Provided is a medicine for safely treating a patient who needs to be treated with the use of pemafibrate, a salt thereof or a solvate of the same (hereinafter called "pemafibrate treatment"). The medicine for treating a patient who needs the pemafibrate treatment comprises pemafibrate, a salt thereof or a solvate of the same as an active ingredient. The treatment using the aforesaid medicine involves a step for avoiding or stopping the concomitant use of the medicine with another medicine comprising an OATP1B inhibitor to prevent an increase in the plasma pemafibrate concentration at the concomitant use of these medicines, or a step for decreasing the dosage of pemafibrate, a salt thereof or a solvate of the same.