The invention provides gemfibrozil for use in adjusting the effect of a CYP2C8 and/or OATP substrate drug, wherein gemfibrozil is administered in an amount of less than 1200 mg/day. The invention also provided a pharmaceutical composition comprising gemfibrozil providing a significant improvement to the plasma profiles of a CYP2C8 and/or OATP substrate drug in a mammal.