Impact of the MHRA safety update on vitamin B12 testing and coding in metformin users: a retrospective primary care analysis

Background Metformin is the most commonly prescribed oral treatment for type 2 diabetes mellitus (T2DM) in the UK. Long-term therapy has been linked to vitamin B 12 deficiency, a concern recognised for decades but not consistently addressed. In June 2022, the UK Medicines and Healthcare products Regulatory Agency (MHRA) classified low vitamin B 12 levels as a common adverse effect of metformin and advised clinicians to consider periodic testing in at-risk patients. Translating such regulatory advice into practice can be challenging, and the extent to which the guidance has influenced testing and diagnostic coding for vitamin B 12 deficiency remains unclear. This study evaluated trends in vitamin B 12 testing and deficiency coding in metformin-treated patients compared with the general population before and after the 2022 MHRA Drug Safety Update. Methods A retrospective quantitative analysis was conducted using Read code data from 148,000 electronic medical records across three Primary Care Networks (PCNs) in the Southwest of England. Vitamin B 12 testing and deficiency coding rates were compared in patients prescribed metformin and the general population across two periods: pre-guidance (2017–2021) and post-guidance (2022–2024). Welch’s t-tests were used to determine statistical significance, with p < 0.05 considered significant. Results Among patients prescribed metformin, vitamin B 12 testing rates rose from 34.5% (SD = 1.8) pre-guidance to 38.2% (SD = 0.4) post-guidance ( p = 0.008). In the general population, testing rates increased from 12.2% to 14.7% ( p = 0.009). However, coding for vitamin B 12 deficiency remained unchanged at 0.25% in the metformin group and decreased from 0.072% to 0.060% in the general population, with no statistically significant difference. The post-guidance period included only two years of data, which limits the ability to assess longer-term or comparative trends between groups. Conclusions This study demonstrated that the release of the MHRA Drug Safety Update was associated with a modest but statistically significant increase in vitamin B 12 testing among patients prescribed metformin, paralleled by a smaller rise in testing within the general population. However, diagnostic coding practices did not change, suggesting limited translation of safety alerts into structured documentation. Further research is warranted to explore barriers and evaluate interventions to improve monitoring and coding compliance in primary care.