Key takeaways:
Phentermine (Adipex-P, Lomaira) is an oral weight-loss medication that works by reducing appetite. In studies, more than 80% or more of people lost at least 5% of their body weight while taking phentermine. It works best when combined with regular exercise and a balanced diet.
Phentermine is FDA approved for short-term use (up to 12 weeks). But some studies suggest it may be safe for longer use if your prescriber approves it. It’s usually taken once daily in the morning or in smaller doses before meals.
Common phentermine side effects include dry mouth and trouble sleeping. It may not be a safe option if you have heart disease, a history of substance misuse, or drink alcohol regularly.
Save on related medications
Newer weight-loss medications, such as semaglutide (Wegovy, Wegovy HD) and tirzepatide (Zepbound), are getting a lot of attention. But there are other options that have been around a lot longer and don’t require an injection.
Phentermine (Adipex-P, Lomaira) is an oral weight-loss medication that was FDA approved in 1959. But how does phentermine work for weight loss? And is it a safe and effective option? Let’s explore the answers to these and more common questions about phentermine to help you make informed choices about your weight-loss treatment options.
1. How does phentermine work for weight loss?
Phentermine is a stimulant medication. Stimulants help speed up the functions of your body.
Search and compare options
Phentermine works by activating your sympathetic nervous system, also known as your fight-or-flight response. This helps reduce your appetite, so you eat fewer calories. Over time, eating less can create an energy deficit — meaning you’re burning more calories than you’re eating — which can lead to weight loss. People also report a boost in energy when they’re taking phentermine, which helps them be more active.
Phentermine works best if it’s combined with regular exercise and a balanced diet. If you don’t make lifestyle changes while taking phentermine, it may not work as well for you. And you may have a higher chance of regaining weight once you stop taking it.
2. How much weight will you lose while taking phentermine?
It’s estimated that average weight loss with phentermine is around 17 lbs to 22 lbs after 3 to 6 months of treatment. But how much weight you’ll lose while taking phentermine can vary. It largely depends on how much excess weight you have and how well you’re able to adjust your diet and exercise habits while taking it.
Experts consider losing 3% to 5% of your body weight to be a clinically meaningful change. This means it’s likely to have a positive impact on your overall health and well-being.
Several studies show that over 80% of people taking phentermine lost at least 5% of their starting body weight. And often, more than half of participants lost 10% or more of their starting body weight.
3. How long does it take for phentermine to work?
In the studies mentioned above, some people experienced weight loss within the first 2 weeks of taking phentermine. For others, it took more than 3 months to start noticing a change in body weight.
Your prescriber will likely check in with you to see how phentermine is working when you start taking it. If you aren’t noticing a change in your weight, they may increase your dosage or try a different weight-loss medication. But you shouldn’t take more phentermine than prescribed on your own. This could raise the risk of side effects.
4. What is the phentermine dosage and the best time to take it for weight loss?
There are several different forms of phentermine. Doses range from 15 mg to 37.5 mg daily. In most cases, you’ll take phentermine once a day in the morning. But the typical dosage for Lomaira is 8 mg (1 tablet) 3 times a day before meals.
Regardless of which form of phentermine you take, you should avoid taking it in the evening. It could interfere with your sleep if you take it too late in the day.
5. How long can you take phentermine?
Your healthcare team will help you decide how long to take phentermine based on your medical history and how well it’s working for you.
Phentermine is only approved for short-term use (12 weeks or less). This is because there wasn’t any data that showed it was safe to take long term when it was initially approved.
But newer studies show that long-term phentermine use (more than 3 months) is not only safe, but may help people lose more weight. Results of an ongoing study about the long-term safety and efficacy of phentermine are expected in early 2027.
6. Is phentermine safe? What are the side effects?
Phentermine is approved for weight loss. This means it’s considered a safe and effective medication when you take it as prescribed. But there are some risks to be aware of.
Phentermine is a controlled substance. This means it carries a risk for misuse or dependence. Studies on phentermine show this isn’t likely to happen when you take phentermine as prescribed for weight management. So as mentioned, it’s important to stick to the phentermine dosage your prescriber recommends.
The most common phentermine side effects are dry mouth, trouble sleeping, and headache. In some cases, it may cause tremors, changes in your mood or behavior, and an increased heart rate. Most phentermine side effects will go away as your body adjusts to the medication.
Good to know: The combination of fenfluramine and phentermine — known as “fen-phen” — was prescribed off-label for weight loss. It led to dangerous heart and lung-related side effects, which led the FDA to remove fenfluramine from the market in 1997. Phentermine alone doesn’t carry the same risks as fen-phen.
7. Who shouldn’t take phentermine?
Phentermine isn’t recommended if you’re living with certain health conditions, including:
Heart disease
High blood pressure that isn’t well managed
Hyperthyroidism (too much thyroid hormone)
Glaucoma (high eye pressure)
High levels of nervousness or anxiety
History of drug misuse
Pregnancy
Breastfeeding
Phentermine may not be the best weight-loss medication for you if you consume alcohol regularly. The combination could increase your risk of alcohol-related harm and interfere with your weight-loss goals.
And phentermine isn’t recommended if you’re taking monoamine oxidase inhibitors (MAOIs). The combination could cause dangerously high blood pressure.
Good to know: Phentermine is only approved for people ages 17 and older with a body mass index (BMI) of 30 or more (or 27 if you’re also living with a weight-related health condition such as diabetes or high blood pressure). But BMI isn’t a perfect tool and may not be an accurate measure of your health. Your healthcare team may decide to prescribe phentermine even if you don’t fall into one of these approved categories. But you shouldn’t take phentermine on your own without the guidance of a healthcare professional.
How to save on the cost of phentermine
Phentermine is one of the most commonly prescribed oral weight-loss medications. This is largely due to the fact that it’s also one of the most affordable options. GoodRx can help you navigate ways to save on the cost of your phentermine prescription:
Phentermine capsules are only available as a generic medication. GoodRx can help you pay as little as $14.54 for a 30-day supply of the 15 mg dose.
Adipex-P is available as both a brand-name and lower-cost generic. You can pay as little as $16.85 for a 30-day supply of the generic 37.5 mg dose with GoodRx.
Lomaira is only available as a brand-name medication. You can pay as low as $29.63 per month with a free GoodRx discount. If you have commercial insurance, you can pay as little as $0.50 per pill with a copay card from the manufacturer.
Frequently asked questions
No, phentermine doesn’t seem to cause withdrawal symptoms when you stop taking it. This is a common concern, since withdrawal symptoms can happen with other stimulant medications. But it doesn’t seem to be an issue with phentermine.
The biggest difference is that phentermine is approved for weight loss, while Ozempic (semaglutide) is approved for Type 2 diabetes. Phentermine also isn’t recommended for people living with heart disease, while Ozempic may benefit this group. And phentermine is a daily oral pill, while Ozempic comes as a once-weekly injection as well as a daily oral pill. When it comes to phentermine versus Wegovy, the form of semaglutide approved for weight loss, the two medications work differently to manage your weight. And Wegovy typically results in more weight loss than phentermine.
People participating in clinical studies didn’t report hair loss as a phentermine side effect. But rapid weight loss is known to cause hair loss. In most cases, this type of hair loss should reverse on its own. But let your prescriber know if hair loss is severe or bothersome. They can help determine the cause and suggest options to manage it.
Caffeine and phentermine are both stimulants. Combining them can raise the risk of heart-related side effects. It can also make you feel jittery or more anxious than usual. You may not need to avoid coffee altogether, but it’s best to limit your daily intake. Talk to your prescriber about how much caffeine, if any, is safe for you while taking phentermine.
No, phentermine doesn’t seem to cause withdrawal symptoms when you stop taking it. This is a common concern, since withdrawal symptoms can happen with other stimulant medications. But it doesn’t seem to be an issue with phentermine.
The biggest difference is that phentermine is approved for weight loss, while Ozempic (semaglutide) is approved for Type 2 diabetes. Phentermine also isn’t recommended for people living with heart disease, while Ozempic may benefit this group. And phentermine is a daily oral pill, while Ozempic comes as a once-weekly injection as well as a daily oral pill. When it comes to phentermine versus Wegovy, the form of semaglutide approved for weight loss, the two medications work differently to manage your weight. And Wegovy typically results in more weight loss than phentermine.
People participating in clinical studies didn’t report hair loss as a phentermine side effect. But rapid weight loss is known to cause hair loss. In most cases, this type of hair loss should reverse on its own. But let your prescriber know if hair loss is severe or bothersome. They can help determine the cause and suggest options to manage it.
Caffeine and phentermine are both stimulants. Combining them can raise the risk of heart-related side effects. It can also make you feel jittery or more anxious than usual. You may not need to avoid coffee altogether, but it’s best to limit your daily intake. Talk to your prescriber about how much caffeine, if any, is safe for you while taking phentermine.
The bottom line
Phentermine (Adipex-P, Lomaira) is an oral weight-loss medication. It’s a stimulant that works by reducing your appetite to help you eat less. You may also feel a boost of energy while you’re taking it. In studies, more than 80% of participants lost at least 5% of their body weight while taking phentermine. But it works best if you combine it with regular exercise and a balanced diet.
Phentermine is only FDA approved for short-term use (12 weeks or less). But studies show it’s likely safe to take it long-term if your prescriber OKs it. In most cases, you’ll take phentermine once daily in the morning or a lower dose 3 times daily before meals.
Common phentermine side effects include dry mouth and trouble sleeping. If you’re living with heart disease, have a history of substance misuse, or consume alcohol regularly, phentermine may not be the best weight-loss medication for you.
Why trust our experts?
References
Bryant Ranch Prepack. (2025). Phentermine hydrochloride- phentermine hydrochloride tablet [package insert].
ClinicalTrials.gov. (2026). Long-term effectiveness of the antiobesity medication phentermine (LEAP). National Library of Medicine.
Connolly, H. M., et al. (1997). Valvular heart disease associated with fenfluramine–phentermine. The New England Journal of Medicine.
Federal Register. (2015). Determination that Pondimin (fenfluramine hydrochloride) tablets, 20 milligrams and 60 milligrams, and Ponderex (fenfluramine hydrochloride) capsules, 20 milligrams were withdrawn from sale for reasons of safety or effectiveness. U.S. Food and Drug Administration.
Fintepla. (n.d.). What is the Fintepla REMS (risk evaluation and mitigation strategy)?
Hall, K. D., et al. (2018). Maintenance of lost weight and long-term management of obesity. Medical Clinics of North America.
Hendricks, E. J., et al. (2011). A study of abrupt phentermine cessation in patients in a weight management program. American Journal of Therapeutics.
Hendricks, E. J., et al. (2013). Addiction potential of phentermine prescribed during long-term treatment of obesity. International Journal of Obesity.
Kang, D., et al. (2024). Telogen effluvium associated with weight loss: a single center retrospective study. Annals of Dermatology.
Kang, J. G., et al. (2010). Randomized controlled trial to investigate the effects of a newly developed formulation of phentermine diffuse-controlled release for obesity. Diabetes, Obesity and Metabolism.
Khera, R., et al. (2017). Association of pharmacological treatments for obesity with weight loss and adverse events. JAMA.
Kim, K. K., et al. (2006). Effects on weight reduction and safety of short-term phentermine administration in Korean obese people. Yonsei Medical Journal.
KVK-Tech. (2023). Lomaira- phentermine hydrochloride tablet [package insert].
Lewis, K. H., et al. (2019). Safety and effectiveness of longer-term phentermine use: Clinical outcomes from an electronic health record cohort. Obesity.
Márquez-Cruz, M., et al. (2021). Three- and six-month efficacy and safety of phentermine in a Mexican obese population. International Journal of Clinical Pharmacology and Therapeutics.
MedlinePlus. (2024). Phentermine.
PubChem. (2025). Phentermine. National Library of Medicine.
Rocha-Gonzalez, H. I., et al. (2021). Weight loss at first month and development of tolerance as possible predictors of 30 mg phentermine efficacy at 6 months. Journal of Personalized Medicine.
Ryan, D. H., et al. (2017). Weight loss and improvement in comorbidity: Differences at 5%, 10%, 15%, and over. Current Obesity Reports.
Tak, Y. J., et al. (2021). Long-term efficacy and safety of anti-obesity treatment: Where do we stand? Current Obesity Reports.
Wadden, T. A., et al. (1998). The fen-phen finale: A study of weight loss and valvular heart disease. Obesity Research.
