Semaglutide and tirzepatide are typically evaluated through dose escalation, yet many patients in routine care do not reach a recommended maintenance dose due to tolerability, access, cost, supply, or individualized goals. Here, we used repeated 0.25 mg only semaglutide and 2.5 mg only tirzepatide prescription records as real-world proxies for sustained treatment at the lowest approved dose, a prescribing pattern that may overlap with, but does not establish, intentional microdosing. We analyzed de-identified electronic health records (EHRs) from a federated system of 29 million patients. Among 490 072 semaglutide-treated patients, 814 met the stringent sustained low-dose criterion of at least three distinct 0.25-mg prescriptions spanning at least 6 months from the first to the last qualifying prescription and among 322 442 tirzepatide-treated patients, 1016 met the analogous criterion of at least three distinct 2.5-mg prescriptions spanning at least 6 months. In a matched head-to-head comparison aided by AI-enabled curation of clinical notes, sustained low-dose tirzepatide showed higher 24-month adverse event rates (P < .05) than sustained low-dose semaglutide for constipation (32.6% vs. 22.4%), acute kidney injury (2.7% vs. 0.4%), muscle cramps (8.3% vs. 4%), lumbar disc disease (3.3% vs. 0.3%), dyspnea on exertion (9.6% vs. 6.8%), lentigo (4.7% vs. 0.8%), and actinic keratosis (3.3% vs. 0.6%), whereas sustained low-dose semaglutide (vs. tirzepatide) had higher rates of including otitis (5.6% vs. 2.1%), diaphoresis (5.5% vs. 3.2%), and ankle swelling (6.9% vs. 3.7%). Sustained low-dose tirzepatide was associated with greater weight loss than semaglutide (mean 12-month percent weight change -5.5% vs. -2.2%; P < .05 at all timepoints). Analyses of prescription patterns supported the findings from clinical notes. For example, renal-supportive and hyperkalemia therapies increased after low-dose tirzepatide (5.6% to 9.5% at 12 months, P = .016) but not after low-dose semaglutide (8.2% to 6.6%, P = .382). Taken together, these observational associations motivate prospective studies to better characterize the outcomes and pharmacology of sustained initiation-dose semaglutide or tirzepatide, and related microdosing studies.
