GLP-1 Medications, Explained: What Semaglutide and Tirzepatide Actually Do
The drugs behind Wegovy, Ozempic, Zepbound and Mounjaro have reshaped obesity care. Here’s what the major trials show, the side effects to know, and the questions to bring to your doctor.
Key takeaways
- In large trials, semaglutide 2.4 mg produced average weight loss of about 15% and tirzepatide up to about 21% over roughly a year and a half.
- Semaglutide cut major cardiovascular events by 20% in people with heart disease and excess weight, even without diabetes.
- Stomach side effects are common; most people regain weight if they stop.
- Protecting muscle with protein and strength training matters while on these drugs.
Few drug classes have changed medicine as quickly as GLP-1 receptor agonists. Originally developed for type 2 diabetes, they now anchor modern obesity treatment, and their benefits are reaching into heart, kidney and liver disease. They are also widely misunderstood. Here is what the evidence actually shows.
How they work
GLP-1 (glucagon-like peptide-1) is a hormone your gut releases after you eat. It prompts insulin release when blood sugar is high, slows how quickly the stomach empties, and acts on appetite centers in the brain. Semaglutide (sold as Ozempic for diabetes and Wegovy for weight) mimics GLP-1 and lasts about a week, which allows once-weekly injections. Tirzepatide (Mounjaro and Zepbound) activates both GLP-1 and a second gut-hormone receptor, GIP.
The practical result is that people feel full sooner and think about food less. That is why the weight loss is substantial, and also why it tends to reverse when the medication stops.
What the major trials found
In the STEP 1 trial, adults with obesity or overweight (without diabetes) who took semaglutide 2.4 mg weekly lost an average of 14.9% of body weight over 68 weeks, compared with 2.4% on placebo. In SURMOUNT-1, tirzepatide produced average losses of 15% to nearly 21% over 72 weeks depending on dose.
The most important result may be cardiovascular. The SELECT trial enrolled more than 17,000 people with established heart disease and overweight or obesity but no diabetes. Semaglutide reduced the combined risk of heart attack, stroke and cardiovascular death by 20% compared with placebo. That moved the conversation from cosmetic weight loss to treating a disease with measurable effects on survival.
Side effects and safety
Nausea, vomiting, diarrhea and constipation are the most common side effects, especially when the dose is increased. They usually ease over weeks, which is why doses are raised gradually. Less common but serious risks include gallbladder problems and pancreatitis. Both drugs carry a boxed warning based on thyroid C-cell tumors seen in rodents; they should not be used by people with a personal or family history of medullary thyroid cancer or multiple endocrine neoplasia type 2.
Because these drugs slow stomach emptying, tell your surgeon or anesthesiologist if you take one before a procedure.
What happens when you stop
Obesity behaves like a chronic condition. In the STEP 4 trial, participants who switched from semaglutide to placebo after 20 weeks regained a substantial share of the weight they had lost, while those who stayed on the drug kept losing. For many people, these are long-term medications, which raises real questions about cost and access worth discussing up front.
Protecting muscle
Some of the weight lost on any calorie-reducing treatment is lean mass, not fat. Eating adequate protein and doing resistance training two or more days a week are the best tools for preserving muscle and strength. Our guide to protein needs after 40 covers how much to aim for.
Questions to ask your doctor
- Do I meet the criteria for a weight-management medication, and which one fits my health history?
- How will we handle side effects as the dose goes up?
- What is the long-term plan, and what happens if I need to stop?
- Should I avoid compounded versions, and why?
Frequently asked questions
Is Ozempic the same as Wegovy?
Both contain semaglutide. Ozempic is approved for type 2 diabetes and Wegovy for chronic weight management, with different dosing.
How much weight do people lose on GLP-1 drugs?
In major trials, average loss was about 15% of body weight with semaglutide 2.4 mg and up to about 21% with the highest dose of tirzepatide, though individual results vary widely.
Do you regain weight after stopping semaglutide?
Most people regain a significant portion of lost weight after stopping, which is why these drugs are often used long term.
Sources
- Wilding JPH et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). N Engl J Med. 2021.
- Jastreboff AM et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). N Engl J Med. 2022.
- Lincoff AM et al. Semaglutide and cardiovascular outcomes in obesity without diabetes (SELECT). N Engl J Med. 2023.
- Rubino D et al. Effect of continued weekly semaglutide vs placebo on weight loss maintenance (STEP 4). JAMA. 2021.