The GLP-1 receptor agonist class has become the most clinically significant development in metabolic medicine in decades. Semaglutide (Ozempic/Wegovy) and tirzepatide (Mounjaro/Zepbound) are the two dominant compounds in this space, and their head-to-head comparison is one of the most clinically relevant questions in metabolic health today.

This guide synthesizes the published clinical trial data - primarily the STEP program (semaglutide) and the SURMOUNT program (tirzepatide) - and compares the two compounds on efficacy, safety, mechanism, and practical use.

Mechanism: Where They Differ

Semaglutide: GLP-1 Receptor Agonist

Semaglutide is a GLP-1 (glucagon-like peptide-1) receptor agonist. GLP-1 is an incretin hormone secreted by intestinal L-cells in response to food intake. Its primary actions are:

Stimulation of glucose-dependent insulin secretion from pancreatic beta cells
Suppression of glucagon from alpha cells
Delayed gastric emptying (slows food movement through GI tract)
Central appetite suppression via hypothalamic GLP-1 receptors

Semaglutide is a modified GLP-1 analogue with ~94% homology to native GLP-1, with modifications that extend its half-life to approximately 7 days, enabling once-weekly dosing.

Tirzepatide: Dual GIP/GLP-1 Receptor Agonist

Tirzepatide is a "twincretin" - a single molecule that activates both the GLP-1 receptor and the GIP (glucose-dependent insulinotropic polypeptide) receptor. GIP is another incretin hormone, previously considered less therapeutically interesting than GLP-1. The addition of GIP agonism is the key mechanistic distinction.

GIP receptor activation in the context of tirzepatide appears to:

Enhance the weight loss effects beyond what GLP-1 agonism alone achieves
Improve tolerability of GI side effects relative to pure GLP-1 agonism at equivalent weight loss
May have direct adipose tissue effects (GIP receptors are expressed in adipocytes)

Efficacy: What the Trials Show

STEP Trials (Semaglutide)

Trial	Population	Dose	Weight Loss	Duration
STEP 1	Obesity, no T2D	2.4 mg/week	-14.9% body weight	68 weeks
STEP 2	Obesity + T2D	2.4 mg/week	-9.6% body weight	68 weeks
STEP 3	Obesity + lifestyle	2.4 mg/week	-16.0% body weight	68 weeks
STEP 4	Sustained treatment	2.4 mg/week	-17.4% body weight (continued arm)	68 weeks

SURMOUNT Trials (Tirzepatide)

Trial	Population	Dose	Weight Loss	Duration
SURMOUNT-1	Obesity, no T2D	15 mg/week (max)	-20.9% body weight	72 weeks
SURMOUNT-2	Obesity + T2D	15 mg/week (max)	-15.7% body weight	72 weeks
SURMOUNT-3	Obesity + lifestyle lead-in	15 mg/week	-26.6% body weight	72 weeks
SURMOUNT-4	Sustained treatment	10-15 mg/week	-25.3% continued arm	88 weeks

Direct head-to-head: The SURPASS-6 trial compared tirzepatide directly to semaglutide in T2D patients. Tirzepatide 15 mg produced approximately 2.9% greater weight loss than semaglutide 1 mg over 40 weeks. At higher semaglutide doses (2.4 mg, the Wegovy dose), the gap narrows but tirzepatide still demonstrates superior weight loss in available comparative data.

Side Effect Comparison

Side Effect	Semaglutide	Tirzepatide	Notes
Nausea	44% (any grade)	33-40%	Both peak during uptitration
Vomiting	24%	18-23%	Tirzepatide slightly lower
Diarrhea	30%	22-30%	Similar between compounds
Constipation	24%	17-23%	Common, underreported
Hypoglycemia	Low (without sulfonylurea)	Low (without sulfonylurea)	Both glucose-dependent mechanism
Pancreatitis	Rare (<0.3%)	Rare (<0.3%)	Class effect; contraindicated with pancreatitis history
Thyroid C-cell effects	Rodent carcinoma signal	Rodent carcinoma signal	Class contraindication: personal/family history MTC or MEN2

The GI side effect burden is generally lower with tirzepatide at equivalent weight loss outcomes - a clinically meaningful difference given that GI side effects are the primary reason for discontinuation in both compound classes.

Dosing Schedules

Semaglutide (Wegovy - weight management indication)

Weeks 1-4: 0.25 mg/week
Weeks 5-8: 0.5 mg/week
Weeks 9-12: 1.0 mg/week
Weeks 13-16: 1.7 mg/week
Week 17+: 2.4 mg/week (maintenance)

Tirzepatide (Zepbound - weight management indication)

Weeks 1-4: 2.5 mg/week
Weeks 5-8: 5 mg/week
Weeks 9-12: 7.5 mg/week (optional plateau)
Weeks 13-16: 10 mg/week (optional plateau)
Weeks 17-20: 12.5 mg/week
Week 21+: 15 mg/week (maintenance)

Regulatory Status and Access

Compound	Brand (T2D)	Brand (Weight Mgmt)	FDA Approval Status
Semaglutide	Ozempic (0.5-2 mg)	Wegovy (2.4 mg)	FDA approved T2D (2017), obesity (2021)
Tirzepatide	Mounjaro (2.5-15 mg)	Zepbound (2.5-15 mg)	FDA approved T2D (2022), obesity (2023)

Which to Choose?

Based on the available evidence, a reasonable framework for compound selection:

For maximum weight loss efficacy: Tirzepatide has consistently demonstrated superior absolute weight reduction in published trials
For GI tolerability: Tirzepatide generally produces lower GI burden at equivalent weight loss
For established long-term data: Semaglutide has a longer post-approval track record and more cardiovascular outcome data (SELECT trial showing 20% reduction in MACE in non-diabetic obese patients)
For cost: Both are expensive without insurance coverage; compounded versions have been available during shortage periods but regulatory status varies by jurisdiction and changes frequently

The critical caveat: this is not a decision to make without physician involvement. Both compounds carry meaningful contraindications (MEN2, medullary thyroid carcinoma history, pancreatitis history, active GI disease) and require appropriate monitoring. The evidence comparison above is educational context, not a prescribing framework.

Semaglutide vs Tirzepatide: Head-to-Head Evidence Comparison