In Roald Dahl’s 1980 masterpiece The Twits, Quentin Blake’s illustrations demonstrate how Mrs Twit’s horrible attitudes eventually ended up deforming her looks. “If a person has ugly thoughts,” wrote Dahl, “it begins to show on the face.”

In her latest book, science writer Helen Pilcher explores this very idea: that negative beliefs “can be physically transformative”. The nocebo effect, as this is known, comes from the Latin for “I will harm”, and strikes when a person’s negative expectations, whether subconscious or conscious, lead to illness.

This Book May Cause Side Effects is a bold attempt to examine the anatomy of this phenomenon. In its simplest form it can be described as follows: “when people are warned to expect symptoms, they become more likely to experience them”. Much like the impossible instruction not to think of a pink elephant, if you are told a drug might make you feel nauseous, it is a compelling psychological invitation to experience it.

In an analysis of 231 placebo-controlled clinical trials, Pilcher notes that 76% of people in the experimental groups reported side-effects, compared with 73% of those who were on a placebo. “Most of us experience funny sensations in the body at times,” she writes, “but the nocebo effect is behind becoming more aware of them, and misattributing them to a medication.”

Beyond drug side-effects, Pilcher’s book explores the nocebo effect as it applies to a range of human conditions including ageing, “hex deaths”, or the deaths of people who believed they had been cursed to die, and mass psychogenic illness.

History is rich with examples of mass psychogenic illness, or MPI, such as collective panic about shrinking genitalia in Asia, first recorded two millennia ago. It’s the nocebo effect at scale. While in the past the pace of symptom contagion was limited by geography, today’s lightning-fast global communication and the existence of social media platforms can make the nocebo effect go viral.

In 2014, social media is thought to have transmitted a mass psychogenic illness across Colombia. Children at a girls’ school began convulsing and fainting, having recently had the HPV vaccine, which protects against cancer of the cervix. Cases spread across the country and, although health officials found no link between the vaccine and the symptoms, public confidence in the vaccine was shattered. From HPV immunisation rates of over 90%, uptake dropped to 5%.

Whether people’s nocebo effect symptoms can ever be physically verified is largely irrelevant. Subjective experience such as pain or fatigue lies behind a veil that we are unable to penetrate. Yet Pilcher also presents an array of research that shows measurable physical changes resulting from the nocebo effect. In one striking example, she cites a study at Stanford in which participants were randomly told – regardless of their actual genetics – that they possessed a gene associated with either low risk or high risk of obesity. GLP-1, synthetic analogues of which include Ozempic, is a natural hormone released by the body that makes us feel full. After a meal, those who were told they had the “skinny” gene showed a significant increase in GLP-1, while those told they had the “fat” gene had no change from their baseline levels.

When interviewing a researcher who inserts electrodes into the brains of cancerous mice in an area associated with reward processing and positive emotion, Pilcher is gripped by the finding that stimulating this area curbs the cancer, and dampening it makes the cancer grow faster. “This is potentially huge. It’s one thing to entertain the idea that mental processes can slow the growth of cancer,” she observes, “It’s quite another, however, to suggest that certain thoughts can make cancer worse.” Pilcher has a dog in the race, revealing on the first page that she herself has a diagnosis of cancer. Yet, despite her caveats clarifying that the stimulation of a neuron in a mouse brain is not equal to a positive thought, the seed has been planted that this is the case. There is a risk that the folk intuition that makes Mrs Twit’s metamorphosis credible in fiction – an intuition that chimes with the growing research on the nocebo effect – may feed into something morally repugnant.

Ultimately, This Book May Cause Side Effects deals with the central philosophical quandaries of humankind: how we conceptualise mind and matter, and to what extent we can shape our own destinies. While Pilcher steers away from tackling the philosophy head-on, this ambitious and fascinating book will add to our understanding of these mercurial and controversial questions. It could also help us avoid the nocebo effect in our daily lives. As side-effects go, that’s a pretty good one.