Hydromorphone is known for its low bioavailability. Currently, there is no recommended conversion ratio when changing from injectable to oral hydromorphone for patients with hepatic impairment. Its overdose may result in serious side effects, particularly respiratory depression, necessitating careful attention to dosage. In the present case, a patient with hepatic impairment developed respiratory depression following a switch from continuous subcutaneous administration of hydromorphone to an oral extended-release form. The prescribed oral dose of hydromorphone was four times higher than the injectable dosage, based on the standard conversion ratio typically considered equivalent in drug potency. However, increased bioavailability due to hepatic impairment is believed to have caused the patient’s respiratory depression. Therefore, in such cases, it is imperative to cautiously monitor dosage due to significant fluctuations in the conversion ratio, which varies based on individual circumstances.