A 51-year-old woman had previously received treatment for breast cancer at another hospital but had refused early and aggressive treatment. Therefore, she was treated with symptomatic therapy. As her disease progressed, the patient wished to receive palliative care, and was transferred to a palliative care hospital. However, based on her general condition, it was determined that aggressive treatment should not be abandoned, and she was referred to our hospital for treatment. During her initial visit, the patient was found to have left breast cancer with chest wall invasion, right breast metastasis, multiple liver and lung metastases, left pleural effusion accompanied by pleural dissemination, and left upper limb edema. There was no evidence of bone metastases. The patient's pain was managed with oral oxycodone sustained-release tablets (320 mg daily), using high-dose (80 mg) oral oxycodone hydrochloride hydrate as rescue medication. The results of immunohistochemical testing, confirmed by her previous hospital, were ER (-), PgR (-) and HER2/neu positive. First-line treatment was initiated with paclitaxel (PTX) plus trastuzumab (Tmab), and the response was rated as stable disease (SD). During the course of treatment, she developed drug-induced interstitial pneumonia, which was probably caused by the taxane. Therefore, the first-line treatment was discontinued and T-DM1 was initiated as second-line treatment. However, beginning with cycle 3 of the T-DM1 treatment, the patient began complaining of joint pain, mainly in the upper limbs. Therefore, the dose of oxycodone sustained-release tablets was increased to 600 mg per day. However, the patient's joint pain showed no improvement and it was considered unlikely that the pain was due to bone metastases. It was suspected that the pain was an adverse reaction to T-DM1, and the dose of T-DM1 was reduced by one step in cycle 7 of treatment. This resulted in a dramatic improvement of the patient's symptoms. Since oxycodone sustained-release tablets was being used at a high dose, sleepiness caused by the drug interfered with her activities of daily living. Consequently, as part of an opioid rotation scheme, topical fentanyl citrate was used concomitantly, and the initial daily oxycodone sustained-release tablets dose of 600 mg was reduced to 40 mg and administered in combination with fentanyl citrate (12mg). These findings suggest that uncontrollable joint pain can occur as an adverse reaction to T-DM1.