Semaglutide vs Tirzepatide: Mechanisms, Efficacy Data & Cost Comparison

Semaglutide and tirzepatide are the two dominant injectable agents in the GLP-1-based weight management space. Both are administered once weekly, both cause significant weight loss, and both come with a GI side-effect profile that demands slow titration. Yet their underlying pharmacology differs in a clinically meaningful way, and the pivotal trial data shows a measurable efficacy gap. This guide compares the two across mechanism, trial outcomes, dosing, tolerability, and cost.

Mechanism: GLP-1 Alone vs. GLP-1 + GIP Dual Agonism

Semaglutide is a selective GLP-1 receptor agonist. It mimics endogenous GLP-1, binding to receptors in the pancreas (stimulating glucose-dependent insulin secretion and suppressing glucagon), the brain (reducing appetite via hypothalamic and brainstem pathways), and the gastrointestinal tract (slowing gastric emptying). The result is reduced caloric intake and improved glycemic control.

Tirzepatide adds agonism at the glucose-dependent insulinotropic polypeptide (GIP) receptor to the same GLP-1 receptor effects. GIP is an incretin released in the proximal gut after fat and carbohydrate ingestion. Its role in energy balance is complex — GIP receptors are expressed in adipose tissue and the central nervous system, and preclinical data suggests GIP co-agonism may enhance the weight-loss effect of GLP-1 agonism beyond simple additive insulin secretion. The molecular mechanism for tirzepatide’s superior weight loss likely involves central appetite suppression, altered energy expenditure in adipose tissue, or both.

Efficacy Data: STEP-1 vs. SURMOUNT-1

The STEP-1 trial (Wilding et al., 2021, NEJM) enrolled 1,961 adults with obesity (BMI ≥30) or overweight (BMI ≥27) with at least one weight-related comorbidity. Participants received semaglutide 2.4 mg SC once weekly vs. placebo for 68 weeks. Results: mean body-weight reduction of 14.9% in the semaglutide group vs. 2.4% in the placebo group. 86.4% of semaglutide participants achieved ≥5% weight loss.

The SURMOUNT-1 trial (Jastreboff et al., 2022, NEJM) enrolled 2,539 adults with obesity or overweight with comorbidities. Participants received tirzepatide 5, 10, or 15 mg SC once weekly vs. placebo for 72 weeks. Results: mean weight loss of 15.0%, 19.5%, and 20.9% for the 5, 10, and 15 mg doses respectively, vs. 3.1% for placebo. At the 15 mg dose, 57% of participants achieved ≥20% weight loss.

Direct head-to-head data comes from the SURPASS-2 trial (Frías et al., 2021, NEJM), which compared tirzepatide to semaglutide 1.0 mg (the diabetes dose, not the obesity dose) in patients with type 2 diabetes. All tirzepatide doses outperformed semaglutide on both HbA1c reduction and weight loss. A direct comparison at the semaglutide 2.4 mg obesity dose has not been published as of this writing.

Dosing Schedules

Both agents are administered once weekly by subcutaneous injection. Semaglutide (Wegovy) titrates: 0.25 mg/wk (weeks 1–4) → 0.5 mg/wk (weeks 5–8) → 1.0 mg/wk (weeks 9–12) → 1.7 mg/wk (weeks 13–16) → 2.4 mg/wk maintenance. Tirzepatide (Zepbound) titrates: 2.5 mg/wk (weeks 1–4) → 5.0 mg/wk (weeks 5–8), with optional further escalation to 7.5, 10, 12.5, or 15 mg every 4 weeks as tolerated. Both schedules are designed to minimize GI adverse events by gradual dose escalation.

Side-Effect Profiles

Both agents share a class GI side-effect profile: nausea, vomiting, diarrhea, and constipation, most common during dose escalation and generally attenuating over time. In STEP-1, nausea affected ~44% of semaglutide participants vs. 16% in placebo. In SURMOUNT-1, nausea affected 25–33% of tirzepatide participants depending on dose. Discontinuation due to adverse events was 7% for semaglutide (STEP-1) and 6.2–7.4% for tirzepatide doses in SURMOUNT-1.

Both carry class warnings for thyroid C-cell tumors (seen in rodents; clinical significance in humans unknown), contraindication in personal or family history of MTC or MEN2, and pancreatitis risk. See our full GLP-1 safety guide at /guides/glp-1-receptor-agonist-safety for complete contraindication and drug interaction details.

Cost Per Milligram

Brand-name list prices (before insurance or rebates, as of early 2025): Wegovy (semaglutide 2.4 mg/week) approximately $1,300–1,400/month; Zepbound (tirzepatide, dose-dependent) approximately $1,000–1,100/month. Compounded versions from 503A/503B pharmacies are significantly lower, commonly $200–500/month depending on dose. The FDA has periodically moved to restrict compounded versions as brand-name supply constraints ease. On a per-milligram basis, compounded tirzepatide has historically been less expensive than compounded semaglutide, though pricing shifts frequently. Check the Pepticker vendor comparison page for current market pricing.