Wellness
April 19, 2025

Semaglutide and Tirzepatide for Weight Loss: What to Know

Semaglutide and tirzepatide are prescription injections that reduce appetite and are FDA-approved for long-term weight management under the brand names Wegovy and Zepbound. They are approved for adults with obesity, or with overweight plus a weight-related condition, alongside a reduced-calorie diet and more physical activity. Both carry a boxed warning about thyroid tumors, and the FDA has raised concerns about unapproved and compounded versions.

This guide explains how the two medicines work, who they are approved for, what results and side effects to expect, and what to check about where the medicine comes from.

What semaglutide and tirzepatide are

Both are once-weekly injections given under the skin.

Semaglutide mimics a gut hormone called GLP-1 that acts on the areas of the brain that regulate appetite. Its brand names are Wegovy for weight management and Ozempic for type 2 diabetes.
Tirzepatide activates the receptors for two gut hormones, GLP-1 and GIP, to reduce appetite and food intake. Its brand names are Zepbound for weight management and Mounjaro for type 2 diabetes.

Both also slow the emptying of the stomach. They are manufactured drugs, not plant extracts or supplements.

Who semaglutide and tirzepatide are approved for

The FDA approved Zepbound in November 2023 for chronic weight management in adults with:

obesity, meaning a body mass index (BMI) of 30 or greater; or
overweight, meaning a BMI of 27 or greater, with at least one weight-related condition such as high blood pressure, type 2 diabetes or high cholesterol.

The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) gives the same thresholds for weight management medicines in general. It also says they are “meant to help people who have health problems related to overweight or obesity,” and advises never taking one only to improve the way you look.

How much weight people lose

In the larger of the two trials behind Zepbound’s approval, adults without diabetes on the highest dose lost on average 18% of their body weight compared with placebo over 72 weeks. In the trial of adults with type 2 diabetes, the figure was 12%.

For weight management medicines as a group, the NIDDK says people lose 3% to 12% more of their starting weight after a year than people in a lifestyle program alone, and that with some medications more than half of participants lose 10% or more. Results vary by medication and by person.

Two points from the NIDDK put those numbers in context:

the medicines work best combined with a lifestyle program, and do not replace healthy eating and physical activity;
you will probably regain some weight after stopping.

Side effects and warnings

MedlinePlus lists the common side effects of both medicines as nausea, vomiting, diarrhea, constipation and stomach pain or upset.

Boxed warning. Both caused thyroid tumors in rats. It is not known whether they do so in humans. The FDA says Zepbound should not be used by anyone with a personal or family history of medullary thyroid cancer or with Multiple Endocrine Neoplasia syndrome type 2. The same applies to semaglutide.

Other warnings. The FDA’s list for Zepbound includes inflammation of the pancreas, gallbladder problems, low blood sugar, acute kidney injury, eye problems in people with type 2 diabetes, and suicidal behavior or thinking. Severe stomach pain that does not go away, or signs of an allergic reaction, need urgent medical attention.

Pregnancy. The NIDDK says never to take weight management medicines during pregnancy or when planning one.

Compounded and unapproved versions

This matters as much as the medicine itself. The FDA says compounded drugs “are not FDA approved,” which means the agency does not review them for safety, effectiveness or quality, and that they should be used only when a patient’s medical needs cannot be met by an FDA-approved drug.

The FDA’s stated concerns include:

Dosing errors. It has received reports of adverse events, some needing hospital treatment, linked to patients measuring the wrong dose of compounded semaglutide.
Salt forms. Some compounded products use semaglutide sodium or semaglutide acetate, which are different active ingredients from the one in the approved drug.
Fraudulent products. Some carry labels naming pharmacies that do not exist or did not make them.
Storage. These injections need refrigeration, and the FDA advises not using one that arrives warm.

As of May 31, 2026, the FDA had received 990 adverse event reports for compounded semaglutide and more than 730 for compounded tirzepatide. It notes that such events are likely under-reported.

The FDA lists warning signs: a claim that a compounded drug is “the same as” an approved one, prices that seem too good to be true, and no screening or prescription by a licensed clinician.

How long treatment lasts

The NIDDK explains that if a medicine is helping and you have no serious side effects, your clinician may advise staying on it indefinitely. If you have not lost at least 5% of your starting weight after 12 weeks on the full dose, they will probably advise stopping.

Questions to ask before starting

Is this the FDA-approved product, or a compounded version? If compounded, why?
Which pharmacy supplies it, and is it state-licensed?
Do I meet the BMI criteria the medicine is approved for?
Do I have any condition, such as a family history of thyroid cancer or past pancreatitis, that needs to be considered first?
What monitoring and follow-up will I have?
What is the plan if I stop?