When stratified by duration of use, the researchers noted a significant increase in breast cancer risk in patients using GLP-1 receptor agonists for 2-3 years which diminished at more than three years of use, but the low number of events and people taking GLP-1 receptor agonists at this stage make it difficult to draw any true conclusions for longer follow-up periods.
An editorial notes that although the potential risks of pancreatic or thyroid cancer with use of GLP-1 receptor agonists are recognised, the risk of breast cancer has also recently arisen as a potential risk with these drugs. The US FDA and European Medicines Agency separately conducted pooled analyses of four weight management trials of 3.0 mg liraglutide (GLP-1 receptor agonist); 12 (0.29%) breast cancer events were reported in the liraglutide arms vs. 2 (0.08%) events in the placebo arms. However given the rare occurrence of these events, it was unclear whether this difference was due to chance alone or a true increase in breast cancer risk. The editorial suggests that before prescribing, clinicians should discuss the balance of risks and benefits with all patients, including a possible small increase in risk of breast cancer. It calls for further studies to determine with greater confidence whether this risk is real, at what doses the risk appears, and whether it is specific to individual drugs within the larger class of GLP-1 receptor agonists. It adds that as these drugs come off patent and are used more widely in the future, outcomes on larger numbers of patients will be available to compare risks of rare events, such as breast cancer, across drug types and doses and over longer exposure periods.