Although there are many changes in the gastrointestinal tract with aging, only those in the liver substantially influence blood concentrations and clearance of drugs. The liver mass, overall function, and blood flow decrease approximately 1% per year after age 40 to 50 years, and accordingly, the hepatic metabolism or clearance of drugs decreases in this proportion. The sensitivity of the gastrointestinal tract to usual concentrations of drugs is increased, and this, in part, accounts for the increased frequency of adverse drug reactions in elderly patients.