There are certain genes which contain instructions to build proteins called enzymes. These enzymes are involved in breaking down many medications in the body, primarily in the liver. People have different versions of these genes which produce enzymes with varying levels of activity (e.g. slow, normal or fast). As a result, people can vary in how slow or fast they break down and clear medications from their body, and this can influence how well a medicine works or the risk of side effects for that individual.