Oral mucositis (OM) is a common adverse effect of docetaxel-containing treatment. This study aimed to assess whether dexamethasone (DEX) dose-dependently attenuates docetaxel-induced OM and dysgeusia. We retrospectively analyzed medical records of patients with breast cancer receiving docetaxel-containing regimens at Hokkaido University Hospital between June 2015 and June 2022. The patients were divided into low-dose and high-dose groups (DEX 4 or 8 mg/day on days 2–4, respectively), and incidence of OM and dysgeusia, and risk factor(s) for OM incidence were evaluated. The incidence of all-grade OM in the first cycle was 57.8% in the low-dose group and 19.2% in the high-dose group (P= 0.0002), which met our primary endpoint. The incidence of OM in all treatment cycles was also significantly lowered by DEX-dose increase (P= 0.01). In contrast, the incidence of dysgeusia was similar between the two groups in the first and all cycles (P= 0.50 andP= 0.28, respectively). These results were also confirmed in a propensity score-matched population. Multivariate logistic regression analysis also suggested that lower DEX dosage was a singular risk factor for all-grade OM incidence. In conclusion, our study suggests that DEX dose-dependently reduces the incidence of OM in docetaxel-containing regimens for breast cancer treatment.