Medically reviewed by Dr. Ehtesham Ghani, Internal Medicine & Bariatric Medicine (ASBP). Last reviewed June 2026.
If you are considering a GLP-1 medication, the first question is almost always the same: how much weight can I actually expect to lose? It is a fair question, but the honest answer is a range, not a guarantee. The numbers reported in clinical trials reflect averages achieved alongside diet, exercise, and ongoing medical supervision. Your own results depend on your starting weight, your health history, the medication chosen, and how consistently the plan is followed. At ThinFast MD, physician-supervised medical weight loss in Illinois since 1984, we set expectations honestly and build the plan around your physiology, not a marketing promise.
How much weight can you lose on GLP-1 medications?
In the major clinical trials, average weight loss with GLP-1 and dual-agonist medications, used together with lifestyle changes, fell into these ranges over roughly 68 to 72 weeks. Semaglutide (the active ingredient in Wegovy and Ozempic) produced average total body weight reduction of about 15% in the STEP 1 trial (NEJM, Wilding et al. 2021). Tirzepatide (the active ingredient in Zepbound and Mounjaro), which targets both GIP and GLP-1 receptors, produced average reductions of roughly 16% to 22% in the SURMOUNT-1 trial, depending on the dose (NEJM, Jastreboff et al. 2022). These are averages: some people lost more, some less. Individual results vary, and these figures apply to people who stayed on the medication and followed the accompanying diet and activity plan under medical supervision.
How does GLP-1 weight loss compare to older appetite suppressants?
GLP-1 and dual-agonist medications generally produce larger average weight loss than older oral appetite suppressants such as phentermine, which is typically associated with around 3% to 7% of body weight over the short term. That does not make the older options obsolete. Phentermine, phendimetrazine, diethylpropion, and metformin remain useful tools, and for some patients they are a better fit based on health history, cost, insurance, tolerance, or treatment goals. Phentermine and related stimulant appetite suppressants are generally intended for short-term use and are not appropriate for everyone, including people with uncontrolled high blood pressure, heart disease, overactive thyroid, or a history of substance use, which is one more reason a medication should be chosen only after a physician exam. A physician exam is what determines which medication, if any, is clinically appropriate for you. We carry the full range so the plan can be matched to the patient rather than the patient forced onto a single product.
How long does it take to see results?
The headline trial averages were measured over roughly 68 to 72 weeks, so meaningful results take months, not days. Doses are titrated upward gradually to help manage side effects, and weight loss tends to build steadily over that period rather than arriving all at once. This is one reason ongoing medical supervision matters: your dose, nutrition, and activity are adjusted along the way, and side effects are monitored by a physician. You can read more about the dual-agonist option on our tirzepatide weight loss page.
Are GLP-1 medications safe? Who should not take them?
GLP-1 and dual-agonist medications are generally well tolerated, but they are prescription medications with real precautions, and they are not appropriate for everyone. According to FDA prescribing information, semaglutide and tirzepatide carry a boxed warning and should not be used by people with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia syndrome type 2 (MEN 2). They are also generally avoided in people with a history of pancreatitis and are not recommended during pregnancy. The most common side effects are gastrointestinal, such as nausea, which is part of why doses are increased slowly. It is also worth knowing that the FDA weight-management indications belong to Wegovy and Zepbound; Ozempic and Mounjaro are FDA-approved for type 2 diabetes, and any use is determined by your physician. A complete history and physical lets us screen for these factors before anything is prescribed, which a mail-order form cannot do.
Will the weight come back if you stop the medication?
This is the part mail-order and telehealth-only services rarely emphasize. In clinical studies, including the STEP 1 trial extension, a meaningful portion of lost weight tended to return when the medication was stopped without a structured plan to maintain the loss. GLP-1 medications treat a chronic condition; they are not a one-time course. That is why we pair every program with a long-term strategy, including our weight-loss maintenance program, OPTIFAST and OptitrimMD meal replacement support, nutrition counseling, and lipotropic and B12 injections where appropriate. The goal is not just to lose weight but to keep it off with medical guidance.
Why does an in-person physician exam matter for GLP-1 results?
A weight number on a website cannot tell you what will happen in your body. Conditions like PCOS, type 2 diabetes or prediabetes, thyroid disorders including Hashimoto’s, and the metabolic shifts of menopause and being over 50 all influence how you respond to treatment. Unlike a telehealth-only pharmacy that ships medication after a brief online form, ThinFast MD performs a true in-person evaluation, including history and physical, labs, EKG when indicated, and a metabolism review, so your plan is grounded in your actual health data. Our medical director, Dr. Ehtesham Ghani, practices Internal Medicine and Bariatric Medicine and is a member of the American Society of Bariatric Physicians. We see patients at four Illinois locations in Hinsdale, Arlington Heights, Brookfield, and Rockford, and we also offer telehealth and online consults for follow-up convenience.
How do you find out what’s realistic for you?
The most reliable estimate of your own weight-loss potential comes from a consultation, not an online calculator. At your visit we review your history, run the appropriate labs, discuss your goals, and recommend whether a GLP-1 medication, an appetite suppressant, a meal-replacement approach, or a combination makes sense, always used with diet, exercise, and medical supervision. Pricing and insurance coverage are reviewed individually at your consultation. To schedule at any of our four Illinois clinics, call (708) 485-4050.
Frequently asked questions
How much weight can you lose on GLP-1 medications on average?
In clinical trials with diet and exercise, semaglutide averaged about 15% total body weight loss (STEP 1) and tirzepatide about 16% to 22% (SURMOUNT-1) over roughly 68 to 72 weeks. Individual results vary.
Is tirzepatide more effective than semaglutide for weight loss?
On average, tirzepatide showed higher weight-loss percentages in trials (about 16% to 22%) than semaglutide (about 15%), but the right choice depends on your health, tolerance, and a physician’s evaluation. The most effective option is the one that fits your individual situation.
How long do you have to take a GLP-1 medication?
GLP-1 medications treat a chronic condition and are generally taken long term. In studies, weight often returned after stopping without a maintenance plan, which is why we build in long-term support.
Who should not take a GLP-1 medication?
Per FDA prescribing information, semaglutide and tirzepatide carry a boxed warning and should not be used by people with a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2, and are generally avoided with a history of pancreatitis or during pregnancy. A physician exam screens for these before any prescription.
How much weight does phentermine cause you to lose?
Phentermine is typically associated with about 3% to 7% of body weight over the short term, generally less than GLP-1 medications, but it remains a useful option for many patients depending on their needs. It is intended for short-term use and is not appropriate for everyone.
Are GLP-1 weight-loss results guaranteed?
No. The figures from clinical trials are averages achieved with diet, exercise, and medical supervision. Individual results vary, and no medication can guarantee a specific outcome.
This page is for educational purposes only and is not medical advice. GLP-1 and other weight-loss medications are prescribed only when clinically appropriate following a physician evaluation, and are intended to be used with diet, exercise, and medical supervision. The weight-loss figures cited are averages from clinical trials; individual results vary and are not guaranteed. Talk with a qualified clinician about your specific situation.
Find your nearest ThinFast MD clinic
We serve patients across the Chicago suburbs and Rockford. Choose your nearest location — Hinsdale, Arlington Heights, Brookfield, or Rockford — or call (708) 485-4050 to book. See all service areas.
