OBJECTIVES: This study aims to evaluate the impact of cancer and its treatment on HbA-values among individuals with colorectal cancer (CRC) using glucose lowering drugs (GLDs). METHODS: Patients with primary CRC (1998-2011) were selected from the Eindhoven Cancer Registry and linked to the PHARMO Database Network including out-patient pharmacy and clinical laboratory data. Patients with more than two years of GLDs use prior to cancer diagnosis were included. Linear mixed effects models were conducted to evaluate changes in HbA for colon cancer (CC) and rectal cancer (RC) patients in the four years around CRC diagnosis. RESULTS: Of all CRC patients (n=4,714), 294 (6%) GLDs users with CC and 144 (3%) with RC were selected. In the crude model, mean HbA at cancer diagnosis was 6.9% (51.6 mmol/mol) among CC patients and 7.1% (53.5 mmol/mol) among RC patients. Among CC patients, HbA decreased with 0.12% per year (p=0.0002) before cancer diagnosis in the adjusted model and after diagnosis it increased with 0.12% per year (p=0.02). In subgroup analyses, effects on HbA were more pronounced in users of anti-anaemic preparations, these preparations are suggested to interfere with HbA decreased before diagnosis with 0.18% per year (p=0.0006), whereas after diagnosis it changed not-significantly. CONCLUSIONS: Among users of GLDs, HbA decreased with 0.12%-0.18% (1-2 mmol/mol) per year before CRC diagnosis. Only among CC patients, HbA increased after diagnosis (0.12% per year; 1.3 mmol/mol). In (un)diagnosed cancer patients the HbA measure to visualise glycaemic control might be influenced by anti-anaemic preparations.