Tirzepatide vs semaglutide: what the head-to-head trials actually found

The actual difference between the two molecules

Both are once-weekly injections that mimic gut hormones, but they do not act on the same receptors. That single difference drives most of what follows.

• Semaglutide is a GLP-1 receptor agonist. It is sold as Ozempic and Rybelsus for type 2 diabetes and as Wegovy for weight management.
• Tirzepatide acts on two receptors — GIP and GLP-1. It is sold as Mounjaro for type 2 diabetes and Zepbound for weight management and obstructive sleep apnoea in obesity.
• Both are injected subcutaneously once a week. Semaglutide also exists as a daily tablet (Rybelsus); tirzepatide does not have an approved oral form.
• Neither is a stimulant or a fat burner. Both slow gastric emptying and act on appetite signaling, which is why food volume drops before the scale does.

What the head-to-head trials measured

Two trials compared them directly rather than comparing separate studies, which is the only fair way to read the numbers.

• SURPASS-2 (type 2 diabetes, 40 weeks) compared tirzepatide 5, 10 and 15 mg against semaglutide 1 mg. All three tirzepatide doses produced larger HbA1c reductions and larger weight reductions than semaglutide 1 mg.
• SURMOUNT-5 (obesity without diabetes, 72 weeks) compared maximum-tolerated tirzepatide against maximum-tolerated semaglutide 2.4 mg, and reported roughly 20% versus roughly 14% mean body-weight reduction.
• In their separate registration trials, tirzepatide 15 mg averaged about 21% weight reduction at 72 weeks (SURMOUNT-1) and semaglutide 2.4 mg about 15% at 68 weeks (STEP 1).
• Averages hide spread. In every one of these trials a meaningful share of participants lost far more, and a share lost very little, on both drugs.

Side effects and tolerability

The side-effect profiles are more alike than the efficacy numbers. Both labels carry the same major warnings.

• Nausea, vomiting, diarrhea and constipation are the most common complaints on both, usually worst in the days after a dose increase.
• Both labels carry a boxed warning for thyroid C-cell tumours seen in rodents, and both are contraindicated with a personal or family history of medullary thyroid carcinoma or MEN 2.
• Pancreatitis, gallbladder disease, kidney injury from dehydration and worsening diabetic retinopathy appear in the labels for both.
• Discontinuation rates for side effects in the head-to-head trials were broadly similar, so 'stronger' does not automatically mean 'harder to tolerate' for any individual.

What to record while you are on either one

Escalation schedules run for months, and the detail that matters at a clinic visit is the thing nobody remembers four weeks later.

• Dose and the date it changed. Both drugs step up on a fixed schedule, and the side effects you feel almost always trace back to a step.
• Injection day and site. Weekly dosing is easy to lose track of once a dose lands on a busy week.
• Nausea and appetite scored the same way each week, so a pattern is visible instead of remembered.
• Weight on the same day of the week, and blood pressure or glucose if your clinician asked for them.
• Other medicines in the same window. Slowed gastric emptying can change how oral medicines are absorbed, which is worth checking rather than assuming.

References

• SURPASS-2: Tirzepatide versus semaglutide once weekly in type 2 diabetes (NEJM 2021)
• SURMOUNT-1: Tirzepatide once weekly for the treatment of obesity (NEJM 2022)
• STEP 1: Once-weekly semaglutide in adults with overweight or obesity (NEJM 2021)
• FDA prescribing information — Zepbound (tirzepatide)
• FDA prescribing information — Wegovy (semaglutide)