Mechanisms of the placebo effect of sweet cough syrups.

The review discusses the large placebo effect associated with cough medicines and speculates on the observation that most cough medicines are formulated as sweet syrups rather than capsules or tablets. The review proposes that the major benefit of cough medicines for treatment of cough associated with common cold is related to the placebo effect rather than the pharmacological effect of an active ingredient. The placebo effect is discussed in terms of physiological effects of cough syrups associated with the taste of the medicine and true placebo effects associated with belief in the therapy. The idea is developed that a sweet taste may modulate cough at the level of the nucleus tractus solitarius, possibly by influencing the production of endogenous opioids.