GLP-1 Weight-Loss Medications: How They Work, Safety, Side Effects, and Expected Results
Ozempic and Mounjaro are among the best-known medicines currently associated with weight loss, but they are not the same drug. Ozempic contains semaglutide, a GLP-1 receptor agonist, while Mounjaro contains tirzepatide, which activates both GIP and GLP-1 receptors.
Both are prescription medicines that may help eligible patients lose weight by reducing appetite and food intake. However, they are not suitable for everyone and can cause side effects and important risks that require medical supervision.
What is the difference between Ozempic, Mounjaro, Wegovy, and Zepbound?
| Active ingredient | How it works | Common brand names | Main approved use |
|---|---|---|---|
| Semaglutide | GLP-1 receptor agonist | Ozempic and Wegovy | Ozempic is mainly approved for type 2 diabetes, while Wegovy is approved for chronic weight management in eligible patients. Approved uses may vary by country. |
| Tirzepatide | Dual GIP and GLP-1 receptor agonist | Mounjaro and Zepbound | Mounjaro is mainly approved for type 2 diabetes, while Zepbound is approved for chronic weight management in eligible patients. Approved uses may vary by country. |
How do GLP-1 medicines help with weight loss?
These medicines act on hormone pathways involved in appetite, blood glucose control, and digestion. They may help by:
- Reducing hunger and food cravings.
- Increasing fullness after meals.
- Lowering overall calorie intake.
- Slowing stomach emptying, especially early in treatment.
- Improving blood glucose control in some people with diabetes.
They do not directly “melt” fat and do not block calorie absorption. Weight loss mainly occurs because the person eats less over time.
How much weight can someone lose?
The numbers below are averages from clinical trials and are not guaranteed results for every patient.
Semaglutide
In the STEP 1 trial, adults with obesity or overweight without diabetes lost an average of about 14.9% of their body weight after 68 weeks with semaglutide 2.4 mg once weekly plus lifestyle changes.
Tirzepatide
In the SURMOUNT-1 trial, average weight loss after 72 weeks ranged from about 15% to 20.9%, depending on the dose.
In a direct comparison study, average weight loss was about 20.2% with tirzepatide versus 13.7% with semaglutide at the doses studied. This does not mean tirzepatide is automatically the best option for every patient.
Results vary according to the dose, treatment duration, tolerance, diabetes status, diet, physical activity, and overall health.
Who may be eligible for these medicines?
Weight-management medicines are commonly considered for adults with obesity, or for adults with overweight plus a weight-related condition such as high blood pressure, type 2 diabetes, or abnormal cholesterol levels.
The decision should not be based only on weight. It requires medical assessment of health history, current medicines, contraindications, and the follow-up plan. These medicines should support, not replace, a sustainable healthy weight-management approach.
What are the common side effects?
Most common side effects involve the digestive system and may be more noticeable when treatment starts or when the dose is increased.
- Nausea.
- Vomiting.
- Diarrhea.
- Constipation.
- Abdominal pain or bloating.
- Indigestion or reflux.
- Reduced appetite.
- Fatigue or dizziness in some people.
Nausea does not mean the medicine is working better. Severe vomiting, inability to eat, or inability to keep fluids down requires medical advice.
What are the important risks?
Pancreatitis
Severe, persistent abdominal pain, especially if it spreads to the back or occurs with vomiting, requires urgent medical evaluation.
Gallstones or gallbladder inflammation
Gallbladder problems may occur, and rapid weight loss itself may increase the risk of gallstones. Pain in the upper-right abdomen, fever, or jaundice should be assessed medically.
Dehydration and kidney problems
Persistent vomiting or diarrhea can cause dehydration and may affect kidney function. Inability to keep fluids down requires medical review.
Low blood sugar
The risk is higher when these medicines are used with insulin or certain diabetes medicines such as sulfonylureas.
Severe gastroparesis or digestive symptoms
Semaglutide and tirzepatide are generally not recommended for people with severe gastroparesis. Severe digestive disorders or previous gastrointestinal surgery should be discussed with the prescriber.
Is there a boxed warning about thyroid cancer?
Yes. Semaglutide and tirzepatide products in the United States carry a boxed warning about the risk of thyroid C-cell tumors.
These tumors occurred in rodents during studies, but it has not been proven that the medicines cause medullary thyroid carcinoma in humans. Even so, they are contraindicated in people with:
- A personal or family history of medullary thyroid carcinoma.
- Multiple endocrine neoplasia syndrome type 2, known as MEN2.
A neck lump, persistent hoarseness, difficulty swallowing, or difficulty breathing should be evaluated by a healthcare professional.
Who should avoid these medicines or use extra caution?
- People who are pregnant or planning pregnancy.
- People with medullary thyroid carcinoma or MEN2.
- People with a previous serious allergic reaction to the active ingredient.
- People with severe gastroparesis.
- People with a history of pancreatitis or gallbladder disease, after medical assessment.
- People using insulin or certain diabetes medicines.
- People who are due to have surgery, anesthesia, or endoscopy.
The anesthesia team should be told about these medicines because they may delay stomach emptying and increase the chance that food remains in the stomach during a procedure.
Can they be used during pregnancy?
Weight-loss medicines should not be used during pregnancy. Current US labeling for semaglutide advises stopping it at least two months before a planned pregnancy.
Tirzepatide may reduce the effectiveness of oral contraceptives when treatment starts or the dose is increased, so the most appropriate contraceptive method should be discussed with a healthcare professional.
What happens after treatment stops?
Weight regain is common after treatment is stopped. In a follow-up to the semaglutide STEP 1 trial, participants regained about two-thirds of the weight they had lost during the following year.
This is why obesity is treated as a chronic condition that may require a long-term plan involving nutrition, physical activity, muscle preservation, and medical follow-up.
Do these medicines cause muscle loss?
Weight loss may include loss of both fat mass and lean mass. To reduce unnecessary muscle loss, adequate protein intake and resistance exercise are important when medically appropriate.
Supplements cannot replace enough food, protein, or fluids. General hydration guidance can support the plan, but persistent vomiting or dehydration needs medical attention. See Vitaminati’s guide to hydration and water intake.
Conclusion
Medicines such as Ozempic, Mounjaro, Wegovy, and Zepbound may help eligible patients achieve meaningful weight loss, but they are powerful prescription medicines rather than quick cosmetic solutions.
Results differ from person to person, and important side effects or risks may occur. They should be used under medical supervision, with gradual dose escalation and ongoing monitoring of symptoms, nutrition, blood glucose, and overall health.
Frequently asked questions
Are Ozempic and Wegovy the same?
Both contain semaglutide, but they have different approved uses and dosing schedules.
Are Mounjaro and Zepbound the same?
Both contain tirzepatide. Mounjaro is mainly approved for type 2 diabetes, while Zepbound is approved for chronic weight management.
Can Ozempic and Mounjaro be used together?
No. Combining them is not recommended and may increase the risk of side effects.
Do these medicines cause thyroid cancer?
This has not been proven in humans. However, they carry a boxed warning because thyroid C-cell tumors occurred in rodents, and they are contraindicated in people with medullary thyroid carcinoma or MEN2.
Does weight return after treatment stops?
A substantial amount of weight may return, which is why a long-term maintenance plan is important.
Scientific references
- US Food and Drug Administration. Semaglutide prescribing information and thyroid C-cell tumor warning.
- US Food and Drug Administration. Zepbound prescribing information.
- Wilding JPH, et al. Once-weekly semaglutide in adults with overweight or obesity. 2021.
- Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity. 2022.
- Aronne LJ, et al. Tirzepatide compared with semaglutide for obesity treatment. 2025.
- Wilding JPH, et al. Weight regain after semaglutide withdrawal. 2022.
Medical disclaimer: This article is for general educational purposes and does not provide a personal diagnosis or treatment plan. These medicines should be used under medical supervision. Do not start, stop, combine, or change the dose of a prescription medicine without consulting your healthcare professional.