Reliability and validity of the Bowel Function Index for evaluating opioid-induced constipation: translation, cultural adaptation and validation of the Portuguese version (BFI-P) - Dueñas, M.; Mendonça, L.; Sampaio, R.; Gouvinhas, C.; Oliveira, D.; Castro-Lopes, J.M.; Azevedo, L.F. (Current Medical Research and Opinion, 2017)