Nocebo Effects and Hypnosis Research

Most people have heard of the placebo effect: expect to get better, and you sometimes do. Fewer know its twin, the nocebo effect: expect to get worse, and you sometimes do too. In hypnosis research, nocebo effects are under-discussed but potentially important.

What a nocebo effect is

The term comes from the Latin “I shall harm,” just as placebo comes from “I shall please.” A nocebo effect occurs when a negative expectation produces a genuine negative outcome [2].

The evidence is clearest in drug trials. When patients are told a sugar pill might cause headaches, a significant proportion report headaches. A review of nocebo phenomena in clinical settings found that the type and frequency of side effects people reported from inert treatments closely matched what they were told to expect [3].

These are not imaginary symptoms. A meta-analysis of nocebo hyperalgesia found that negative verbal suggestions produced small-to-moderate increases in pain that were measurable and consistent across studies [4]. Brain imaging studies show that nocebo-induced pain activates overlapping neural regions with physically induced pain [1].

How nocebo effects could affect hypnosis research

In the control group

Many hypnosis trials use a no-treatment or wait-list control. Participants assigned to these groups know they are not receiving the active treatment. If they expected hypnosis and were disappointed, their negative expectation could worsen their outcomes, making hypnosis look more effective by comparison. This is one reason why wait-list comparisons tend to overestimate treatment effects.

Through hypnotizability testing

Some trials screen participants for hypnotizability before treatment. If low scorers are told they are unlikely to respond well, that information itself could reduce their response. Irving Kirsch’s response expectancy theory argues that expectations directly generate experiences, not just reports of experiences [6]. If that theory is correct, telling someone they are low in hypnotizability is not just a prediction but a potential cause of poor response.

Through suggestion wording

Hypnotic suggestions are usually framed positively: “You will notice the discomfort becoming less.” But poorly worded suggestions could inadvertently draw attention to symptoms or imply that improvement will be difficult. A suggestion like “Try not to think about the pain” may function as a nocebo by directing attention toward the pain.

The relationship with placebo effects

Placebo and nocebo effects are not opposites so much as two manifestations of the same principle: that expectations shape experience [2]. In hypnosis research, this creates a challenging situation. Hypnosis itself may work partly through positive expectation. If that is the case, then the placebo component is not a confound to be eliminated but part of the mechanism.

This makes it difficult to design a fair comparison group. For the broader problem, see Hypnosis and the Placebo Question. For how expectations shape results in general, see Expectation Effects in Hypnosis.

Clinical implications

If negative expectations can worsen outcomes, then how a clinician frames hypnosis matters. Presenting it as “unlikely to help” or “only for highly hypnotizable people” could itself reduce its effectiveness. Conversely, presenting it as a guaranteed cure creates a different problem: if it fails to deliver, the disappointment may be its own nocebo.

A careful framing might be: “Many people find this helpful, and we will see how it works for you.” This sets a positive expectation without making claims that cannot be supported.

What the research still needs

Gap	Why it matters
No direct study of nocebo in hypnosis trials	The discussion is theoretical, not empirical
Hypnotizability framing effects unstudied	We do not know if telling people their score changes their response
Suggestion wording effects rarely isolated	Hard to separate content from delivery
Mechanism overlap with placebo unclear	Makes it difficult to know what is being measured

What this means in practice

Nocebo effects are real, measurable and relevant to any treatment that involves expectations, which certainly includes hypnosis. Researchers should be aware that their comparison conditions, their screening procedures and even their consent forms can influence outcomes through negative expectations. Clinicians should be thoughtful about how they frame hypnosis, avoiding both dismissive and overblown descriptions.

For how expectations influence outcomes more broadly, see Expectation Effects in Hypnosis. For the practical challenge of designing a fair test, see Why Comparison Groups Matter.

A note on suggestion safety. There is no evidence that standard clinical hypnosis causes lasting harm through nocebo mechanisms. However, poorly trained practitioners who make alarming suggestions or imply that symptoms will worsen should be avoided. Hypnosis should be delivered by someone with appropriate training and professional registration.