What Is the Placebo Effect and How Has It Been Studied?

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The placebo effect is the measurable, observable, or perceived improvement in a health condition that is not attributable to an actual medical treatment. A placebo, as used in research, is an inactive substance or procedure used as a control.

When a patient is given a known inactive substance (e.g., sugar pill, distilled water, or saline solution) rather than a substance or device having true medical value, they may improve merely because their expectation to do so is strong.

To eliminate the effect of positive thinking on clinical trials, researchers often run blind, placebo-controlled studies whereby the patient is unaware if they are given the placebo or the actual treatment.

The word placebo literally means "I will please" in Latin. The first known double-blind placebo-controlled trial was done in 1907. The FDA doesn't require that a drug study include a placebo control group, however, placebo-controlled trials have long been the standard.

Placebo Effect Has Been Extensively Studied

In fact, the placebo effect has been of interest to researchers for decades. Placebos are not only helpful for conditions with a psychological aspect, such as depression, anxiety, or pain, they lessen physical symptoms associated with inflammatory diseases, Parkinson's disease -- and they have even shrunk tumors! Researchers have determined that the placebo effect may bubble up for reasons that go beyond an expectation that a treatment will work (i.e., a conscious belief in the drug or treatment).

There may actually be subconscious connections that lead a patient from treatment to wellness (i.e., subliminal conditioning). It may be a doctor's white coat, or just seeing the syringe, that provokes subconscious feelings. Researchers used rat studies to prove that unconscious, associative learning can provoke the placebo effect.

It does not have to be belief or the expectation of a positive result.

Researchers continued to explore behavioral conditioning and subconscious placebo responses and found that they could control immune responses, as do immunosuppressant drugs. Furthermore, researchers dug deeper and studied the neurological basis for conditioned placebos.

Researchers concluded that while belief and expectation contribute to the placebo effect, that mostly pertains to symptoms that the patient perceives (subjective symptoms), such as pain.  So, it seems there may be two parts to the placebo effect, an expectation effect and a conditioned effect.

Findings Bring Questions

All of the findings seem to bring more questions. For example, does a patent's personality play a role in the placebo effect? Researchers have not identified personality traits that are tied to the placebo effect. Certainly, personality would have little effect with subconscious conditioning.

Interestingly, the price of a medication or the complexity of a treatment may provoke either the placebo effect or the nocebo effect (i.e., provokes feelings of illness). Low cost medications are generally viewed as ineffective. Similarly, simple treatments are generally regarded as less effective than complicated treatment options.

The Bottom Line

Whether it is achieved consciously, subconsciously, or unconsciously, a good result that is not due to the actual treatment or drug itself is attributed to the placebo effect. In clinical trials, the placebo is called the control or sham treatment.


Placebo Effect: A Cure in the  Mind. Scientific American. Maj-Britt Niemi. February 1, 2009.

Placebo Effect, Robert Todd Carroll, The Skeptic's Dictionary, Skepdic.com

The Mysterious Placebo Effect, by Carol Hart. American Chemical Society. July/August 1999.

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