As we age, our ability to process medication changes. The kidneys and liver clear drugs more slowly, so drug levels in the blood remain higher for a longer time. People also gain fat and lose muscle mass over time. Both these changes affect the way drugs are distributed to and broken down in body tissues. And because these drugs are stored in body fat, they can continue to produce effects days after people stop taking them, especially in people with a higher proportion of body fat. In addition, older people tend to take more prescription and over-the-counter medications, each of which has the potential to suppress or enhance the effects of the others.