Key takeaways
Phentermine is a generic weight loss medication and has a standard daily dose of 15-37.5 milligrams (mg).
Phentermine is FDA-approved only for short-term use as an adjunct to lifestyle changes like diet and exercise.
Phentermine is considered generally safe and effective, but it can cause side effects like high blood pressure and rapid heartbeat. The drug is also a controlled substance due to a potential risk of abuse or misuse.
Blockbuster GLP-1 agonists, including Wegovy and Zepbound, have gotten a lot of buzz lately, but they’re not the only weight loss treatments on the market. Take phentermine, a well-established generic prescription weight-loss drug.
Before starting phentermine, it’s important to understand how it works, who should take it, and its side effects and risks.
What is phentermine and how does it work?
Phentermine is a prescription generic medication also sold under the brand names Adipex-P and Lomaira. Phentermine is approved by the FDA as a short-term adjunct to weight loss treatment plans involving a reduced-calorie diet and exercise. It belongs to a class of drugs called anorectics (appetite suppressants), but it is also chemically related to central nervous system (CNS) stimulants, such as Adderall or Vyvanse.
“Phentermine works by increasing the release of the chemical, norepinephrine, from the brain, thereby suppressing appetite and reducing food intake,” says Cynthia Odogwu, MD, a board-certified family medicine physician in Maryland.
Note: Phentermine may pose a risk for abuse and misuse. Because of this, phentermine is a Schedule IV controlled substance and should be avoided in people with a history of substance use disorder.
Who is phentermine 37.5 mg for?
Phentermine is FDA-approved for short-term weight loss in people with a body mass index (BMI) of 30 or greater (obese) and 27 or greater (overweight) with other risk factors (controlled hypertension, diabetes, or hyperlipidemia). It should not be used by anyone under 16 years of age because its safety and efficacy have not been established in pediatric patients.
Because it acts in the brain rather than the digestive system, phentermine 37.5 mg works well for patients with “an exaggerated drive for food,” says Dr. Chrissie Ott, MD, a healthcare advocacy consultant who is board-certified in pediatrics, internal medicine, and integrative medicine. She says she would consider prescribing it to a patient who has been unable to sustain weight loss with lifestyle interventions alone, adding that the ideal candidate may be someone ”not able to access GLP-1 medications due to cost, contraindications, or intolerance.”
Phentermine HCl 37.5 mg cost
Without insurance, the current retail price for phentermine HCl is $50 for 30, 37.5-mg tablets. This comprises a one-month supply.
Insurance coverage can dramatically reduce the out-of-pocket cost of phentermine. The exact cost of phentermine with insurance varies depending on the insurance plan, copay, and, in some cases, whether the annual deductible has been met.
People who are uninsured or underinsured can use a free SingleCare coupon to lower the cost to $10 for 30, 37.5 mg tablets of phentermine.
Save up to 80% on Phentermine with SingleCare
Different pharmacies offer different prices for the same medication. SingleCare helps find the best price for you.
Dosing and administration
Phentermine is available in oral dosage forms, such as capsules or pills. It is available in doses of 8 mg, 15mg, 30mg, and 37.5mg. A standard dose of phentermine is 15-37.5 mg per day, sometimes divided into two doses. Phentermine 37.5 mg per day is also the maximum dosage approved by the FDA. “People with severe kidney dysfunction should only take up to 15 mg a day,” Dr. Odogwu says.
Patients need to work with a healthcare provider to determine the best starting and maintenance doses. Dr. Ott often starts her own patients on just 8 mg of phentermine to test for efficacy and tolerance. “Starting at too high a dose can lead to uncomfortable symptoms including dizziness or palpitations,” she says.
Because phentermine can cause addiction and other serious side effects, it’s important to stay in contact with a healthcare professional throughout the entire course of phentermine treatment. Never adjust the phentermine dosage or stop taking it without consulting a healthcare provider.
Potential side effects
According to DailyMed, the most common side effects of phentermine include:
- Elevated blood pressure (hypertension)
- Dry mouth
- Overstimulation
- Dizziness
- Constipation
Dr. Odogwu says phentermine can also cause trouble sleeping (insomnia) if taken in the afternoon or evening.
Serious side effects of phentermine include:
- Valvular heart disease (decreased blood flow to the heart)
- Pulmonary hypertension
- Physical or psychological dependence
It’s important to seek medical advice for any severe or concerning side effects. Seek urgent medical attention if you develop chest pain, trouble breathing, psychosis, or signs of an allergic reaction (throat constriction, facial swelling, or rapid-onset hives) after taking phentermine.
Who should avoid phentermine?
Phentermine is not right for everyone.
Per the drug label on DailyMed, you should not use phentermine if you have the following conditions:
- History of cardiovascular disease (e.g., congestive heart failure, coronary artery disease, stroke, arrhythmia, uncontrolled hypertension)
- Hyperthyroidism
- Glaucoma
- Agitated state
- History of drug abuse
- Pregnancy, trying to become pregnant, or breastfeeding
- History of allergic response to any of the medication’s ingredients
You should also not use phentermine within 14 days of taking a monoamine oxidase inhibitor (MAOI), such as selegiline or Parnate (tranylcypromine).
Due to a lack of data on possible drug interactions, phentermine should not be taken with:
- Other prescription weight loss medications
- Over-the-counter weight loss supplements or herbal products
- Selective serotonin reuptake inhibitors (SSRIs), such as fluoxetine, sertraline, fluvoxamine, or paroxetine
- Other serotonergic medications, such as fenfluramine
Never take phentermine 37.5 mg without a current prescription from a licensed healthcare provider. Talk to your provider to help determine if phentermine is right for you.
The role of lifestyle
Phentermine can be a powerful tool for weight loss, but it is not a magic pill. This medication is intended to be used in combination with lifestyle changes, such as diet and exercise.
A reduced-calorie diet and regular exercise help create a calorie deficit, leading to body weight reduction. In addition, making diet and exercise changes helps create habits that sustain weight loss or weight management after phentermine is discontinued.
Additional strategies for weight loss include increasing protein intake, optimizing sleep quality, and reducing alcohol intake. For personalized medical advice on weight loss with or without phentermine, it’s a good idea to talk to your prescribing healthcare provider or a nutritionist.
Long-term use and dependency concerns
Phentermine is prescribed as a short-term treatment because of the potential risk of dependency. Phentermine is not an amphetamine, but it is chemically similar. Because of concerns over this similarity, phentermine is defined as a Schedule IV controlled substance.
That said, one study on long-term phentermine use found that patients did not experience psychological dependence (addiction), phentermine cravings, or phentermine withdrawal after abrupt discontinuation of the medication. Another large, observational study found that long-term phentermine use was safe, effective, and not associated with a greater risk of death or heart disease.
Ultimately, the risks of long-term phentermine treatment are not well-established. Because of the drug’s chemical similarity to highly addictive drugs, it is only approved as a short-term adjunct therapy. Anyone interested in taking phentermine long-term should discuss the risks versus benefits with their healthcare provider.
The risk of withdrawal symptoms after quitting phentermine is also not well-established. Anyone who wishes to discontinue phentermine or switch to a different weight loss medication should do so under the guidance of a licensed healthcare provider.
Is phentermine right for you?
Only a healthcare provider familiar with your medical history, health conditions, and other medications can determine whether phentermine is right for you.
When consulting with a healthcare provider about whether or not to take phentermine, be sure to discuss the following:
- Personal and family medical history
- Your medications, including over-the-counter drugs and supplements
- Personal risk or concerns about substance use disorder
- Heart health concerns
- Previous weight loss strategies
- Tolerance for side effects
- Proposed timeline for phentermine use
- Potential risks and benefits of short-term versus long-term phentermine use
“My advice is to start low and go slow, and make sure you have a physician who is watching out for side effects with you,” Dr. Ott says.
The SingleCare prices in this article are the most accurate at the time of publishing in ZIP code 23666 as of Sep. 3, 2026. Prices vary by pharmacy. Visit our coupon page for updated drug prices at pharmacies near you.
- Phentermine: A sympathomimetic amine with a long history, Obesity Medicine Board Review Questions (2025)
- Phentermine hydrochloride tablet, DailyMed (2025)
- Optimal diet strategies for weight loss and weight loss maintenance, Journal of Obesity & Metabolic Syndrome (2020)
- Clinical evidence and mechanisms of high-protein diet-induced weight loss, Journal of Obesity & Metabolic Syndrome (2020)
- Sleep deprivation: Effects on weight loss and weight loss maintenance, Nutrients (2022)
- Alcohol intake and weight loss during an intensive lifestyle intervention for adults with overweight/obesity and diabetes, Obesity (2019)
- Addiction potential of phentermine prescribed during long-term treatment of obesity, International Journal of Obesity (2014)
- Safety and effectiveness of longer-term phentermine use: Clinical outcomes from an electronic health record cohort, Obesity (2019)
Featured experts
- Cynthia Odogwu, MD, a board-certified family medicine physician in Maryland
- Chrissie Ott, MD, a healthcare advocacy consultant who is board-certified in pediatrics, internal medicine, and integrative medicine
- Medically accurate: SingleCare’s Medical Review Board analyzes all of our content to confirm it’s in line with current medical advice.
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