Examining the use of Metformin to prevent weight gain in patients taking antipsychotics

Previous studies have shown that patients who are prescribed antipsychotic medication are at higher risk of excessive weight gain and developing type 2 diabetes (1). This in turn puts them at increased risk of cardiovascular and cerebrovascular mortality, morbidity and is an increasing public health concern.

Certain antipsychotics, such as clozapine and olanzapine, are thought to pose a particularly high risk of weight gain. However, these medications are also often the most effective in treating psychotic symptoms.

Metformin is prescribed as a treatment for type 2 diabetes mellitus and can be used to prevent or delay the onset of diabetes in individuals at high risk of the disease. Metformin can be effective in preventing or treating weight gain in individuals taking antipsychotics. However, the use of metformin to prevent or treat weight gain is not common practice in many clinical settings where antipsychotics are used (2), and patterns of metformin prescription in NWL are not well described in the literature.

Study aims

Understanding the patterns of prophylactic metformin prescription

The aims of this study were to perform retrospective analysis of the longitudinal NWL Discover dataset to establish the prevalence, efficacy, and safety of prophylactic prescribing of metformin in individuals taking antipsychotics who do not have type 2 diabetes.


The benefits of linked data

The Discover dataset is an important advance in the provision of data banks in the UK as it contains linked coded data from primary care, secondary care, community, mental health, and social care and is therefore uniquely able to provide a whole system view of the patient journey throughout their medical care. 

This means that, while all data is depersonalised, we are still able to see the outcomes for patients prescribed with metformin.

Using a four-year data period, the cohort included all patients prescribed with antipsychotics who were not recorded as having a diagnosis of type 2 diabetes before their antipsychotic prescription.

Within this cohort two groups were identified and compared:

  1. Those prescribed with metformin
  2. Those not prescribed with metformin

The groups were then further split into individuals taking antipsychotics in the following categories:

  1. High risk of weight gain
  2. Moderate risk of weight gain
  3. Low risk of weight gain
  4. Those who switched medication during the study period

Further subgroup analysis included a comparison of outcomes between those who had been prescribed metformin before, at the same time and after antipsychotic prescription.

Males and females over 18 were included in the study.

The Discover linked dataset enabled examination of the following:

  • Healthcare (HC) usage including A&E attendances, non-elective admissions, elective admissions, outpatient appointments, community appointments, primary care appointments and HC usage costs.
  • Procedures and interventions including dialysis, percutaneous coronary interventions, and lower limb amputations.
  • Clinical outcomes and comorbidities including development of type 2 diabetes, major adverse cardiac events, end stage renal disease, bone fractures, diabetic ketoacidosis, severe hypoglycaemia, HbA1C, BMI and all-cause mortality.
  • Demographics including age, sex, ethnicity, frailty index and indices of multiple deprivation (IMD).


The results for this study are currently being reviewed. Understanding the prescribing patterns, and outcomes for patients prescribed with metformin to prevent weight gain and type 2 diabetes may help to determine how best to care for these patients.

Next steps

West London NHS Trust are planning to complete further work to refine the exclusion criteria to understand the issue in further detail. The aim will then be to submit the data generated in this study for publication in a peer reviewed journal. The results will be made available to the wider healthcare system, allowing clinicians to review their prescribing practices.


This was a novel type of project for us and the use of the Discover dataset enabled us to investigate patterns of real-world prescribing and outcomes in a very powerful way. The ICHP team were very helpful in guiding us through the steps of conducting the study and producing the results in a clear format for us to interpret.”

Dr Nicholas Stokes
Consultant Clinical Psychologist, West London NHS Trust


  1. Barton BB, Segger F, Fischer K, Obermeier M, Musil R. Update on weight-gain caused by antipsychotics: a systematic review and meta-analysis. Expert opinion on drug safety. 2020 Mar 3;19(3):295-314.
  2. Moin T, Schmittdiel JA, Flory JH, Yeh J, Karter AJ, Kruge LE, Schillinger D, Mangione CM, Herman WH, Walker EA. Review of metformin use for type 2 diabetes prevention. American journal of preventive medicine. 2018 Oct 1;55(4):565-74.


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