Patients with diabetes become less adherent to their diabetes medication regimen following a diagnosis of cancer, concluded a new study in Diabetologia (2015; doi:10.1007/s00125-015-3497-8).

Cancer patients with diabetes have a significantly higher overall mortality risk compared with cancer patients without diabetes. Most research on diabetes and cancer has focused on the influence of diabetes and glucose lowering drugs (GLDs) on outcomes after a cancer diagnosis; yet cancer itself might affect outcomes associated with diabetes, in part by affecting adherence to prescribed GLDs.

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 patients without cancer. The research was led by Marjolein Zanders, MD, of the Netherlands Comprehensive Cancer Organisation in Eindhoven, the Netherlands, and by Jeffrey Johnson, PhD, of the School of Public Health at the University of Alberta, Edmonton, Canada.

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All new users of GLDs (1998-2011) who lived in the Eindhoven Cancer Registry-PHARMO Database Network (which includes outpatient pharmacy data) catchment area were selected. Of the 52,228 GLD 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. 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 not covered by the use of GLDs (ie, patients did not take their diabetes medications on 3 days in that month). For every month the MPR for cases was compared with the MPR for matched controls.

The data showed that before cancer diagnosis the MPR increased by 0.10% per month. Besides a significant decrease in MPR at the time of cancer diagnosis (–6.3%), there was an ongoing, yet lower, monthly decline in MPR (–0.20%) after diagnosis. The largest declines in MPR at the time of diagnosis (range 11% to 15%) were seen among patients with stage IV cancer and gastrointestinal or pulmonary cancers.

Different effects were seen for the various tumor types studied: no important decline in MPR was seen at the time of diagnosis for patients with prostate (+2.1%) and breast cancer (–0.5%); however, large declines were seen among patients with esophageal, stomach, pancreas, or liver cancer (­–12.5%) and pulmonary cancers (–15.2%). Among those patients with large declines in adherence, the MPR after a cancer diagnosis decreased approximately 0.5% monthly, indicating ongoing declining medication adherence in such cases.

Within cancer subgroups, the largest declines in MPR at the time of diagnosis were seen for liver and esophageal cancers (–35% and –19%, respectively), and for each extra month after a cancer diagnosis, the largest decline, almost 1% each month, was seen among patients with pancreatic cancer (–0.97%).

Greater declines in medication adherence occurred with cancers at more advanced stages. Among patients with stage IV cancers, the decrease was –10.7%, and each extra month after diagnosis the MPR declined an additional –0.64%.

The authors suggest that the devastating effects of a serious cancer diagnosis could relegate the importance of taking medication for diabetes or other conditions.

“In future studies, the reason for the decline in MPR needs to be further elucidated among the different cancer types—is it the patient who prioritizes the fight against cancer or the advice of the physician to stop the treatment?” concluded the authors.