They share a class name, not a receptor profile. Tirzepatide delivers ~6 percentage points more average weight loss than semaglutide in pivotal trials. Tolerance, insurance coverage, and state stock often flip the decision anyway. Here's what to weigh before the provider call - and when the right answer is neither.
The mechanism gap that matters
Semaglutide activates GLP-1 receptors. Tirzepatide activates GLP-1 plus GIP - the extra receptor tightens glucose response and pushes trial weight loss to ~20.9% at 72 weeks on 15 mg (SURMOUNT-1), vs ~14.9% on 2.4 mg semaglutide at 68 weeks (STEP-1).
In T2D trials, the HbA1c drop tracks the same way: tirzepatide brings ~2.0-2.4% reduction at max dose (SURPASS), semaglutide brings ~1.5-1.8% (SUSTAIN). If your A1c sits at 7.5%+ and you're trying to avoid adding insulin, the extra half-point matters.
The trade-off: less real-world history. Semaglutide has a longer post-market safety tail, with tens of millions of patient-years behind it. See the panels that would frame this call.
Side effects: more similar than marketing suggests
Both cause nausea, reflux, and constipation in weeks 1-4 of each dose step. SURMOUNT-5 (the only head-to-head trial) showed tirzepatide is marginally easier on most GI fronts - roughly 20% vs 24% nausea incidence - but neither is gentle during titration.
Fatigue and injection-site tenderness appear on both. Severe or persistent GI symptoms trigger a dose hold or a switch under provider review - not a tough-it-out plan.
Contraindications are identical: MTC, MEN2, pregnancy, pancreatitis, severe gastroparesis. Thyroid history screens the same way regardless of molecule. Run the eligibility check before shopping the molecule.
What actually decides the call
Insurance: if coverage lands on Wegovy or Ozempic, start with semaglutide. If Zepbound is covered, start with tirzepatide. Out-of-pocket: compounded tirzepatide ran $325-500/month in Q1 2026; compounded semaglutide ran $225-400/month.
State stock: compounded access is tightening nationally through 2026. Some states restrict one or both. We confirm availability before any script gets written.
Tolerance history: if a prior GLP-1 course went well on semaglutide, most members re-start there. If nausea was the dealbreaker, tirzepatide is the natural re-entry.
What to do now
Book the assessment with the Advanced panel attached. A1c, fasting insulin, ApoB, and a hormone screen answer "which molecule" faster than any algorithm, and catch the cases where neither is the right first step.
If insurance is the constraint, message a provider before starting so we sequence the script with whichever formulation actually lands covered.