What is the placebo effect?

A placebo is a substance or other kind of treatment that looks just like a regular treatment or medicine, but it is not. It is actually an inactive “look-alike” treatment or substance. This means that it is not a medicine. The person who is getting a placebo does not know for sure that the treatment is not real. Sometimes the placebo is in the form of a “sugar pill,” but a placebo can also be an injection, a liquid, a procedure, or any other type of therapy that doesn’t directly affect the illness being treated.

Even though placebos do not act on the disease, they seem to have an effect in about 1 out of 3 patients. A change in a person’s symptoms as a result of getting a placebo is called the placebo effect. Usually the term “placebo effect” speaks to the helpful effects of a placebo in relieving symptoms. This effect usually lasts only a short time, and is thought have something to do with the body’s own chemical ability to briefly relieve pain or certain other symptoms.

But sometimes the effect goes the other way, and causes unpleasant or worse symptoms. These may include headaches, nervousness, nausea, or constipation, to name a few of the possible “side effects.” The unpleasant effects that happen after getting a placebo or an inactive treatment are sometimes called the nocebo effect.

Together, these 2 types of outcomes are sometimes called expectation effects. This means that the person taking the placebo may experience something along the lines of what he or she expects to happen. If a person expects to feel better, that may happen. If the person believes that he or she is getting a strong medicine, the placebo may be thought to cause the side effects. The placebo does not cause any of these effects directly. Instead, the person’s belief in or experience of the placebo helps change the symptoms, or change the way the person perceives the symptoms.

Along with the placebo or nocebo effect, incidental events (unrelated effects that may have happened without the placebo) may also be linked to the placebo because of their timing. For example, a headache or rash that happens soon after taking a placebo may be caused by something else entirely, but the person may think the placebo caused it. The same can be said for good outcomes: if a person happened to start feeling better after taking a placebo, that improvement may be thought to be due to the placebo.

Some patients can have the placebo effect without getting a pill, shot, or procedure. Some may just feel better from visiting the doctor or doing something else they believe in. That type of placebo effect seems most related to the degree of confidence and faith the patient has in the doctor or activity.

How are placebos used?

Placebos may be used in clinical trials. Clinical trials are research studies testing new drugs or other treatments in volunteers. Before a new treatment is used on people, it is studied in the lab. If lab studies suggest the treatment will work, the next step is to test it on animals. If that also gives promising results, it may then be tested in clinical trials to see if it has value for patients. The main questions the researchers want to answer are:

  • Does this treatment work?
  • Does it work better than what we’re now using?
  • What side effects does it cause?
  • Do the benefits of the treatment outweigh the risks?
  • Which patients are most likely to find this treatment helpful?

If standard treatments for the disease are already available, the new treatment is usually compared to one of these treatments. This tells researchers if the new treatment is as good as or better than the one that is currently available.

If there is no approved treatment for an illness or condition, some people in the study may be given a placebo, while others get the new treatment being tested. The main reason to have a placebo group is to be sure that any effects that happen are actually caused by the treatment and not some other factor. The placebo looks, tastes, or feels just like the actual treatment, so that the patient’s expectations alone are not responsible for the outcome. In a “double-blind controlled study,” neither the volunteers taking part in the study nor their doctors know who is getting which treatment. This study design helps avoid biases in measuring outcomes that can be caused by the researchers’ or the patients’ expectations about the treatment.

If you would like to know more about clinical trials, see our document called Clinical Trials: What You Need to Know.

People in a study that involves a placebo should always know that there is a chance they could be getting a placebo. It is not OK to give someone a treatment in a clinical trial and not mention that it could be a placebo. Please see our document Informed Consent for more information on this topic.

Those who get placebos in medical studies serve an important role. Their responses help provide a good way to measure the actual effect of the treatment being tested. The placebo group provides an important baseline with which to compare the treatment group. It helps researchers see what would have happened without the treatment, though the placebo group may still have some short-term effects based on what the patient expects. For instance, illnesses that sometimes go away on their own might be thought to get better because of the medicine, unless there is a placebo group and those people get better too. On the other side, bad effects that were going to happen anyway, or that occur from some unrelated cause, may be blamed on the treatment unless they also happen to people in the placebo group.

How does the placebo effect work?

In the past, some researchers have questioned whether there is convincing proof that the placebo effect is a real effect. But there are studies showing that the placebo effect is real. For example, scientists have recorded brain activity in response to placebo.

Many think the placebo effect occurs because the patient believes in the substance, the treatment, or the doctor. The patient’s mind somehow causes short-term physical changes in the body. The patient expects to feel better, and so he or she does feel better for some time. But even if a person feels better after taking a placebo, it doesn’t mean the person’s illness or symptoms were not real.

Since many scientific tests have shown the placebo effect, it is one way we know for sure that the mind and body are connected.

Some scientific evidence suggests that the placebo effect may be partly due to the release of endorphins in the brain. Endorphins are the body’s natural pain killers. But there is probably more to it than this.

What is commonly called the placebo effect even plays a role in mainstream medicine. Many people feel better after they get medical treatments that they expect to work. But the opposite can also happen, and this seems to support the idea of the expectation effect even more. For example, in one study, people with Alzheimer’s disease were less affected by pain medicines. These patients required higher doses — possibly because they had forgotten they were getting the drugs, or they forgot that the pain medicines had worked for them before.

This suggests that past experiences also play into the placebo effect. In one study that looked at the placebo effect in pain relief, one group got a real pain medicine and the other did not. In the following days, both groups were given a placebo that looked like the real pain medicine. Those who had gotten the real pain medicine were able to tolerate more pain than those who had not gotten pain medicines before. In the same study, people who were given a drug that raised a certain hormone level beforehand actually had a similar (but smaller) hormone response when they were given a placebo later. Those who had not gotten the real drug beforehand had no change in their hormone levels when they got the placebo, even though they were told that they would. This helped to separate out the power of the researcher telling them they would have an effect from the learned experience of having the effect in the past. This type of learned response after personal experience is called the conditioning effect. It seems to be part of what we call the placebo effect.

The nocebo effect, in which a person has more symptoms or side effects after a placebo, is still being studied. Researchers believe it may be partly explained by a substance in the body that sends messages through the nerves. When a person is anxious, for instance, the substance is activated and the person feels more pain than a person who isn’t anxious. The nocebo effect shows up in the brain: brain-imaging studies have shown that pain is more intense when a person expects more pain than when there is a neutral expectation. This is linked to changes in certain brain regions on the imaging studies.

Although we may not know exactly how it works, the idea that the mind can affect the body has been around for years and is well-proven in certain situations. Many ancient cultures depended on mind-body connections to treat illness. Shamans or medicine men would not have viewed their efforts as placebos. But their healing powers may have worked in the same way, partly through the patient’s strong belief that the shaman’s treatments would restore health. Or it could be that the sick person was going to get better anyway, but the recovery was thought to be because of the treatment — which may have really done nothing for the illness.

Because placebos often have an effect, even if it does not last long, some people think that the placebo produced a “cure.” But placebos do not cure. And in studies where doctors are looking at whether a tumor shrinks, placebos would be expected to have very little, if any, effect.

Still, placebos clearly can help relieve certain symptoms such as pain, anxiety, and trouble sleeping in some people. In earlier times, placebos were sometimes given by doctors out of frustration or desperation, because nothing else was available or seemed to work. In one 2008 study, nearly half of doctors polled said that they used a placebo when they felt that it might help the patient feel better.

Sometimes if the placebo looks more “real,” the person may think it is an active medicine or treatment and believe in its power even more. For example, a larger pill may look more powerful than a small pill. In some people, an injection may have a stronger placebo effect than a pill.

Some believe that placebos seem to work because many illnesses improve over time even without treatment. People may also take better care of themselves by exercising, eating healthier, or resting if they are taking a placebo. Just as natural endorphins may relieve pain once they are released, some research shows the brain may respond to an imagined scene much as it would to something it actually sees. A placebo may help the brain remember a time before the symptoms and bring about a chemical change. This is a theory called remembered wellness.

Some scientists believe that the effects of many alternative therapies may simply be a placebo effect. If the patient believes in the treatment and wants it to work, it can seem to do so, at least for a while. If this effect worked on an illness that usually would not get better on its own, and it lasted, it would be considered a real cure, not a placebo effect.

Finally, there is evidence to suggest that what a patient expects about real medicines can influence how the patient feels after the medicine is taken. Even though responses from real drugs aren’t typically thought of as placebo effects, some short-term effects are affected by expectations — good ones as well as bad.

There is still much to find out about the placebo effect and all the ways it may work. Researchers continue to study it in order to learn more about it, and whether it may be used to help people feel better.

Additional resources

More information from your American Cancer Society

The following related information may also be helpful to you. These materials may be viewed on our Web site or ordered from our toll-free number, 1-800-227-2345.

Clinical Trials: What You Need to Know (also available in Spanish)

Complementary and Alternative Methods for Cancer Management (also available in Spanish)

Dietary Supplements: How to Know What Is Safe

Guidelines for Using Complementary and Alternative Methods

Learning About New Cancer Treatments

Informed Consent

American Cancer Society Operational Statement on Complementary and Alternative Methods of Cancer Management

Along with the above, information on many different types of complementary and alternative treatments are available at no cost to you from the American Cancer Society. You can find them on our Web site or request from our toll-free number as noted above.

Other organizations and Web sites*

Today there is a great deal of interest in complementary and alternative therapies. Mass communication, especially the Internet, makes it possible for people to share ideas and information very quickly. But too often information on the Internet is written by promoters of useless treatments. Along with the American Cancer Society, the following is a partial list of Web sites and phone numbers of reputable groups that provide information on complementary and alternative therapies:

National Cancer Institute

Web site: www.cancer.gov

Toll-free number: 1-800-422-6237 (1-800-4-CANCER)

TTY: 1-800-332-8615

National Center for Complementary and Alternative Medicine (NCCAM)

Web site: http://nccam.nih.gov

Toll-free number: 1-888-644-6226

TTY: 1-866-464-3615

Memorial Sloan Kettering Cancer Center

About Herbs and Botanicals

Web site: www.mskcc.org/mskcc/html/11570.cfm

United States Department of Agriculture, Food and Nutrition Information Center

Web site: http://fnic.nal.usda.gov

Choose “Dietary Supplements” from the left menu bar

United States Food and Drug Administration

Web site: www.fda.gov/Food/DietarySupplements/default.htm

Toll-free number: 1-888-INFO-FDA (1-888-463-6332)

To report side effects or other adverse events from a dietary supplement, call Medwatch at 1-800-FDA-1088

National Council Against Health Fraud

Web Site: www.ncahf.org


Web Site: www.quackwatch.org

*Inclusion on this list does not imply endorsement by the American Cancer Society

No matter who you are, we can help. Contact us anytime, day or night, for information and support. Call us at 1-800-227-2345 or visit www.cancer.org.


Benedetti F, Arduino C, Costa S, et al. Loss of expectation-related mechanisms in Alzheimer’s disease makes analgesic therapies less effective. Pain. 2006;121:133-144.

Benedetti F, Pollo A, Lopiano L, et al. Conscious expectation and unconscious conditioning in analgesic, motor, and hormonal placebo/nocebo responses. J Neurosci. 2003;23(10):4315-23.

Beyerstein BL. Distinguishing science from pseudoscience. 1995. Victoria, B.C.: The Centre for Curriculum and Professional Development. Accessed at www.sfu.ca/~beyerste/research/articles/02SciencevsPseudoscience.pdf on November 8, 2007. Content no longer available.

Boström H. Placebo — the forgotten drug. Scand J Work Environ Health. 1997;23 Suppl 3:53-7.

Colloca L, Benedetti F. Nocebo hyperalgesia: how anxiety is turned into pain. Curr Opin Anaesthesiol. 2007;20(5):435-9.

Colloca L, Benedetti F. Placebo analgesia induced by social observational learning. Pain. 2009 Mar 9.

Haour, F. Mechanisms of the placebo effect and of conditioning. Neuroimmunomodulation. 2005;12:195-200.

Hrobjartsson A, Gotzsche PC. Is the placebo powerless? Systematic review with 52 new randomized trials comparing placebo with no treatment. Journal of Internal Medicine. 2005;257:394-396.

Kaptchuk TJ. The placebo effect in alternative medicine: Can the performance of a healing ritual have clinical significance? Ann Intern Med. 2002;136:817-825.

Kaptchuk TJ, Kelley JM, Conboy LA, et al. Components of placebo effect: randomised controlled trial in patients with irritable bowel syndrome. BMJ. 2008;336(7651):999-1003.

Link J, Haggard R, Kelly K, Forrer D. Placebo/nocebo symptom reporting in a sham herbal supplement trial. Eval Health Prof. 2006;29:394-406.

Pollo A, Amanzio M, Arslanian A, et al. Response expectancies in placebo analgesia and their clinical relevance. Pain. 2001;93(1):77-84.

Wampold, BE, Minami T, Tierney SC , Baskin TW, Bhati, KS. The placebo is powerful: Estimating placebo effects in medicine and psychotherapy from randomized clinical trials. Journal of Clinical Psychology. 2005;61:835-854.

Scott DJ, Stohler CS, Egnatuk CM, et al. Individual differences in reward responding explain placebo-induced expectations and effects. Neuron. 2007;55:325-336.

Sherman R, Hickner J. Academic physicians use placebos in clinical practice and believe in the mind-body connection. J Gen Intern Med. 2008;23(1):7-10.

Victoria (Australia) Better Health Channel. Placebo effect. Last reviewed October 2009. Accessed at www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Placebo_effect?OpenDocument on August 23, 2010.

Wager TD, Rilling JK, Smith EE, et al. Placebo-induced changes in fMRI in the anticipation and experience of pain. Science. 2004;303:1162-1167.