How each medication works
Semaglutide is a GLP-1 receptor agonist. It mimics a hormone your gut releases after eating, which slows stomach emptying, reduces appetite signals in the brain, and supports more stable blood sugar. This mechanism is also the basis for semaglutide's earlier approval for type 2 diabetes management, under different brand names and dosing than the versions studied specifically for weight management.
Tirzepatide is a dual GIP and GLP-1 receptor agonist. It activates the GLP-1 pathway described above and also engages the GIP receptor, a second gut hormone pathway thought to further influence appetite and how the body processes fat and sugar. Because it works on two pathways rather than one, researchers have studied whether this combination produces a larger average effect. Scientists are still studying the precise interplay between these two receptor systems, including why combined activation appears, on average, to produce a stronger effect on body weight than GLP-1 activation alone.
Dosing and administration
- Both medications are delivered as a once-weekly subcutaneous injection, typically in the abdomen, thigh, or upper arm.
- Both use a gradual dose-escalation schedule over several weeks to months, which is intended to reduce gastrointestinal side effects.
- Maintenance doses differ by medication and brand, and are set by a prescribing clinician based on tolerance and response, not chosen by the patient.
- Neither medication is a daily pill; both require ongoing clinical follow-up to monitor progress and side effects.
What the semaglutide trial (STEP 1) found
The STEP 1 trial, published in the New England Journal of Medicine in 2021, followed adults with obesity (or overweight with a weight-related condition) who did not have diabetes over 68 weeks. Participants taking semaglutide 2.4 mg weekly, combined with lifestyle intervention, had a mean weight reduction of about 14.9% of body weight, compared with about 2.4% in the placebo group.
These are group averages from a clinical trial, not a promise of individual results. Some participants lost more, some lost less, and about 5% discontinued treatment due to gastrointestinal adverse events. The trial also included regular dietitian-led lifestyle counseling for all participants, which is part of why clinicians often recommend pairing medication with nutrition and activity support rather than viewing medication alone as the entire plan.
What the tirzepatide trial (SURMOUNT-1) found
The SURMOUNT-1 trial, published in the New England Journal of Medicine in 2022, studied adults with obesity or overweight with a weight-related condition, without diabetes, over 72 weeks. At the highest studied dose (15 mg weekly), participants achieved a mean weight reduction of about 20.9%, compared with about 3.1% for placebo, again as trial averages alongside lifestyle intervention.
As with any trial, results reflect averages across a study population under close medical supervision; they are not a guarantee of what any one person will experience.
The head-to-head trial: SURMOUNT-5
Because the two medications had only been compared indirectly across separate trials, researchers conducted SURMOUNT-5, a head-to-head trial published in the New England Journal of Medicine in 2025 that directly compared tirzepatide and semaglutide in the same study population. Tirzepatide was associated with a significantly greater average weight reduction than semaglutide over the study period.
Head-to-head data like this is more informative than comparing two separate trials, because it controls for differences in study design and population. Even so, it remains an average result, and a physician must weigh a person's full medical history, other medications, and goals before recommending either option.
Who typically isn't eligible, and why medical oversight matters
Eligibility is not based on weight goals alone. A physician considers body mass index, weight-related health conditions, prior treatment history, pregnancy status, and other medications before recommending either option. People with a personal or family history of certain thyroid cancers, active pancreatitis, or significant gastrointestinal disease are generally not candidates.
Ongoing medical oversight also matters because both medications require dose titration and monitoring for side effects, and because stopping either medication is typically associated with some regain of weight over time, which is an important part of setting long-term expectations with your physician.
Side effects and the boxed warning
- The most common side effects reported with both medications are gastrointestinal: nausea, diarrhea, vomiting, constipation, and decreased appetite, especially during dose escalation.
- Both drug classes carry a boxed warning regarding thyroid C-cell tumors observed in rodent studies; it is unknown whether this risk occurs in humans, but both medications are contraindicated for people with a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2.
- Other reported risks include gallbladder problems, pancreatitis, and injection-site reactions; a physician reviews personal and family history before prescribing either medication.
A note on compounded versions
Compounded semaglutide and tirzepatide products are not FDA-approved. The FDA has issued public warnings about safety risks associated with some compounded GLP-1 products, including dosing errors, the use of salt forms not reviewed by the agency, and contamination concerns at certain facilities.
Anyone considering these medications should ask their provider whether a product is FDA-approved, and discuss the source and oversight of any medication before starting treatment.
Who determines eligibility
Neither medication is appropriate for everyone. A physician reviews medical history, current medications, lab work when indicated, and personal goals to determine whether semaglutide, tirzepatide, another approach, or no medication at all is the right next step. This article is educational and does not replace an individualized medical evaluation.
Because both medications are prescription-only, no reputable program should offer them without a documented medical evaluation. If a source offers either medication without any review of your health history, that is a signal to ask more questions about who is overseeing your care and whether the product itself is legitimate and FDA-approved.
Frequently asked questions
Is tirzepatide always more effective than semaglutide?
On average, trial data including the SURMOUNT-5 head-to-head study show larger average weight reductions with tirzepatide. However, these are group averages, and individual response depends on many factors a physician evaluates directly.
Can I just choose which medication I want?
No. A licensed physician reviews your health history and determines whether either medication is appropriate for you, and which one may align best with your medical profile and goals.
Are compounded semaglutide or tirzepatide the same as the brand-name versions?
No. Compounded versions are not FDA-approved. The FDA has warned about safety concerns with some compounded GLP-1 products, including dosing inconsistencies, use of unapproved salt forms, and inconsistent sterility practices at certain compounding facilities. Patients should ask whether any product they are offered carries FDA approval before beginning treatment.
What is the boxed warning about?
Both medication classes carry a boxed warning related to thyroid C-cell tumors seen in rodent studies. Whether this risk applies to humans is unknown, and both are contraindicated in people with certain thyroid cancer or endocrine histories.
Do these medications replace diet and exercise?
No. Both trials studied these medications alongside lifestyle intervention, and ongoing nutrition and activity habits remain an important part of any medically supervised weight management plan.
Sources
Treatment requires evaluation by a licensed provider. Eligibility is determined by a physician. Individual results vary. This article is educational and is not medical advice.
