To provide a pharmaceutical for safely treating a patient requiring a treatment with pemafibrate or a salt thereof or their solvates (called as "pemafibrate treatment").SOLUTION : A pharmaceutical for treating a patient requiring a pemafibrate treatment contains pemafibrate or a salt thereof or their solvates as an active ingredient. The treatment includes the step for avoiding or stopping the combinational use of a pharmaceutical containing an OATP1B inhibitor with pemafibrate in order to suppress a rise in in-plasma concentration of pemafibrate, or the step for reducing an amount of administration of pemafibrate or a salt thereof or their solvates.SELECTED DRAWING : None