Tirzepatide vs Semaglutide: What the SURMOUNT and STEP Trials Actually Show

The GLP-1 Revolution

GLP-1 agonists have transformed obesity treatment. But with tirzepatide showing seemingly superior results, is the comparison fair? Let's look at the actual data.

Weight Loss: Head-to-Head

Semaglutide (STEP Trials)

• STEP 1 (2.4mg weekly): -14.9% body weight at 68 weeks
• STEP 2 (diabetes): -9.6% at 68 weeks
• STEP 3 (with lifestyle): -16.0% at 68 weeks
• STEP 5 (long-term): -15.2% at 2 years

Tirzepatide (SURMOUNT Trials)

• SURMOUNT-1 (15mg): -22.5% at 72 weeks
• SURMOUNT-2 (diabetes): -14.7% at 72 weeks
• SURMOUNT-3 (with lifestyle): -26.6% at 84 weeks
• SURMOUNT-4 (withdrawal): Significant regain after stopping

The Caveats

Trial Duration Differences

SURMOUNT trials generally ran 4 weeks longer than STEP trials. Weight loss curves hadn't fully plateaued, so the difference matters.

Dose Comparison Issues

Semaglutide maximum dose is 2.4mg. Tirzepatide goes up to 15mg. The 5mg tirzepatide dose shows results more comparable to semaglutide 2.4mg.

Direct Head-to-Head

SURPASS-2 compared tirzepatide to semaglutide 1mg (not 2.4mg) — an unfair comparison criticized by the field. No direct comparison to semaglutide 2.4mg exists.

Metabolic Outcomes Beyond Weight

| Outcome | Semaglutide 2.4mg | Tirzepatide 15mg | |---------|-------------------|-------------------| | HbA1c Reduction | -1.6% | -2.1% | | Blood Pressure | -5 mmHg systolic | -8 mmHg systolic | | Triglycerides | -20% | -25% | | Liver Fat (MASH) | -60% | -55% | | Cardiovascular Events | -20% (SELECT) | Pending (SURPASS-CVOT) |

Side Effect Profiles

Both drugs share similar GI side effects (nausea, vomiting, diarrhea). Tirzepatide may cause slightly more GI disturbance at higher doses but has a more gradual titration schedule.

Our Assessment

Tirzepatide likely produces greater weight loss at maximum doses, primarily through the added GIP receptor agonism. However, semaglutide has more long-term safety data, proven cardiovascular benefits (SELECT trial), and wider availability. The "best" choice depends on individual factors — metabolic profile, insurance coverage, and tolerability.