Medically reviewed by Dr. Ehtesham Ghani, Internal Medicine & Bariatric Medicine (ASBP). Last reviewed June 2026.
If you are weighing your weight-loss medication options, two names come up again and again: semaglutide and phentermine. They are very different drugs, work through different mechanisms, and suit different patients. At ThinFast MD, our physicians have helped patients across Illinois choose between them since 1984, basing the decision on an in-person exam rather than a checkbox on a website. Here is how the two compare.
What is the difference between semaglutide and phentermine?
The short answer: they belong to entirely different drug classes and were designed for different purposes. Phentermine is a stimulant appetite suppressant that has been used in the United States for decades. Semaglutide is a GLP-1 receptor agonist, a newer class of medication that mimics a gut hormone your body already makes. Both can support weight loss when combined with diet, exercise, and medical supervision, but they are not interchangeable, and the right choice depends on your health history, goals, and how long you plan to stay on treatment.
How does phentermine work for weight loss?
Phentermine is a sympathomimetic (stimulant) that suppresses appetite by acting on the central nervous system. It is FDA-approved for short-term use, generally about 12 weeks, as an adjunct to a reduced-calorie diet and increased physical activity. In clinical use described in the FDA phentermine (Adipex-P) prescribing information and clinical reviews, short-term phentermine combined with a reduced-calorie diet has been associated with average weight loss in roughly the 5 to 10 percent range over about 12 weeks, though individual results vary. Because it is a stimulant, phentermine can raise heart rate and blood pressure. It is contraindicated in people with cardiovascular disease such as uncontrolled high blood pressure, coronary artery disease, arrhythmias, or heart failure, as well as hyperthyroidism, and it should not be used within 14 days of a monoamine oxidase (MAO) inhibitor, among other cautions. That is exactly why a physician exam matters before starting it. You can learn more on our appetite suppressants page.
How does semaglutide work for weight loss?
Semaglutide is a GLP-1 receptor agonist. It works by slowing how quickly your stomach empties, reducing appetite, and helping you feel full longer. Unlike phentermine, it is intended for longer-term use as a chronic weight-management tool. In the manufacturer’s STEP clinical trial program for the branded product Wegovy, adults without diabetes lost on average about 15 percent of their body weight over 68 weeks when semaglutide was combined with lifestyle changes, according to the FDA Wegovy prescribing information and the New England Journal of Medicine STEP 1 trial results. Results vary from person to person, and the medication is always used alongside diet, exercise, and medical supervision. Semaglutide also carries an FDA boxed warning: in rodent studies it caused thyroid C-cell tumors, and although the relevance to humans has not been established, it is contraindicated in anyone with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia syndrome type 2 (MEN 2). Our GLP-1 semaglutide page goes deeper on what to expect.
Which produces more weight loss, semaglutide or phentermine?
On average, semaglutide is associated with greater total weight loss in clinical trials, roughly 15 percent over 68 weeks, compared with the roughly 5 to 10 percent typically seen with short-term phentermine over about 12 weeks. However, “more weight loss on average” does not automatically mean it is the better choice for you. Phentermine can be an effective, lower-cost, short-term option for some patients who need a jump-start and have no contraindications. Semaglutide may be a better fit for someone who needs sustained, longer-term support and qualifies for GLP-1 therapy. The honest answer is that the best medication is the one that is both safe for your body and matched to your goals, which is a clinical decision, not a marketing one.
Who is a candidate for each medication?
Phentermine candidates are generally adults who need short-term appetite control and do not have cardiovascular disease, uncontrolled high blood pressure, hyperthyroidism, or other stimulant-related contraindications. Semaglutide candidates are typically adults who meet body-mass-index criteria for chronic weight management, want or need a longer-term approach, and have no personal or family history of medullary thyroid carcinoma or MEN 2. Conditions like PCOS, prediabetes or type 2 diabetes, thyroid issues, and menopause can all influence which path makes sense. At ThinFast MD, our physicians review your history and physical, labs, EKG, and metabolism before recommending either medication, because eligibility for these drugs is genuinely individual.
Why see an in-person physician instead of a mail-order app?
Many telehealth-only and mail-order services ship medication after a brief online questionnaire. That model skips the very steps that make these drugs safe: a hands-on exam, baseline labs, an EKG when appropriate, and a real review of your metabolism, thyroid history, and cardiovascular health, which matters especially with a stimulant like phentermine and with the thyroid contraindications tied to GLP-1 therapy. ThinFast MD has four Illinois locations in Hinsdale, Arlington Heights, Brookfield, and Rockford, with physician supervision from medical director Dr. Ehtesham Ghani. We do offer telehealth follow-ups for convenience, but your care starts with a genuine medical evaluation, not a checkout cart. Pricing and insurance are reviewed at your consultation.
Frequently asked questions
Can you take semaglutide and phentermine together?
In some cases a physician may consider combining or sequencing therapies, but this is an individualized decision that requires medical supervision and is not appropriate for everyone. Discuss it with a ThinFast MD physician rather than starting any combination on your own.
Is phentermine safe long term?
Phentermine is FDA-approved for short-term use, generally about 12 weeks. It is a stimulant and is contraindicated with cardiovascular disease, uncontrolled high blood pressure, and hyperthyroidism, which is why ongoing physician monitoring is important.
How much weight can you lose on semaglutide?
In the manufacturer’s STEP trials reported in the FDA Wegovy prescribing information and the New England Journal of Medicine, adults without diabetes lost on average about 15 percent of body weight over 68 weeks alongside lifestyle changes. Individual results vary.
Does semaglutide have any serious warnings?
Yes. Semaglutide carries an FDA boxed warning for thyroid C-cell tumors seen in rodent studies, and it is contraindicated in anyone with a personal or family history of medullary thyroid carcinoma or MEN 2. A physician reviews your thyroid and family history before prescribing it.
Which is cheaper, phentermine or semaglutide?
Phentermine is generally a lower-cost option, while GLP-1 medications like semaglutide can be more expensive and have variable insurance coverage. Exact pricing and insurance are reviewed at your ThinFast MD consultation.
How do I know which one is right for me?
The right choice depends on your health history, contraindications, goals, and how long you plan to treat. A ThinFast MD physician determines this through an in-person exam, labs, and an EKG when appropriate. Call (708) 485-4050 to schedule at any of our four Illinois locations.
This page is for educational purposes only and is not medical advice. Weight-loss medications are prescribed only when clinically appropriate after a physician evaluation, and are used together with diet, exercise, and medical supervision. Individual results vary. Please consult a ThinFast MD physician 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.
