Semaglutide vs Phentermine for Weight Loss: Key Differences

You have probably seen both options discussed in weight loss conversations: phentermine, a stimulant medication prescribed for decades, and semaglutide, a newer GLP-1 medication that has become widely discussed. Both can support weight loss. But they work in different ways, have very different evidence behind them, and are labeled for very different treatment timelines.

If you are comparing these options, here is what the FDA labels and clinical trials actually show. One thing to know up front: no trial has compared semaglutide and phentermine directly, so this page compares each medication against placebo in its own studies.

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What phentermine is

Phentermine is a sympathomimetic amine and a Schedule IV controlled substance, first approved by the FDA in 1959^[1].

Its label describes pharmacologic activity similar to amphetamine. It is commonly called an “anorectic” (appetite-reducing) drug, although the label notes it has not been established that appetite suppression is its primary action in treating obesity; other central nervous system or metabolic effects may be involved^[1]. It is approved only as a short-term adjunct, which the label describes as “a few weeks”, in a weight-reduction plan based on exercise, behavioral changes, and calorie reduction, for adults with a BMI of 30 or higher, or 27 or higher with other risk factors^[1].

Because phentermine is a controlled substance, it requires a prescription from a licensed provider, and federal and state rules on controlled substances apply. It is available as a generic medication.

Phentermine is also one of two ingredients in Qsymia (phentermine and topiramate extended-release), a separate FDA-approved product with its own label. Unlike phentermine alone, Qsymia is indicated to reduce excess body weight and maintain weight reduction long term^[2].

What semaglutide is

Semaglutide is a GLP-1 receptor agonist, a peptide similar to the glucagon-like peptide-1 hormone your body makes. GLP-1 helps regulate appetite and calorie intake^[3].

Semaglutide was first approved by the FDA in 2017 for type 2 diabetes, and in 2021 for chronic weight management, as Wegovy^[3][4]. Wegovy is labeled to reduce excess body weight and maintain weight reduction long term, so it is meant for ongoing use rather than a short course^[3].

Semaglutide for weight management is available as FDA-approved brand-name products (a weekly injection and a daily tablet). Some telehealth programs also offer compounded semaglutide, which is not FDA-approved and is not the same as brand-name products (see the disclosures below).

Transformation Health does not prescribe or sell either medication. This comparison is informational, to help you understand how the two approaches differ before you talk with a licensed provider.

How they work: side-by-side comparison

Sources: phentermine label^[1]; Wegovy label^[3]; STEP 1 and STEP 4^[5][7].

What the evidence shows: weight loss

No randomized trial has compared semaglutide with phentermine head to head (PubMed search, October 16, 2026). The figures below come from each medication’s own placebo-controlled evidence, which differs in length and design, so they should not be read as a direct comparison.

Phentermine: The phentermine label summarizes the evidence for its drug class: in relatively short-term trials, adults treated with these drugs plus dietary guidance lost more weight on average than those on placebo, but the extra weight loss was “only a fraction of a pound a week.” The label adds that the studies lasted only a few weeks and that the impact over diet alone “must be considered clinically limited”^[1].

Semaglutide: The STEP 1 trial, published in the New England Journal of Medicine in 2021, enrolled 1,961 adults without diabetes and followed them for 68 weeks. Participants on once-weekly semaglutide 2.4 mg (the dose in Wegovy) plus lifestyle intervention had an average body weight change of -14.9%, compared with -2.4% on placebo^[5]. Nausea and diarrhea were the most common side effects, and 4.5% of semaglutide participants stopped treatment because of gastrointestinal events, compared with 0.8% on placebo^[5].

The STEP 1 results describe participants taking the brand-name semaglutide product studied; compounded versions were not tested. They are not a prediction for any individual. Results vary.

Side effects and tolerability

Phentermine (from its FDA label)^[1]:

• Heart and blood vessels: palpitations, fast heart rate, elevated blood pressure, ischemic events; rare cases of primary pulmonary hypertension and serious heart valve disease have been reported
• Nervous system: overstimulation, restlessness, insomnia, dizziness, tremor, headache, euphoria or dysphoria, and in rare cases psychosis
• Digestive: dry mouth, unpleasant taste, diarrhea, constipation
• Other: hives, impotence, changes in libido
• May impair your ability to drive or operate machinery; risk of abuse and dependence
• Contraindicated with a history of cardiovascular disease (such as coronary artery disease, stroke, arrhythmias, heart failure, or uncontrolled high blood pressure), during or within 14 days of MAO inhibitors, with hyperthyroidism, glaucoma, agitated states, a history of drug abuse, pregnancy, or nursing

Semaglutide (from the Wegovy label)^[3]:

• Boxed warning: semaglutide caused thyroid C-cell tumors in rodents; it is unknown whether it causes these tumors, including medullary thyroid carcinoma (MTC), in humans
• Most common side effects include nausea, diarrhea, vomiting, constipation, abdominal pain, headache, fatigue, indigestion, and dizziness, among others listed on the label
• Serious warnings include acute pancreatitis, gallbladder disease, low blood sugar when used with insulin or a sulfonylurea, acute kidney injury, severe gastrointestinal reactions, serious allergic reactions, increased heart rate, and pulmonary aspiration during anesthesia or deep sedation
• Contraindicated with a personal or family history of MTC, Multiple Endocrine Neoplasia syndrome type 2 (MEN 2), or a prior serious allergic reaction to semaglutide

Phentermine is not suitable for people with cardiovascular disease or other conditions listed on its label. Semaglutide is not suitable for people with a personal or family history of MTC or MEN 2. Which one a person tolerates better depends on their medical history and side effect sensitivity, and a licensed provider is the one to weigh that.

Key differences: duration of treatment

This is one of the most important distinctions.

Phentermine is labeled as a short-term adjunct, “a few weeks”, in a weight-reduction plan^[1]. Using it longer is off-label. A provider might do this if they judge the benefit outweighs the risks for your situation, but it is not the labeled use. The label also says tolerance to the appetite effect usually develops within a few weeks^[1].

Semaglutide, as Wegovy, is labeled for long-term use to reduce excess weight and maintain weight reduction^[3]. In STEP 1, participants stayed on it for 68 weeks^[5].

If a short course of medication alongside a weight-reduction plan fits your situation, phentermine is labeled for that use. If you are working toward sustained weight loss over many months, semaglutide is labeled for long-term use.

Controlled substance status

Phentermine is a Schedule IV controlled substance^[1]. This means:

• Under federal rules, a Schedule IV prescription cannot be filled or refilled more than six months after it was issued, or refilled more than five times^[8]
• States can add their own rules for prescribing controlled substances
• The label says the smallest practical amount should be prescribed at one time to reduce the risk of overdose^[1]
• Phentermine is contraindicated in people with a history of drug abuse^[1]

Semaglutide is not a controlled substance and does not have these restrictions.

Cardiovascular outcomes

The SELECT trial (2023) enrolled 17,604 adults aged 45 or older with established cardiovascular disease and overweight or obesity, but without diabetes. Over a mean follow-up of 39.8 months, a major cardiovascular event (cardiovascular death, nonfatal heart attack, or nonfatal stroke) occurred in 6.5% of people on semaglutide 2.4 mg and 8.0% on placebo, about a 20 percent lower relative risk (hazard ratio 0.80). Adverse events led 16.6% of semaglutide participants to stop treatment, compared with 8.2% on placebo^[6]. Based on this, the Wegovy label includes reducing the risk of major cardiovascular events in adults with established cardiovascular disease and either obesity or overweight^[3].

No comparable cardiovascular outcome trial exists for phentermine. Its label lists a history of cardiovascular disease as a contraindication and advises caution even with mild high blood pressure^[1].

If you have heart disease or cardiovascular risk factors, bring this up with a licensed provider; it is one of the clearest places where the two medications’ labels differ.

Cost comparison

Prices vary by program, pharmacy, and dose, and they change, so this page does not list them. Instead, compare the total cost of each path:

• Medication: generic phentermine, brand-name semaglutide, or compounded semaglutide are priced very differently, and brand-name and compounded prices often change with the dose
• Dose increases: semaglutide is started low and increased every 4 weeks, and some programs charge more at higher doses
• Provider visits: some programs include follow-ups; others bill each visit separately
• Labs: included in some programs, billed separately in others
• Shipping and membership fees: check whether the program charges them, and what the cancellation terms are
• Insurance: coverage for either medication depends on your plan; ask your insurer

Also think about duration. Phentermine is labeled for a few weeks, while semaglutide is labeled for long-term use, so a monthly price alone does not tell you the total cost of the path you are considering. More importantly, when a short course of phentermine ends, you face the question of what comes next.

Why phentermine is limited to short-term use

Phentermine’s short-term labeling reflects how the medication works and the limits of the evidence behind it.

Tolerance develops. The label says tolerance to the appetite effect usually develops within a few weeks, and that when it does, the dose should not be increased; the drug should be stopped instead^[1].

The evidence is short-term. The label notes that the trials behind drugs of this class lasted only a few weeks, while obesity is measured over years, so the added benefit over diet alone “must be considered clinically limited”^[1].

Abuse and dependence risk. Phentermine is related to the amphetamines, and its label warns about the risk of abuse and dependence^[1].

The question after a short course. When you stop phentermine, its effect on appetite stops too. Any weight you lost is easier to keep off if you have built habits that last, but regain after stopping a weight-loss medication is common.

Is phentermine ever prescribed longer? Some providers prescribe it off-label for longer periods. A different, long-term option that contains phentermine is Qsymia, which combines it with topiramate extended-release and has its own FDA approval and label^[2]. If your goals extend well beyond a few weeks, ask a licensed provider about medications labeled for long-term use.

Long-term weight management: why the treatment timeline matters

Here is the practical difference between the two approaches:

Phentermine path: A short course of medication alongside a weight-reduction plan, then the medication stops. What comes after is up to you and your provider.

Semaglutide path: Ongoing medication that keeps working on appetite for as long as you take it. Semaglutide is labeled for long-term use, and in STEP 1 participants were treated for 68 weeks^[3][5].

Stopping matters for semaglutide too. In the STEP 4 trial, adults who had taken semaglutide for 20 weeks were randomly assigned either to keep taking it or to switch to placebo. From week 20 to week 68, those who continued had an average weight change of -7.9%, while those switched to placebo had +6.9%^[7]. Results vary.

The cost and timeline trade-off is real: phentermine is usually less expensive month to month but is labeled for a short course. Semaglutide usually costs more month to month and is labeled for people with long-term weight management goals.

If you have already tried phentermine in the past and the weight returned, that does not mean you failed at diet or exercise. Phentermine is labeled for short-term use, and weight regain after stopping a weight-loss medication is common, including with semaglutide.

Who might choose phentermine

• You and your provider want a short course of medication to support a specific weight-reduction plan
• You want a lower-cost option and are willing to try short-term treatment
• Cost is a significant barrier and you want to see how medication-supported weight loss works for you
• Your provider recommends it based on your medical history
• You cannot take GLP-1 medications for medical reasons
• You prefer a once-daily oral medication for a short period

Phentermine is a long-established, FDA-approved option that is usually less expensive. Its limits are the short labeled duration, the modest effect described on its label, and its cardiovascular and abuse-related risks.

Who might choose semaglutide

• You are aiming for sustained weight loss over many months
• You have type 2 diabetes (semaglutide also has FDA-approved products for blood sugar control)
• You have established cardiovascular disease along with overweight or obesity
• You want a medication labeled for long-term use
• You want to avoid stimulant side effects, or phentermine is contraindicated for you
• Diet and exercise alone have not been enough for you over many years

Semaglutide is labeled for long-term weight management with ongoing provider oversight.

Can you use both together?

The phentermine label says it is indicated only as short-term monotherapy and that taking it with other drug products for weight loss is not recommended, because the safety and effectiveness of such combinations have not been established^[1]. Some providers may still discuss combining medications off-label, based on your health history, blood pressure, heart rate, and goals.

If you are considering combination therapy, this requires a licensed provider’s oversight. Do not combine these medications on your own.

How the process should work

If you are evaluating whether semaglutide, phentermine, or another weight-loss medication is appropriate for you, here is what a proper process looks like.

At most telehealth programs and in-person practices, you share your health history, current weight and height, goals, and any weight-loss medications you have tried. A licensed provider (a physician, nurse practitioner, or physician assistant) reviews that information and decides whether a prescription is appropriate. Not every program prescribes phentermine, because it is a controlled substance; ask before you sign up.

Before you choose a program, check:

• Which medications it prescribes, and whether they are brand-name or compounded
• The total cost: medication, dose increases, visits, labs, shipping, membership fees, and cancellation terms
• How follow-up works, including who monitors side effects, blood pressure, and heart rate
• The pharmacy that fills the prescription, and for compounded medications, its type and licensing

The takeaway

Phentermine and semaglutide can both support weight loss, but they are different medications labeled for different situations.

Phentermine is an older, generic stimulant labeled for short-term use. Its own label describes the added weight loss in its short trials as modest, and it carries cardiovascular and abuse-related risks.

Semaglutide is labeled for long-term weight management. In its own placebo-controlled trials it produced larger average weight loss over a longer period, and it has cardiovascular outcome data in people with established heart disease. It also has a boxed warning, common gastrointestinal side effects, and a higher monthly cost, and weight tends to return after stopping. No trial has compared the two directly.

The choice depends on your medical situation, your goals, your timeline, and the side effects and risks you can accept. An honest conversation with a licensed provider about both options will point you toward what fits your situation.

Important disclosures

This content is for general education and is not medical advice. Talk with a licensed provider about your situation.

Results vary. Trial results describe study participants and are not a prediction for any individual. The STEP 1, STEP 4, and SELECT figures on this page describe participants taking semaglutide 2.4 mg, the FDA-approved brand-name product, not compounded semaglutide^[5][6][7].

Compounded medications are not FDA-approved. The FDA does not review them for safety, effectiveness, or quality. Compounded semaglutide is not the same as brand-name products such as Wegovy and may differ in formulation, purity, and potency. On February 21, 2025, the FDA determined that the semaglutide injection shortage was resolved; it said its 503A enforcement discretion period ended after an April 24, 2025 court decision (the original end date was April 22, 2025) and its 503B period ended May 22, 2025. That FDA page was most recently updated April 1, 2026; check it for the current statement^[9].

Wegovy is a trademark of Novo Nordisk A/S. Qsymia is a trademark of VIVUS LLC. Transformation Health is not affiliated with or endorsed by these companies.

Transformation Health does not accept payment from providers. How we make money.

Citations

[1] U.S. Food and Drug Administration. ADIPEX-P (phentermine hydrochloride USP) capsules and tablets, CIV, prescribing information, sections 1, 2, 4, 5, 6, 9, 12, and 14. Revised 01/2012 (most recent ADIPEX-P label posted on Drugs@FDA, checked 2026-10-16). https://www.accessdata.fda.gov/drugsatfda_docs/label/2012/085128s065lbl.pdf (Supports: Schedule IV; initial U.S. approval 1959; short-term (“a few weeks”) adjunct indication and BMI criteria; activity similar to amphetamine; tolerance within a few weeks; contraindications; reported adverse reactions; coadministration with other weight-loss drugs not recommended; clinical studies summary.)

[2] U.S. Food and Drug Administration. QSYMIA (phentermine and topiramate extended-release capsules), CIV, prescribing information. Revised 03/2026. https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/022580s032lbl.pdf (Supports: combination of phentermine and topiramate; indicated to reduce excess body weight and maintain weight reduction long term.)

[3] U.S. Food and Drug Administration. WEGOVY (semaglutide) injection and tablets, prescribing information, Boxed Warning and sections 1, 2, 4, 5, 6, and 12. Revised 06/2026. https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/215256s025lbl.pdf (Supports: initial U.S. approval of semaglutide 2017; indications including long-term weight reduction and cardiovascular risk reduction; dose escalation every 4 weeks; boxed warning; contraindications; warnings; common adverse reactions; mechanism.)

[4] U.S. Food and Drug Administration. Drugs@FDA: NDA 215256 (WEGOVY), original approval 06/04/2021, and NDA 209637 (OZEMPIC), original approval 12/05/2017. https://www.accessdata.fda.gov/scripts/cder/daf/index.cfm?event=overview.process&ApplNo=215256 (Accessed 2026-10-16. Supports: years semaglutide was first approved for type 2 diabetes and for weight management.)

[5] Wilding JPH, Batterham RL, Calanna S, et al. “Once-Weekly Semaglutide in Adults with Overweight or Obesity.” N Engl J Med. 2021;384(11):989-1002. https://pubmed.ncbi.nlm.nih.gov/33567185/ (Supports: STEP 1 design, 1,961 adults without diabetes, 68 weeks; mean body weight change -14.9% vs -2.4% with placebo; nausea and diarrhea most common; 4.5% vs 0.8% discontinuation for gastrointestinal events.)

[6] Lincoff AM, Brown-Frandsen K, Colhoun HM, et al. “Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes.” N Engl J Med. 2023;389(24):2221-2232. https://pubmed.ncbi.nlm.nih.gov/37952131/ (Supports: SELECT design and population; primary endpoint 6.5% vs 8.0%, hazard ratio 0.80; discontinuation for adverse events 16.6% vs 8.2%.)

[7] Rubino D, Abrahamsson N, Davies M, et al. “Effect of Continued Weekly Subcutaneous Semaglutide vs Placebo on Weight Loss Maintenance in Adults With Overweight or Obesity: The STEP 4 Randomized Clinical Trial.” JAMA. 2021;325(14):1414-1425. https://pubmed.ncbi.nlm.nih.gov/33755728/ (Supports: weight change from week 20 to 68 of -7.9% with continued semaglutide vs +6.9% after switching to placebo.)

[8] Code of Federal Regulations, 21 CFR 1306.22, “Refilling of prescriptions.” https://www.ecfr.gov/current/title-21/section-1306.22 (Accessed 2026-10-16. Supports: Schedule III and IV prescriptions may not be filled or refilled more than six months after issue or refilled more than five times.)

[9] U.S. Food and Drug Administration. “FDA clarifies policies for compounders as national GLP-1 supply begins to stabilize.” Content current as of 04/01/2026. https://www.fda.gov/drugs/drug-alerts-and-statements/fda-clarifies-policies-compounders-national-glp-1-supply-begins-stabilize (Accessed 2026-10-16. Supports: semaglutide shortage resolution date and end of enforcement discretion periods.)