How they work — and why the mechanism matters
Wegovy (semaglutide 2.4mg, by Novo Nordisk) is a GLP-1 receptor agonist. It mimics glucagon-like peptide-1, a hormone your gut produces after eating. GLP-1 slows gastric emptying, signals satiety to the brain, and reduces appetite. Injected once weekly, it builds to therapeutic levels over a titration period of about 16–20 weeks.
Zepbound (tirzepatide, by Eli Lilly) is a dual agonist — it activates both GLP-1 receptors and GIP (glucose-dependent insulinotropic polypeptide) receptors. Adding GIP receptor activation appears to produce greater appetite suppression and metabolic effects than GLP-1 alone. This is the mechanism behind the larger average weight loss numbers in clinical trials.
Both are once-weekly subcutaneous injections. Both require titration from a low starting dose. Both produce side effects in a similar category (primarily GI: nausea, constipation, vomiting) though the severity profiles differ somewhat.
What the clinical trial data actually shows
The headline numbers from the two pivotal trials:
| Metric | Wegovy (STEP 1) | Zepbound (SURMOUNT-1) |
|---|---|---|
| Study duration | 68 weeks | 72 weeks |
| Avg weight loss (top dose) | ~15% body weight | ~22% body weight |
| % achieving ≥5% loss | 87% | 91% |
| % achieving ≥20% loss | ~30% | ~57% |
Zepbound's advantage is real. At top doses, the average patient loses about 7 percentage points more body weight. For a 250-pound patient, that's roughly 17 additional pounds at the end of the trial period.
The important caveat: these are averages. Individual response varies significantly. Some patients achieve 25%+ on Wegovy; some plateau below 10% on Zepbound. The trial numbers tell you where the population center is, not where you'll land. Your physician's job is to monitor your actual response and adjust accordingly.
These trials were funded by the manufacturers and compare each drug against placebo, not directly against each other. A head-to-head trial exists (SURMOUNT-5) and early results favor tirzepatide, but the full picture is still emerging. The gap is real; the exact magnitude is still being studied.
Side effects: more similar than different
Both medications share a similar side effect profile dominated by GI symptoms, particularly during dose escalation:
- Nausea — most common, usually improves after the first 2–4 weeks at each dose level
- Constipation — common, manageable with hydration, fiber, and sometimes a stool softener
- Vomiting — less common than nausea, typically dose-related
- Fatigue — particularly in the first weeks of a new dose
- Reduced appetite — this is partly the point, but can be pronounced
Serious side effects (pancreatitis, gallbladder disease, thyroid concerns) are real but uncommon. We screen for risk factors before starting and monitor with regular labs.
Some patients tolerate one better than the other — this is individual variation, not a predictable pattern. If you have significant nausea on Wegovy, switching to tirzepatide doesn't guarantee improvement. Dose adjustment within the same medication is usually the first intervention.
Cost and insurance: often the deciding factor
Cash price for both medications at a US pharmacy runs roughly $1,000–$1,300/month without insurance. This is often the deciding factor in which medication a patient uses — not clinical preference.
Insurance coverage varies significantly:
- Many commercial plans cover both with prior authorization for qualifying BMI and comorbidity criteria
- Medicare Part D as of 2026 covers Wegovy for cardiovascular risk reduction (not weight loss per se) for qualifying patients — coverage for Zepbound is more limited
- Medicaid coverage varies by state
Both manufacturers have patient assistance programs that can substantially reduce out-of-pocket cost for commercially insured patients who don't hit their deductible. Our program includes insurance coordination support — we help with the prior authorization documentation, though we can't guarantee approval.
One practical note: if your insurance covers one but not the other, that medication is the right choice for you, clinical trial averages aside. A medication you can afford and take consistently outperforms the technically superior medication you can't sustain.
How to actually choose between them
In order of what usually matters most in the real world:
1. Insurance coverage. Check what your plan covers before anything else. If one is covered and the other isn't, that's your answer in most cases.
2. Your physician's assessment. Certain medical histories, current medications, and metabolic profiles make one a better fit than the other. This isn't something you can self-assess from trial data — it's what the initial medical evaluation is for.
3. Clinical data if you're paying cash. If cost is comparable and there's no medical reason to prefer one, Zepbound's trial data suggests it's the higher-probability option for greater weight loss at equivalent tolerable doses. That's a reasonable tiebreaker.
4. Personal history. If you've tried one before and had significant side effects or poor response, switching to the other is a reasonable next step.
We discuss both medications at every weight loss consultation and make a recommendation based on your individual situation — insurance coverage, medical history, prior GLP-1 experience, and goals. We don't default to one because it's on promotion or because one has a better commission structure.