For his last birthday, I gave my husband a monthly beer box subscription. While he saw it as a generous and delicious present, it spawned a mischievous idea on my part. One evening, as I watched him drain the last bottle, I opened my email. “We’ve just had a message from the beer people,” I said. “They’re issuing a recall on the last batch.”

“What’s the problem?” he answered. “Some sort of contamination issue,” I replied. My husband’s face fell. “Are you OK? You look a bit peaky,” I said.

“Actually, I feel a bit sick,” he said.

There was, of course, no email, and I am a terrible wife. For the past few years, I’ve been writing a book, This Book May Cause Side Effects, about how our thoughts influence ill health. You may have heard of the placebo effect, when positive expectations lead to positive health outcomes. But my interest is in its evil twin. The nocebo effect occurs when dismal expectations lead to negative health outcomes. The phenomenon can create, exacerbate and prolong symptoms. When these symptoms coalesce, people become ill – not from disease, but from the intimate relationship that exists between mind and body.

The beer box ruse was a crude experiment. I wanted to see how easy it is to conjure the nocebo effect – and the answer is “very”. Sometimes, all it takes to make someone feel genuinely unwell is a few carefully chosen words.

You don’t just have to take my word for it. There is a plethora of peer-reviewed studies confirming this idea. In one, patients fresh from minor keyhole surgery received a harmless saline infusion that they were told would temporarily increase their pain. It did just that. In another, 40 asthmatic adults breathed in water vapour from an inhaler they were told contained an irritant. Nineteen went on to feel wheezy. Twelve had a full-blown asthma attack.

These are artificial situations, but the nocebo effect is out there in the real world too. Whenever we have negative expectations about health, they can generate a self-fulfilling prophecy.

If you’ve felt lousy after having the Covid-19 vaccine, there’s a good chance your symptoms weren’t caused by the vaccine. Combining data from 12 separate clinical trials involving more than 45,000 participants, scientists found that large numbers of people who got placebo shots had adverse side-effects, leading them to conclude that the nocebo effect accounted for a whopping 76% of all common adverse reactions to the jab.

And if you’ve ever developed side-effects to a prescription medication, there’s a reasonable chance the phenomenon was responsible for at least part of your suffering. The nocebo effect can also be part of the reason why some people find it hard to tolerate gluten. Blinded to their diet and covertly fed the offending ingredient, some find they can eat regular bread without incident.

The nocebo effect affects us individually, but it can also occur at the level of populations when it spreads like a virus. The phenomenon may be the driving force behind countless seemingly inexplicable “mystery illnesses” – from the dancing plagues of the middle ages to the more recent phenomenon of Havana syndrome, where American diplomats have developed intense symptoms after believing they were struck down by some sort of unidentified covert weapon. During the pandemic, the nocebo effect was responsible for an outbreak of tics that was propagated when young people saw videos of them on TikTok. It became known, fittingly, as the TikTok tics. Now, researchers believe we live in era in which, increasingly, social media are turbocharging the spread of nocebo-generated symptoms.

I believe that the nocebo effect is also responsible for a significant proportion of “medically unexplained symptoms” – sensations such as pain, fatigue and dizziness that cause suffering yet have no discernible organic cause. In the absence of a “proper diagnosis”, people with these symptoms are often accused of being “hypochondriacs”. This is an outdated term that has rightly been dropped by the medical profession because it implies that suffering is feigned or exaggerated. Similarly, in writing my book, I’ve received pushback from those who believe that nocebo-generated symptoms either don’t exist or are somehow less valid than “real” symptoms.

This view is categorically wrong. And if the lived experiences of those who suffer from them aren’t enough, I point towards the detailed body of literature showing that thoughts and neural activity can and do precipitate physical change.

The work of Harvard’s Ellen Langer has shown, for example, that when people with diabetes are made to sit in front of a clock that runs at double, regular or half speed, their blood glucose levels rise and fall with the perceived passing of time, rather than the actual passing of time. Alia Crum at Stanford has shown that when people drink identical milkshakes labelled as “high-calorie” or “diet”, levels of the “hunger hormone” ghrelin drop three times faster after consuming the drink they believed would make them fuller faster.

Animal studies go further still, mapping out the chain of events that link activity in the brain to sometimes dramatic effects in the body. Asya Rolls from the Technion-Israel Institute of Technology and colleagues have shown that activating specific brain areas in mice triggers changes to the immune system, which can then speed the recovery from heart attacks or slow the growth of cancer. Writing in Nature Communications, they say “these findings introduce a physiological mechanism whereby the patient’s psychological state can impact anti-tumour immunity and cancer progression”. They’re not saying that negative thinking can worsen cancer, or that positive thinking can cure it – but they are saying there’s a link between neural activity and disease that deserves further exploration.

Four hundred years ago, the French philosopher René Descartes proposed that mind and body are separate, non-interacting entities. It spawned the dogma of Cartesian dualism and our modern medical model that still defaults to the premise that physical symptoms must have physical roots. While this may be true sometimes, it’s not true all the time. The work of Rolls and others hints at deeper layers of complexity.

I believe that if we truly want to be well, it’s important first to understand the many ways that we become ill. The nocebo effect – underestimated and overlooked – is a key part of this puzzle.

• Helen Pilcher is a science writer and the author of This Book May Cause Side Effects