If a doctor gives you a sugar pill and tells you it will make your headache feel better and it does, that's called the placebo effect. But what happens if they tell you the pill will make your headache worse?
The nocebo is the evil twin of the placebo and it's thought to be responsible for everything from wind turbine syndrome, to electro-sensitivity, to some instances of gluten intolerance.
Auckland University psychologist Dr Keith Petrie has recently been awarded the Mason Durie Medal for his studies of placebos, nocebos and other ways patients' perceptions of illness can affect their health.
Listen to Dr Petrie's interview with Nine to Noon's Kathryn Ryan:
He told Nine to Noon that, while most people are more familiar with the placebo effect, which is responsible for a percentage of the total benefits of prescription drugs, fewer people know as much about the nocebo effect.
"The nocebo effect applies to real drugs and real treatments as well, due to our expectations that we may get negative effects," he said.
"We can get negative effects due to the treatment but also due to our expectations of getting symptoms.
"[When] we tell someone a pill's going to help them, people will then switch from focusing on their symptoms and how sick they feel to signs of improvement and how well they're feeling.
"Our brains tends to be very susceptible to expectations and influences in terms of noticing things that we want to notice, and at the same time focusing on negative things as well."
Watch a video explaining the nocebo effect:
Dr Petrie said another example of how the nocebo effect worked was when people believed rumours, and even stories in the media, that then negatively influenced them.
"Mass pyschogenic illness is one extreme example, where some will get sick in a closed social circumstance such as a school, a factory or even an airport.
"If someone falls down dramatically, other people around them will start noticing symptoms - and soon a kind of a wave of non-specific symptoms will take over and people can get quite dramatically sick quite quickly."
Pills, rumours - and wine
Dr Petrie said the placebo effect worked in the same subliminal way, and both also came into play with how different products were branded or how much they cost.
"You see that not only in pills but with bottles of wine, for example if you tell people this bottle is $45 rather than $10 people will report that it's more pleasurable to drink.
"The pleasure centre in their brain lights up more than if you tell them it only costed $10."
The placebo and nocebo effects were helping medical students understand why some patients did not like taking their medication, he said.
"We often forget about the ideas that the patient has about their illness, and the ideas that a patient has about their medicines," Dr Petrie said.
"Both of those things can really either improve the power of a drug, or work against it, and in particular [when they] believe they're going to get side effects or have concerns about medicines, they're generally not going to take them.
"That's probably the main problem facing medicine today - low adherence."
Dr Petrie said doctors have known for quite some time about how the placebo effect could make their patients feel relaxed, and how it affected their outcomes.
"We know from placebo studies that those doctors who show empathy and concern have a much better response to treatment."
He said, over his 20 years of teaching at Auckland Medical School, students were now much more engaged and interested in that side of medicine.
"I think the generation that's coming through medical school now are much more sophisticated in terms of their understanding of those interpersonal aspects, and because of that their treatments actually will become more effective."