The present invention provides a pharmaceutical composition comprising naltrexone or an analogue thereof for use in the treatment of a subject having a tumour/cancer; wherein the naltrexone or the analogue thereof is to be administered to the subject in a first treatment phase, followed by a recovery phase; wherein, following the recovery phase, a small molecule signalling inhibitor selected from the group consisting of PI3-kinase inhibitors, AKT inhibitors, taxanes, antimetabolites, alkylating agents and cell cycle inhibitors is to be administered to the subject in a second treatment phase; the recovery phase being characterised by the absence of administration of the naltrexone or the analogue thereof and the small molecule signalling inhibitor. Also envisaged are a diagnostic test to assess the response of a subject to the first treatment phase, an in vitro method of testing the efficacy of a small molecule signalling inhibitor when for use in combination with naltrexone, and the use of naltrexone in such a method when in vivo.