For decades, scientists have followed the well-known path of new drug discovery and development. After initial synthesis and in vitro testing, they performed in vivo experiments on animals. Animals were usually male, since researchers avoided females fearing that their reproductive cycle and hormones would interfere with the experiments. Similarly, most of the subjects in clinical trials were men, women often being inadequately represented. One of the consequences of this practice is the fact that women more often report adverse drug reactions. Scientists have slowly begun to realize that physiologic differences between men and women affect drug activity, namely pharmacokinetics and pharmacodynamics. Absorption, distribution, metabolism and elimination are influenced by factors such as body weight, gastrointestinal motility, intestinal enzyme activity, CYP450 enzymes in the liver and glomerular filtration. Pharmacodynamic differences include higher sensitivity of some biological drug targets. Some of the drugs with significantly different activity in men vs women are CNS agents (antipsychotics, anxiolytics, hypnotics, antidepressants, opioid analgesics, drugs of abuse) and cardiovascular agents (beta-blockers, ACE-inhibitors, digoxin, acetylsalicylic acid, diuretics). Today, equal participation of men and women in clinical trials is a standard. The next step is equal inclusion of male and female laboratory animals in the experiments, as well as male and female cells. Eventually, these fundamental changes in the research should bring more accurate therapeutic strategies for both sexes.