Second Obesity commentary sets out the safety and evidence gaps in tirzepatide dose splitting

A second commentary published online August 9, 2026, in the journal Obesity lays out what its author calls unresolved safety and evidence gaps around "microdosing" tirzepatide — the practice of drawing a smaller-than-labeled amount out of a multi-dose pen ^[1]. The piece responds to an earlier commentary in the same journal, published in June 2026 by a separate group of authors, which first raised the issue ^[2].

Tirzepatide, sold as Zepbound for obesity and Mounjaro for type 2 diabetes, is delivered through pens described in Eli Lilly's FDA prescribing information ^[1]. The design of multi-dose pens, unlike single-use autoinjectors, makes it mechanically possible for a patient or caregiver to withdraw a partial dose with a syringe, something the label does not authorize ^[1][2]. The new letter, written by an endocrinologist at Hospital Universitario Príncipe de Asturias in Madrid, is a Letter to the Editor rather than a new research study — it presents no new patient data or trial results of its own ^[1].

The letter's central argument rests on what tirzepatide's approved doses were actually tested against in clinical trials. The SURMOUNT-1 trial established the drug's weight-loss effect in adults with obesity across its approved dose range ^[1]. A separate 2024 trial found benefit for obstructive sleep apnea at labeled doses ^[1]. And the SURMOUNT-4 trial showed that continuing tirzepatide at higher approved doses sustained weight loss, while stopping the drug led to regain ^[1]. None of these trials tested doses lower than the drug's lowest approved starting dose, so there is no controlled trial evidence on whether a smaller, off-label dose preserves any of these documented effects ^[1].

The commentary also points to a parallel debate that has already played out with semaglutide, the molecule behind Ozempic, Wegovy and Rybelsus. A 2025 exchange in the journal Diabetes Care addressed the same microdosing question for semaglutide multi-dose pens used in diabetes care, suggesting this is not a tirzepatide-only issue but one that touches the whole GLP-1 drug class ^[1]. The letter also cites general literature on the documented harms of off-label drug use, arguing that the physical ease of dose manipulation does not by itself make the practice safe ^[1].

Why it matters for patients

Multi-dose pens make it physically simple to take less medicine than the label calls for, whether to save money, stretch a supply, or reduce side effects. But the trials that proved tirzepatide's benefits for weight loss, sleep apnea, and long-term weight maintenance tested only the approved doses ^[1]. There is currently no published trial evidence showing whether a reduced, self-selected dose delivers the same benefit, a lesser benefit, or none at all. Patients and clinicians weighing this trade-off are, by the letter's own account, working without controlled data to guide them ^[1].

What happens next

The letter was received and revised on June 11, 2026, and accepted on June 22, 2026, before its August 9, 2026 online publication — a fast turnaround for a formal reply ^[1]. The sources reviewed here do not describe any planned clinical trial specifically testing below-label tirzepatide dosing, and it is not yet known whether regulators, Eli Lilly, or independent researchers will launch one. Whether the debate prompts changes to pen design, labeling, or physician guidance also remains unclear from the published record.