Book excerpt: Variability in Human Drug Response examines why individual patients differ significantly in their response to drug administration. This book is devoted mainly to pharmacokinetics and covers topics such as drug absorption, distribution, metabolism, and excretion. The sensitivity of tissues of the body to drugs and the importance of monitoring drug therapy are also discussed. This book is comprised of 10 chapters and begins with an introduction to variability in clinical response to administration of defined drugs, as well as the importance of closely matching dosage to the individual patient's requirement to achieve an optimal response to drug administration. The chapters that follow highlight the pharmacokinetic origin of most variability in the clinical response to drugs, along with the difficulties inherent in predicting the effect of drug administration in an individual patient. The role of genetic and environmental factors, disease, and the concomitant administration of other drugs in determining an individual's response to any therapeutic maneuver is also examined. The last chapter describes two methods of monitoring drug therapy: monitoring drug effects or monitoring the plasma levels of drugs. This monograph will be of interest to practicing clinicians and senior medical students.