Over the past year, the nocebo effect and its impact on our responses to medical interventions has been highlighted by the wave of publicity surrounding the Covid-19 vaccines, particularly the intensity of media attention surrounding cases of adverse reactions.

Last month, psychologists at the Beth Israel Deaconess Medical Centre in Boston conducted a study that suggested that the nocebo effect may be behind a sizeable proportion of side effects following Covid-19 vaccination. As part of the study, 22,578 people were injected with a sterile solution which they believed to be a Covid-19 vaccine, with 35 per cent of them reporting symptoms of headache, fatigue and arm pain several hours later.

Robert Baloh, a professor of neurology at the University of California, Los Angeles, is an expert in the nocebo effect, which he dubs “placebo’s evil twin”.

“The nocebo effect is much more common than most people think, including doctors,” he says. “It explains many of the side effects of drugs and vaccines, many cases of gluten sensitivity, environmental chemical sensitivity, electromagnetic sensitivity, and ultrasound sensitivity. People with negative perceptions of these things, through prior personal experiences and beliefs or suggestions from friends, family and the media, are more likely to focus and ruminate on routine body symptoms and then attribute the symptoms to an external source.”

It is only in recent years that scientists have begun to fully appreciate the human propensity for subconsciously thinking ourselves unwell, and the frequency with which this occurs. Some estimates have suggested that up to 30 per cent of GP appointments may be with patients whose symptoms have a psychosomatic origin. Surveys have also found that up to a third of patients in an average general neurology clinic have symptoms that cannot be explained by any medical test or examination.

We now know that along with the nocebo effect, there are multiple different types of psychosomatic illness. One example is conversion disorder, where patients experience severe neurological symptoms such as paralysis or blindness which cannot be explained by medical evaluation. Another is health or illness anxiety where relatively innocuous symptoms become the subject of anxious thoughts, which then cause the symptoms to get worse.

All of these forms of psychosomatic problems can worsen with stress and declining mental health. Sophie Eastwood, a GP and clinical epidemiologist at University College London, says that it is important to recognise that while they may have a psychological origin, the physical symptoms very much exist.

“It’s not so much a question of whether muscular aches or fatigue are genuine, if someone is experiencing them, they are certainly real,” she says.

Because of nocebo, and the knowledge that talking about side effects leads to more side effects, doctors have even suggested giving patients far less information about the potential consequences of a drug or vaccine. However, psychologists believe that more openness is actually the best approach.

“I think it is important to explain the nocebo effect to patients before they receive a treatment,” says Julia Haas, a psychologist at the Beth Israel Deaconess Medical Centre. “This includes informing them that many people experience symptoms even after placebo treatment and that these effects are probably caused by worries, negative expectations or misattribution of symptoms that may have occurred anyway. If patients know that not all symptoms they experience after treatment are necessarily caused by the drug, this can reduce their worries and maybe even the symptoms.”