What are the placebo and nocebo effects?
A placebo is a pill or injection with no active ingredient. When people who get a placebo feel better, that's the placebo effect. The flip side—developing real headaches, muscle aches or fatigue because you were told a drug "may cause side effects," or simply because you're anxious—is the nocebo effect (Latin for "I shall harm"). Changes in a placebo group also include natural recovery over time and regression to the mean, not just expectation.
Why do they happen?
- Expectation: imaging and drug studies suggest that believing you'll improve affects pain-control regions of the brain and the release of the body's own opioids.
- Conditioning: after repeatedly feeling better from a pill, the body can respond to similar cues alone.
- Attention: focusing on minor bodily sensations can make pre-existing symptoms feel like side effects.
Key studies
In knee osteoarthritis pain research, many people improve on placebo alone: in the 2006 GAIT trial, 60.1% of the placebo group met the threshold of 20% pain improvement. In the 2020 SAMSON study in NEJM, people who had quit statins rotated through months of statin, placebo and no pill. Symptom scores were 8.0 with no pill, 15.4 on placebo and 16.3 on statins—about 90% of statin symptoms showed up on placebo too.
Why it matters for trials and care
A new treatment has to beat the placebo effect to count as truly effective, which is why randomized controlled trials use identical-looking placebos and double-blinding. Giving only a placebo is ethically restricted when an effective treatment already exists. In the clinic, being honest about side effects while explaining them in a way that doesn't fuel unnecessary worry may help reduce nocebo effects. For statins and cholesterol, see LDL cholesterol.
FAQ
- Does the placebo effect mean the symptoms were imaginary?
- No. Placebo and nocebo responses involve measurable changes in the brain and body; the symptoms are real even though the pill is inert.
Sources reviewed: 2026-10-25
References
- Benedetti, F. (2014). Placebo effects: from the neurobiological paradigm to translational implications. Neuron, 84(3), 623–637.
- Wood, F. A., et al. (2020). N-of-1 trial of a statin, placebo, or no treatment to assess side effects. New England Journal of Medicine, 383(22), 2182–2184.
- Clegg, D. O., et al. (2006). Glucosamine, chondroitin sulfate, and the two in combination for painful knee osteoarthritis. New England Journal of Medicine, 354(8), 795–808.