Importance of placebo effect in cough clinical trials.

Cough is a unique symptom because, unlike sneeze and other symptoms, it can be under voluntary control and this complicates clinical trials on cough medicines. All over-the-counter cough medicines (OTC) are very effective treatments because of their placebo effect. The placebo effect is enhanced by expectancy related to advertising, brand, packaging, and formulation. This placebo effect creates a problem for the conduct of clinical trials on OTC cough medicines that attempt to demonstrate the efficacy of a pharmacological agent above that of any placebo effect. Up to 85% of the efficacy of some cough medicines can be attributed to a placebo effect. The placebo effect apparent in clinical trials consists of several components: natural recovery, regression of cough response toward mean, demulcent effect, effect of sweetness, voluntary control, and effects related to expectancy and meaning of the treatment. The placebo effect has been studied most in the pain model, and placebo analgesia is reported to depend on the activation of endogenous opioid systems in the brain; this model may be applicable to cough. A balanced placebo design may help to control for the placebo effect, but this trial design may not be acceptable due to deception of patients. The placebo effect in clinical trials may be controlled by use of a crossover design, where feasible, and the changes in the magnitude of the placebo effect in this study design are discussed.