Expectations May Affect Placebo Response

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Individuals with Parkinson's disease were more likely to have a neurochemical response to a placebo medication if they were told they had higher odds of receiving an active drug showing that expectations may affect the placebo response.

Placebo Effect

Placebo effects can arise not only from a conscious belief in a drug but also from subconscious associations between recovery and the experience of being treated.

"The promise of symptom improvement that is elicited by a placebo is a powerful modulator of brain neurochemistry," the authors write as background information in the article. "Understanding the factors that modify the strength of the placebo effect is of major clinical as well as fundamental scientific significance." In patients with Parkinson's disease, the expectation of symptom improvement is associated with the release of the neurotransmitter dopamine.

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Sarah C. Lidstone, Ph.D., of Pacific Parkinson's Research Centre at Vancouver Coastal Health and the University of British Columbia, Vancouver, Canada, and colleagues studied 35 patients with mild to moderate Parkinson's disease undergoing treatment with the medication Levodopa. Patients who were told they had a 75-percent chance of receiving active medication demonstrated a significant release of dopamine in response to the placebo, whereas those in the other groups did not.

Patients reactions to the active medication before the first scan were correlated with their response to placebo. "Importantly, whereas prior medication experience (i.e., the dopaminergic response to Levodopa) was the major determinant of dopamine release in the dorsal striatum, expectation of clinical improvement (i.e., the probability determined by group allocation) was additionally required to drive dopamine release in the ventral striatum," the authors write.

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 a placebo effect. Studies continue to confirm that people who are told that medication can help and provide relief from symptoms confirm that expectations may affect the placebo response.

"Our findings may have important implications for the design of clinical trials, as we have shown that both the probability of receiving active treatment—which varies in clinical trials depending on the study design and the information provided to the patient—as well as the treatment history of the patient influence dopamine system activity and consequently clinical outcome," the authors conclude. "While our finding of a biochemical placebo response restricted to a 75 percent likelihood of receiving active treatment may not generalize to diseases other than Parkinson's disease, it is extremely likely that both probability and prior experience have similarly profound effects in those conditions."

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