than those who only received the traditional treatment.

Ted Kaptchuk, an expert in placebos from Harvard Medical School who co-authored the study, says "these findings turn our understanding of the placebo effect on its head".

It's not like the patients just didn't understand what placebos were. The 97 participants were given 15-minute explanations about them.

After three weeks, the group which took placebos reported a 30 per cent reduction in both usual pain and maximum pain.

The "treatment as usual" group, by contrast, reported reductions of 9 per cent and 16 per cent.

The group that took placebos also had a 29 per cent decrease in pain-related disability — the other group saw barely any improvement by that measure.

So why did the placebos work?

Professor Kaptchuk puts it down to the ritual of care:

"It's the benefit of being immersed in treatment: interacting with a physician or nurse, taking pills, all the rituals and symbols of our healthcare system. The body responds to that," he said.

He points out that there are limits to what placebos can do, saying that "you're never going to shrink a tumour or unclog an artery with placebo intervention".

But he says "open label" placebo treatments could help with other kinds of pain, fatigue and depression, as well as common digestive or urinary symptoms.

"It's not a cure-all, but it makes people feel better, for sure. Our lab is saying you can't throw the placebo into the trash can. It has clinical meaning, it's statically significant, and it relieves patients," he said.

Lead author Dr Claudia Carvalho says the study shows that the benefits of placebos can be delivered without deception, thus avoiding that ethical dilemma.