Cancer patients with diabetes have a significantly higher overall mortality risk compared with patients without diabetes. Most research on diabetes and cancer has focussed on the influence of diabetes and glucose lowering drugs (GLDs) on outcomes after cancer diagnosis; yet cancer itself might affect outcomes associated with diabetes, in part by affecting adherence to their prescribed GLDs. Although one previous study has found that breast cancer patients were less likely to take their diabetes drugs as directed after being diagnosed with cancer, this study lacked a control group without cancer. In this new study, the authors aimed to evaluate changes in adherence to GLDs following a cancer diagnosis, taking into account changes in adherence to GLDs among similar diabetic patients without cancer.
All new users of GLDs (1998-2011) who lived in the Eindhoven Cancer Registry-PHARMO Database Network (which includes out-patient pharmacy data) catchment area were selected. From the 52,228 GLDs users selected, 3,281 cases with cancer and 12,891 controls without cancer during follow-up were included in the study, with a mean age of 68 years in each group. The Medication Possession Ratio (MPR) was used as an indicator for medication adherence. The MPR represents the amount of medication patients had in possession over a certain time period. Thus, a 10% decline in MPR translates to a difference of 3 days in a 30-day month that are not covered by the use of GLDs (that is, 3 days in that month where the patients did not take their diabetes medications). For every month the MPR for cases was compared with the MPR for matched controls, which represented the overall trend among individuals with diabetes but without cancer.
The data showed that before cancer diagnosis the MPR increased by 0.10% per month. Besides a significant drop in MPR at the time of cancer diagnosis of -6.3%, there was an ongoing, yet lower, monthly decline in MPR (-0.20%) after cancer diagnosis. The largest drops in MPR at the time of cancer diagnosis, in the range of 11-15%, were seen among patients with stage IV cancer disease and gastrointestinal or pulmonary cancers.