The placebo: from specificity to the non-specific and back.

A brief historical outline is provided of the concept of specificity in biology and medicine. The advent of scientifically based therapeutics, and especially clinical pharmacology, has directed increasing attention to the role of non-specific factors in the field of mental disorders. The actions and effects of placebos are discussed in relation to the specific/non-specific dichotomy with particular reference to both pharmacological and psychological modes of treatment.