Background and objective: The Pharmacy Department at the Western General Hospital in Edinburgh prepares a topically applied product in which morphine sulphate is incorporated into Instillagel® for use in reducing pain associated with rectal and other cancers. A stability indicating method was required for assessment of the stability of this combination. Methods: A gradient high performance liquid chromatography method was developed to assess stability and an LC-MS method was used to characterize the degradants from forced degradation of the components in the formulation. Results and discussion: The method was found to have acceptable inter- and intra-day precisions. Under all storage conditions investigated morphine sulphate remained within 98·1% and 103·2% of the initial concentration, lidocaine hydrochloride within 96·07% and 104·9% and chlorhexidine gluconate within 97·3% and 105·5%, except for samples stored at 37 °C beyond 240 h. A sample of the admixture was stored up to 7 months at 37 °C and was found to be reasonably stable with only the chlorhexidine concentration declining appreciably to 92% of the initial concentration. Some of the degradants of chlorhexidine and morphine were characterized by liquid chromatography mass spectrometry. It could be concluded that the admixture was stable for over 22 days at 4 °C protected from light, over 22 days at 25 °C exposed to normal light, and for 7 months at 37 °C protected from light.