You can, and it will do less than the headline says. In matched cohorts from ordinary care, first-time users lost 10.2 kg on tirzepatide and 6.1 kg on semaglutide at twelve months. People who had used a GLP-1 before lost 7.9 kg and 3.7 kg. Both drugs did less the second time.
HbA1c moved the same way. First-timers went −1.3% and −0.9%. The experienced went −0.9% and −0.6%.
Dose is the confound to watch in any comparison of the two. One real-world study gave semaglutide at 1.7 mg and tirzepatide at 5 mg, and tirzepatide came out 2.87 kg ahead [2]. Nobody matched those doses, which is the problem with most head-to-heads on this market.
Switching is normal in this market. Prices move, sellers disappear, a plan ends, a molecule goes unavailable. Behind every switch is one assumption. The new drug will do what the headline says. For someone who has taken one before, it will not. This study puts a number on the difference. It sits underneath the discontinuation figures in two-thirds stop within a year.
What was compared
Researchers built matched cohorts from health records of adults with type 2 diabetes starting either drug. They split them by whether a GLP-1 had been used before [1]. Each naive cohort held 10,702 matched patients and each experienced cohort 5,577.
Weight was recorded for far fewer. Among the naive, 454 tirzepatide and 432 semaglutide patients. Among the experienced, 296 and 224. Those are the numbers behind the weight results, and they are much smaller than the matched cohorts they came from.
What it found
Among first-time users, twelve-month weight change was 10.2 kg on tirzepatide against 6.1 kg on semaglutide. Among those who had used one before, it was 7.9 kg against 3.7 kg. HbA1c moved the same way: −1.3% against −0.9% for the naive, and −0.9% against −0.6% for the experienced (p<0.001 throughout).
What it cannot establish
Why. A cohort study can show that prior exposure predicts a smaller subsequent change. It cannot say why. Physiology, the easy weight already gone, or switchers differing from non-switchers all fit. All three are plausible and none is tested here.
It is a diabetes population rather than a weight-loss one. It is matched rather than randomized. That is the limit on every observational comparison, set out in a number too good to use.
What it means before changing anything
That the case for switching rests on price, availability or tolerability. Not on the new drug delivering what the first did not. The premium between the molecules at one seller is set out in what the other molecule costs, . The severe side-effect profiles were indistinguishable in the largest head-to-head, covered in paying more for fewer side effects.
If the reason to switch is cost, that is a good reason. The arithmetic is in what a GLP-1 actually costs. If the reason is disappointment, this study suggests the second attempt starts from a lower ceiling.