Structural Biochemistry/Response to Medicine
Scientists and researchers believe that medicines produce an effect for less than half of those who take it. This is partly due to lifestyle and environmental factors, but it is primarily the result of genetic variants. The genes that make cytochrome P450 proteins are responsible for a medicinal effect in the body. These proteins metabolize hormones that our body produces and those that are foreign. Because every human consists of a different genomic sequence, the proteins that genes encode for also differ. This genetic variant can affect the way in which cytochrome P450 reacts and how the drug elicits an effect. Ultimately, the cytochrome P450 proteins process many of the drugs we take, therefore explaining the different responses to medicine between individuals. 
The above picture is the Dose-Response curve that tells how much a drug (x-axis) causes effect in the body (y-axis). The region in between the blue and green curve is the desired effect and the region on the right side of the green curve is the side effect. Scientists study the relationship between the medicine and its effect on human body. Like how the image is showing, at first there is no effect when no drug is put in. The researcher then adds more drug into the system to see the effect on the body. If too much drug is taken, the body will reach its overdosing point and may face side effects. This image demonstrates the right amount of dose researchers are looking for while working with a drug. 
Berg, Jeremy M., ed. (2002), Biochemistry (6th ed.) New York City, NY: W.H. Freeman and Company
Davis, Alison Davis. Medicines By Design. The Office of Communications and Public Liaison. 2006.