Semaglutide and tirzepatide are the two dominant GLP-1 medications — but they work differently, produce different side effect profiles, and suit different patients. This side-by-side comparison covers what the clinical data shows about tolerability, not just efficacy.
Mechanism of Action: The Key Difference
Semaglutide (Ozempic, Wegovy) targets the GLP-1 receptor only. Tirzepatide (Mounjaro, Zepbound) targets both GLP-1 and GIP receptors — making it a dual agonist. This dual mechanism explains both the higher average weight loss with tirzepatide and its somewhat different side effect profile.
| Side Effect | Semaglutide | Tirzepatide |
|---|---|---|
| Nausea | ~44% (STEP trials) | ~31% (SURMOUNT trials) |
| Diarrhea | ~30% | ~23% |
| Vomiting | ~24% | ~12% |
| Constipation | ~24% | ~11% |
| Hair loss | ~2.5% vs 1.0% placebo | ~5.7% vs 0.9% placebo (15mg) |
| Injection site reactions | ~3.2% | ~7.2% |
| Avg weight loss (highest dose) | ~15–17% | ~20–26% |
Reading the Numbers
Side effect percentages come from clinical trials where patients were carefully monitored and dose-titrated on schedule. Real-world rates may differ — slower titration often reduces GI side effects significantly. Most side effects are transient, peaking in the first 4–8 weeks and declining as the body adjusts.
GI Side Effects: Semaglutide vs Tirzepatide
Both medications cause gastrointestinal side effects — nausea, diarrhea, vomiting, constipation — through their shared GLP-1 mechanism (delayed gastric emptying, appetite suppression). However, the SURMOUNT trials generally showed lower GI side effect rates for tirzepatide compared to semaglutide in the STEP trials. The GIP receptor component may have a moderating effect on gastric emptying that partially offsets the GLP-1-driven GI disruption.
In clinical practice, the most important factor for GI tolerability isn't which drug you choose — it's how you titrate. Slow, gradual dose increases (staying at each tier for 4+ weeks rather than the minimum 2 weeks) dramatically reduce nausea and vomiting for both medications.
Hair Loss: Tirzepatide Has a Higher Signal
Hair loss (typically telogen effluvium, not pattern baldness) was reported at higher rates with tirzepatide — 5.7% at the highest dose vs 2.5% for semaglutide. This likely correlates with the greater magnitude of weight loss rather than a direct drug effect. Faster, larger weight loss creates more metabolic stress, pushing more hair follicles into the resting phase simultaneously.
Which Should You Choose?
Consider semaglutide if: you want the most extensively studied GLP-1 option (larger safety database), you're sensitive to injection site reactions, or you prefer a single-receptor mechanism with a longer real-world track record.
Consider tirzepatide if: you want maximum weight loss potential, you've plateaued on semaglutide, you prefer potentially lower GI side effects, or the dual-agonist mechanism appeals to you clinically.
Important: Neither medication should be chosen based on side effect tables alone. Your medical history, contraindications, insurance coverage, and clinical goals all factor in. A licensed physician should help you decide — and be available to switch you if the first choice doesn't work.
Providers Offering Both Medications
All providers are US-licensed telehealth platforms. Availability varies by state.
⚕️ Compounded medications are not FDA-approved. They are prepared by licensed pharmacies under physician supervision.
⚕️ Compounded medications are not FDA-approved. They are prepared by licensed pharmacies under physician supervision.
⚕️ Compounded medications are not FDA-approved. They are prepared by licensed pharmacies under physician supervision.
Sources & References
- Wilding JPH, et al. "STEP 1." NEJM. 2021;384:989–1002.
- Jastreboff AM, et al. "SURMOUNT-1." NEJM. 2022;387:327–340.
- Novo Nordisk. Wegovy Prescribing Information. Adverse reactions table.
- Eli Lilly. Zepbound Prescribing Information. Adverse reactions table.
- FDA FAERS. GLP-1 Adverse Event Reports. 2024–2025.