The "nocebo" effect: How the way you think can actually make you sick.

In clinical trials for a new drug, scientists usually split the patients into two groups: the group receiving the real medicine, and the group getting a fake, called a “placebo”. The general idea is to test whether the medicine works by comparing patients who’ve taken it with patients who haven’t. But in the past few decades, scientists have noticed something interesting: the patients taking the placebos often show marked improvements in their symptoms, even though there’s no medical reason why the fake medicine should have helped them.

That’s what we call the “placebo effect” – the process whereby a person’s belief that they’re taking medicine that might cure them helps the body to heal itself.

But what if, as well as having the power to heal, those sugar pills also had the power to make people sick?

In the past few years, scientists have begun to notice a concerning phenomenon. It’s called the “nocebo effect”, and it works in like the placebo effect, but in the opposite direction: when people believe that they’re taking medicine that might make them sick, their bodies can begin to break down.

In a clinical trial where nobody knows if they’re getting the real drug or not, everyone has to be informed of the potential side effects – but for some people, the mere suggestion of certain side effects can be enough to bring them on in patients. A 2013 study into the nocebo effect, published in journal Nature, documented cases of placebo patients dropping out of clinical trials because the “side effects” of their sugar pills were so debilitating. To be clear – patients were experiencing horrible symptoms, just because they believed they should be.

It's not the first time the nocebo effect has been documented, either.

Sometimes called the "voodoo effect", the concept that the body might be suggested into illness or even death has been around for centuries.

Reports of people dying after being cursed or hexed by a witch were once widespread, with the most popular story revolving around a man who grew incredibly ill after being "cursed" by a witch in the town square. A doctor, convinced that the patient's ailment was all in his mind, told the patients that the witch had planted a lizard inside him to eat him from the inside. He forced the patient to vomit copiously, dropping a plastic lizard in the vomit when the patient's back was turned. The curse, he declared, was lifted - and the patient went on to make a full recovery.

Then there's the case of Sam Shoeman, who was diagnosed with terminal liver cancer in the 1970s and died - only for doctors to discover, upon autopsy, that his cancer wasn't terminal at all, and hadn't spread since the original diagnosis. The problem, doctors at the time observed, was not that Sam was actually going to die, but that he believed so sincerely that he would.

In another story, a patient in a hospital who attempted suicide with a bottle of placebo pills almost died, his body going into shock as if he had taken a real overdose. He didn't begin to recover until doctors revealed to him that he hadn't taken real medicine.

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The nocebo effect can be individual, as in the cases above, or communal, spread through social and cultural groups by fear.

Take, for example, "electrosensitivity", the name given to the idea that electronic devices can make people sick. Although the "disease" has never been recognised by medical professionals, sufferers reporting chronic symptoms like headaches, sleep disturbances, numbness and decreased resistance to infections. (This "condition" is depicted in the TV show Better Call Saul.)

The consensus among scientists is that "electrosensitivity" is an example of the nocebo effect - a communicated disease that "infects" suggestible people who hear about it.

In the medical profession, understanding of the nocebo effect is growing. Doctors are beginning to realise that the way that they describe side effects to certain drugs can have a profound effect on a patient's health. Even the spread of certain intolerances, like allergies to MSG and gluten, has been attributed to the nocebo effect. If people believe that something will make them sick, it often will, even producing dramatic physical symptoms like vomiting and diarrhoea.

The lesson? Your thoughts have power over your health - even if you don't realise it.