Opioid-induced constipation in patients with cancer pain in Japan (OIC-J study): a post hoc subgroup analysis of patients with gastrointestinal cancer. - Harada T; Imai H; Fumita S; Noriyuki T; Gamoh M; Okamoto M; Akashi Y; Kizawa Y; Tokoro A (International journal of clinical oncology, 2021)