Even in this age of space flight and nuclear medicine, the physicianss main criterion for deciding drug dosage is still what it has been for the past half century-a patient's weight. Nor are the physician's guidelines for selecting a medication that much more sophisticated. He can check the Physician's Desk Reference, hark back to his pharmacology textbook from his second year in medical school or dredge up past clinical experience he and his colleagues have had with the drug. That's pretty much all there is to drug prescribing, or all there has been-until recently. Quietly, low-key but definitively, pharmacology is moving from cursory description to finely honed measurement of bodily drug events-much as the entire field of biology has turned during the past century from description to measurement to alteration of biological reactions. A major advance pharmacologists are now serving up to physicians has to do with pharmacogenetics, the study of genetically triggered individual responses to drugs. More and more clinicians are tuning in to pharmacogenetics. It should help them prescribe more rationally. Calling on pedigree studies, pharmacology-toxicology assays and the latest biochemical techniques, pharmacogeneticists are coming up with some exciting results. For one, there is definite proof that some adverse drug reactions can be inherited (SN: 1 / 30/68, p. 551). Pedigree studies have shown that certain adverse drug reactions run in families. Dose response curves reveal astonishing variations in individual responses to drugs. For example, Drs. Elliot S. Vesell and John G. Page, originally working at the National Institutes of Health, have observed that blatant individual variations in response to drugs vanish in identical twins but remain largely preserved in fraternal twins. The drugs they studied included ethanol, dicumarol (an anticoagulant), halothane (an Hospital Practice