What they are, and why they're different drugs
Semaglutide is a GLP-1 receptor agonist. It mimics one gut hormone that slows stomach emptying, reduces appetite, and improves insulin response. Tirzepatide does that too, but it also activates a second receptor, GIP, which appears to add an independent metabolic effect. They're related. But calling tirzepatide 'stronger semaglutide' misses the point, the second receptor makes it a dual-mechanism drug.
Both were originally developed for type 2 diabetes before their weight-loss effects led to separate approvals under different brand names for that indication.
What the head-to-head trial found
The SURMOUNT-5 trial, published in 2025, directly compared tirzepatide against semaglutide in the same study population rather than across separate trials, which is the only reliable way to compare two drugs. At roughly 72 weeks, tirzepatide produced meaningfully greater average weight loss than semaglutide in that head-to-head comparison.
Before that trial, comparisons relied on cross-trial data: semaglutide's STEP program versus tirzepatide's SURMOUNT program. That's weaker evidence, because the populations, dosing schedules and trial designs differ between separate trials. The direct trial closed that gap. It's the reason tirzepatide is now generally described as the more effective of the two for weight loss specifically.
Side effects: where they're similar and where they differ
Both drugs share the same core side-effect profile: nausea, constipation, and diarrhea, most pronounced during dose increases and easing over time for most people. Neither is risk-free, and both carry the same boxed warning around thyroid C-cell tumors seen in rodent studies, which is why a personal or family thyroid cancer history rules both out.
Discontinuation rates for GI side effects are broadly similar between the two in trial data, tirzepatide's larger effect size doesn't come with a proportionally worse tolerability profile, though individual response varies.
How the decision actually gets made
This isn't a supplement-aisle choice: both require a prescription, medical screening, and ongoing monitoring, and price, insurance coverage, and availability often matter as much as the trial data in practice. Telehealth prescribing has made access easier, but the same screening (thyroid history, pancreatitis history, pregnancy status) applies regardless of how the prescription is obtained.
If cost or access make one available and not the other, that's frequently the deciding factor over the modest average-weight-loss difference between them.
