Key Takeaways
- Phentermine and Zepbound® work through completely different mechanisms. Phentermine is a stimulant that suppresses appetite by increasing norepinephrine in the brain, while Zepbound® mimics two gut hormones that regulate blood sugar, appetite, and fat storage.
- Zepbound® shows significantly stronger weight loss results in clinical trials, with participants losing up to 20.9% of their body weight over 72 weeks compared to around 9% over 12 weeks with phentermine.
- Phentermine is FDA-approved for short-term use only and can raise heart rate and blood pressure, making it unsuitable for people with cardiovascular conditions. Zepbound® is approved for long-term use but carries its own contraindications around thyroid and endocrine history.
- Cost is a real differentiator. Phentermine can cost as little as $10 for a month's supply, while Zepbound® ranges from $299 to $699 without insurance coverage.
Phentermine and Zepbound® are both FDA-approved prescription medications for weight management, used alongside a structured diet and exercise program. Past that, they have little in common. Phentermine is a stimulant, first approved back in 1959. Zepbound®, on the other hand, is a GLP-1 medication, approved more recently in 2023.
So, which one might be the right choice for you? Read on as we compare phentermine versus Zepbound® so you can make an informed choice.
How do phentermine and Zepbound® work? Key differences explained
Navigating weight loss medications can be confusing, but understanding how treatments like phentermine and Zepbound® work can help you decide.
How phentermine works
Phentermine is a generic drug that belongs to a class of medications called sympathomimetic amines. These medicines mirror the effects of your body’s natural alert and survival system (sympathetic nervous system). They do this by stimulating the release or activity of certain chemical messengers (neurotransmitters) in your brain.
The main neurotransmitter that phentermine prompts is called norepinephrine. It’s primarily responsible for keeping you alert, focused, and energized. High norepinephrine levels signal your brain’s appetite center (hypothalamus) to lower hunger. While we know phentermine primarily works by increasing norepinephrine levels, scientists still don’t understand the full extent of how the drug works.
How tirzepatide (Zepbound®) works
We have a broader understanding of another weight loss medication, tirzepatide (the active ingredient in the brand Zepbound®). Tirzepatide mimics two hormones your body naturally produces, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP).
Insulin allows sugar from your food to enter your cells to be used for energy. GLP-1 and GIP are both incretin hormones, meaning they prompt your pancreas to release insulin in response to rising blood sugar levels after a meal. This prevents your blood sugar levels from spiking dangerously high.
Beyond managing insulin, GLP-1 and GIP have other jobs:
- GLP-1 acts on both your hypothalamus and your stomach to suppress appetite and slow down how fast your stomach empties after a meal. This makes you feel fuller faster and for longer.
- GIP increases the effectiveness of GLP-1. GIP also works directly on your body’s fat (adipose) tissue, helping under-the-skin (subcutaneous) fat expand to handle incoming nutrients. In doing this, GIP prevents excess fat from accumulating around your organs as harmful visceral fat.
Phentermine vs. tirzepatide: At a glance
Here’s a quick side-by-side comparison of both medications:
Which one is more effective for weight loss, phentermine or Zepbound®?
Currently, there aren’t any studies directly comparing phentermine and tirzepatide. However, Zepbound® overall has better weight loss results in individual trials.
Tirzepatide’s efficacy
The 2022 SURMOUNT-1 trial tracked participants with a body mass index (BMI) of 30 or greater, or a BMI of 27 or greater with at least one weight-related condition like high blood pressure. Researchers prescribed each a weekly injection of tirzepatide (5 mg, 10 mg, or 15 mg) or a placebo. Over 72 weeks, participants reached these weight loss thresholds:
These outcomes demonstrate tirzepatide’s effectiveness is a key contributor to Zepbound’s®
FDA approval for chronic weight management.
Phentermine’s efficacy
Phentermine’s body of research evidence mainly includes older, smaller, and shorter studies. One, published in Yonsei Medical Journal in 2006, split participants with a BMI of 25 or greater into two groups: a 37.5 mg dose of phentermine or placebo. Over 12 weeks, participants reached these weight loss thresholds:
Beyond weight loss: Zepbound® for obstructive sleep apnea
The FDA approves phentermine and tirzepatide for weight management. However, in 2024, the FDA expanded Zepbound’s® approval to cover moderate-to-severe obstructive sleep apnea (OSA) in adults with a BMI of 30 or higher. OSA is a disorder where your airway repeatedly closes during sleep, which can lead to daytime fatigue, high blood pressure, and heart problems.
One SURMOUNT-OSA trial tested tirzepatide against placebo among participants using a breathing device, called positive airway pressure (PAP) therapy. Another other made the same comparison among non-PAP users. Here’s what researchers found:
| Non-PAP users | PAP users | |||
|---|---|---|---|---|
| Outcome metric | Zepbound® | Placebo | Zepbound® | Placebo |
| Reduction in breathing disturbances | 25 fewer events per hour | 5 fewer events per hour | 29 fewer events per hour | 6 fewer events per hour |
| Cut apnea-hypopnea index* (AHI) by more than half | 61.2% of participants | 19% of participants | 72.4% of participants | 23.3% of participants |
| Symptom remission (or mild symptoms) | 42.2% of participants | 15.9% of *The apnea-hypopnea index is the standard metric clinicians use to diagnose and grade obstructive sleep apnea. It measures the number of apneas (breathing pauses) and hypopneas (shallow breathing) a person experiences per hour of sleep. The tirzepatide groups also experienced weight loss. Researchers noted that this outcome may be a key driver of OSA relief. Who should take phentermine vs. Zepbound®?When choosing between phentermine and Zepbound®, treatment duration, side effects and contraindications, and cost are all key pieces of the equation. FDA ApprovalThe FDA approves phentermine for short-term use (up to 12 weeks), while Zepbound® can be used long-term with ongoing monitoring. Many doctors prescribe phentermine off-label for longer-term use. However, because the drug is a central nervous system stimulant, phentermine can raise blood pressure and increase heart rate. ContraindicationsIt’s not recommended to use medications if you have any contraindications, conditions or circumstances that make a medication unsafe to use. For phentermine, contraindications include:
For Zepbound®, contraindications include a personal or family history of:
Your doctor will assess your full health picture to determine the right path for you. Side effectsBoth phentermine and Zepbound® can cause side effects. Common phentermine side effects include:
Common Zepbound® side effects include:
Cost and insurance coverageMedication cost may also factor into your choice. Phentermine is a low-cost option. Without insurance, prices vary, but typically sit in the ballpark of $10 for 30 tablets. Discount programs can also bring the cost down. Zepbound® typically costs more, ranging from $299 to $699 a month. With insurance, Zepbound® costs can drop meaningfully (as low as $25). Find the right path for your body with Maven ClinicChoosing between phentermine and Zepbound® (or any other weight loss medication) requires a careful understanding of how each one might fit into your unique care plan. At Maven Clinic, our specialists will walk you through your options to design a plan that fits your body. As one of our patients, Sarah, put it, “My provider was incredibly helpful. Understood my goals and is helping me meet them. Incredibly affirming, kind, supportive, and makes sure I have the facts and data." Discover our approach to GLP-1 Care.  "I tried a GLP-1 through another service first. Got the prescription, got nothing else. I'm in my late 40s, my body was changing, and I was asking AI my follow-up questions. Maven was the first time I had a clinician who knew what was going on with me and helped me feel like me again." Priya  "My clinician covered things I hadn't thought to ask, like how GLP-1s can affect birth control. I felt like I was making an informed decision instead of just following a trend." Diane  "There's so much noise online, and I wanted someone I could actually trust. With Maven, I understood my options, why I was taking what I was taking, and what to expect before I ever started. It felt clear from day one." Sarah FAQHow does Adipex® compare to Zepbound®? Does phentermine work better than tirzepatide? What’s the relationship between phentermine and weight loss? Is Zepbound® an alternative to phentermine? Get GLP-1 care that accounts for all of youGet startedRelated articlesWant to stay in the loop?Sign up for the latest content and updates from Maven.  | ||
