Olaparib, an anticancer drug, requires daily administration, frequently causing nausea. Elucidation of the influential factors for nausea is important for continuing treatment. We retrospectively examined 23 patients who received olaparib treatment and were divided into nausea and no-nausea groups, according to antiemetic prescriptions during treatment. We compared the patients' background and laboratory values between the 2 groups. Nine patients developed nausea and 14 did not, with mean body weights at treatment initiation of 49.9±9.8 kg and 60.0±13.9 kg, respectively. Body weights were significantly lower in the nausea than in the no-nausea group. Four weeks after olaparib administration, the logarithmic difference in the fluctuation of the neutrophil count was -0.145±0.154 and 0.095±0.242, while the fluctuation of the lymphocyte count was -0.169±0.053 and -0.060±0.110 in the nausea and no-nausea groups, respectively, with the former significantly lower than the latter. The treatment period for the nausea group was significantly longer than that for the no-nausea group. Since olaparib is administrated as a flat dose, the dose per body weight increased in underweight patients. Thus, being underweight might have impacted the efficacy of olaparib, including the development of side effects such as nausea and hematotoxicity.