LISBON – People with type 2 diabetes who take metformin for many years are more likely to develop anemia than are those who do not, according to the results of a large analysis of data from an observational, population-based study with 20 years of follow-up.
“Metformin treatment was associated with a 6% higher risk of anemia for every cumulative year of metformin exposure,” Louise Donnelly, PhD, and her associates reported in a poster presentation at the annual meeting of the European Association for the Study of Diabetes.
In an interview, Dr. Donnelly, a postdoctoral research assistant at the University of Dundee (Scotland), explained why they looked at the use of metformin and anemia risk in people with type 2 diabetes.
“The Diabetes Prevention Program (DPP) study showed that long-term metformin use in individuals with impaired glucose tolerance was associated with an increased risk of anemia, and this was independent of vitamin B12 status,” she said ( ). “Anemia is a common finding in people with type 2 diabetes, but the impact of long-term metformin use on anemia hasn’t been studied.”
Dr. Donnelly and her associates obtained detailed information on metformin prescribing and hematology measures from electronic patient medical records from the cohort, based in Scotland. This database contains information on individuals with type 2 diabetes and matching controls and is available to researchers worldwide.
For the analysis, the team looked for people diagnosed from 1996 onward who had a baseline hemoglobin measurement. Of 6,440 individuals with type 2 diabetes in the GoDARTS cohort, just over half had a hemoglobin measurement.
“We used a definition of ‘moderate’ anemia and we excluded patients with mild anemia or worse at diabetes diagnosis,” Dr. Donnelly observed. Anemia was considered to be a hemoglobin level of less than 12 g/dL in women and less than 13 g/dL in men. In all, 280 individuals with anemia were excluded from further analysis as the aim was to follow people until they developed anemia, died, left the area, or until the end of the follow-up period, which was set at September 30, 2015. A discrete-time failure analysis was used to model the effect of cumulative metformin exposure on anemia risk.
After a median follow-up of 8 years and a median number of 11 hemoglobin measurements per patient, 2,487 study subjects (71%) had some exposure to metformin and 1,458 of the whole sample (41.8%) had become anemic. Of those who developed anemia, 745 (51%) were current metformin users, 194 (13%) were former users, and 519 (36%) had never taken metformin.
“Cumulative metformin use was independently associated with an increased risk of anemia,” Dr. Donnelly noted (odds ratio [OR], 1.06; 95% confidence interval [CI], 1.02-1.09; P = .0006). This association was not seen when they examined the data based on sulfonylurea use (OR 1.0; 95% CI 0.97-1.04, P = .8), she added.
“Anemia risk was higher with age at diagnosis, duration of diabetes, lower hemoglobin at baseline, and lower eGFR [estimated glomerular filtration rate],” she observed. ORs for first anemia event were 1.03 (95% CI, 1.02-1.04) for every year of increasing age, 1.05 (95% CI, 1.03-1.08) for every additional year since diabetes diagnosis, 0.70 (95% CI, 0.66-0.74) per 1 g/dL of hemoglobin at diagnosis, and eGFR 0.98 (95% CI, 0.98-1.01) per additional 1 mL/min per 1.732 (P less than .0001 for all).
Why cumulative metformin use is associated with an increased of anemia is unclear, however, and Dr. Donnelly noted that this needs further investigation. “We do have data from two other clinical trials now, showing similar results, and maybe through those data we might be able to untangle it.”
The team does not think the anemia is related to B12 deficiency, however, as people who developed anemia while taking metformin were more likely to develop microcytic (12% vs. 7.3%) than macrocytic (7.6% vs. 12.3%), anemia, compared with people with anemia who were not exposed to metformin (P less than .0001).
“In terms of mechanism, we can only conjecture,” , MB, senior author of the study and professor of medicine at the University of Dundee, said during a discussion at the poster presentation. “It is important to stress that metformin is a great drug and we shouldn’t stop it because of a potentially increased risk of anemia.”
The Medical Research Council supported the work. Dr. Donnelly reported having no financial disclosures.