We encountered a case in which the general condition of a patient receiving cabazitaxel worsened with concomitant use of clarithromycin. Cabazitaxel is metabolized mainly by CYP3A4, and the frequency of adverse events is known to increase with increasing exposure. Although these drugs are not often quantified in daily practice, we quantified them because we considered it possible that the blood concentration of cabazitaxel had increased due to CYP3A4 inhibition of clarithromycin and that cabazitaxel-related adverse events had occurred. However, the concentration of cabazitaxel was not increased and we attributed the patient's deterioration to decreased tolerability of cabazitaxel. At least at a trough concentration of 70 ng/mL, which is the trough concentration when a normal dose of clarithromycin is administered, clarithromycin does not appear to have a significant effect on the blood concentration of cabazitaxel. This case suggests that the administration of the normal dose of clarithromycin might be relatively safe in patients receiving cabazitaxel.