Pharmacodynamics (PD) is a branch of pharmacy that deals with the study of the biochemical and physiological effects of drugs on biological system. Pharmacodynamics and pharmacokinetics are the main branches of pharmacology, concerned with the study of the interactions between both endogenous and exogenous chemical substances with living organisms. Pharmacodynamics is the study of how a drug affects an organism, whereas pharmacokinetics is the study of how the organism affects the drug.