ABSTRACT
Analgesics were once classified into two broad groups: centrally and peripherally acting drugs. This chapter presents a brief description of the history of analgesic research and measurement of pain as well as the general relationships between the intensity of action of analgesics and their concentrations in biological fluids. It discusses the application of some of the models to analgesic data. The chapter summarizes the pharmacodynamics (PD) and pharmacokinetics (PK)-PD studies for both opioid and nonopioid analgesics. Studies on the PD of analgesic drugs have lagged behind PK studies, largely because the measurement of the analgesic effect of drugs is inherently much less accurate than plasma concentration measurements. Rapid increases in plasma concentrations are important in determining the magnitude of analgesic effect. The majority of the PK-PD studies have been performed with single doses of analgesics but because these drugs are usually taken in multiple doses the results may be misleading.
