A University of Pennsylvania team is warning that stop-and-start use of GLP-1 weight-loss drugs could backfire, at least in the lab. In a new preclinical study, mice that were cycled on and off semaglutide mostly regained fat during the off stretches and, after several rounds, did not slim down as much when the drug was restarted. By the end of the experiment, those on-again, off-again mice were significantly heavier than mice that stayed on the medication the whole time.
Study design and key findings
The research letter, published in JCI Insight on March 31, 2026, followed diet-induced obese mice for about four months. One group received continuous semaglutide, while another followed a "two weeks on, two weeks off" schedule. The intermittently treated mice piled weight back on quickly during the withdrawal periods and, by the second cycle, showed a weaker response to the drug. Even after 62 straight days of semaglutide at the end of the study, they stayed roughly 20 percent heavier than the animals that never took a break, according to JCI Insight.
How researchers explain the effect
The Penn team points to shifts in body composition as a likely culprit. GLP-1-driven weight loss often includes a sizable loss of lean mass, and the repeated off-periods appeared to leave the mice with relatively more fat and less ability to regain muscle. "The effectiveness of these medications may depend heavily on consistency," Thomas H. Leung said in a statement, per Penn Medicine.
Local reporting and researcher comments
Philadelphia TV crews quickly picked up the story, zeroing in on the Leung Lab's concern that common stop-start prescribing patterns could set up a kind of therapeutic resistance. Graduate student Emmanuel Rapp summed it up bluntly, saying the data "shows this drug develops some sort of therapeutic resistance that makes it not as effective anymore." As reported by 6ABC, the mouse experiments ran over a four-month period.
How common is stop-start use?
GLP-1 use has exploded across the country: about one in eight U.S. adults now say they have taken one of these medications, according to a KFF Health Tracking Poll from KFF. Sticking with the drugs is another story, and a large cohort analysis reported that more than half of patients discontinue GLP-1 therapy within 24 months, per JAMA Network Open. Those real-world patterns, driven by access, cost, and side effects, helped motivate the Penn group to test intermittent dosing in mice in the first place.
Limits and next steps
The authors are careful to note that this is a preclinical study in male mice, not a verdict on what happens in people. The JCI Insight paper calls for follow-up work in humans and in female animal models before anyone changes treatment recommendations. The team also urges more mechanistic research to see whether the same kind of "diminishing returns" might show up with newer dual-agonist drugs such as tirzepatide, according to JCI Insight.
What clinicians are saying
In its write-up of the study, Penn Medicine suggests that clinicians work with patients to protect lean mass through nutrition and exercise while they are on GLP-1 therapy. The release also encourages doctors and patients to talk upfront about these drugs as a potential long-term commitment, not a quick trial run. Conversations about affordability, side effects, and the realistic chances of staying on a medication remain central to deciding whether GLP-1s are the right long-term strategy for any individual patient.
