Phentermine for Weight Loss
Phentermine is the oldest weight-loss medication still in clinical use and, until Wegovy and Zepbound arrived, the most prescribed. It has been FDA-approved since 1959. It is inexpensive, it works, and at BeLite it is usually prescribed together with 5-HTP and carbidopa, a combination our Medical Director has published on.
What phentermine is
Phentermine is an oral medication that increases norepinephrine activity in the parts of the brain that control appetite. It is chemically related to amphetamine, but with an important difference: amphetamine releases both dopamine and norepinephrine, and the dopamine release is what drives its addictive potential. Phentermine releases norepinephrine only. Dependence on phentermine is rare in clinical practice, and a 2019 study of nearly 14,000 patients found that longer-term use produced more weight loss with no increase in cardiovascular events. We review that evidence in Is Phentermine Safe for Longer-Term Weight Loss?.
What to expect
In published trials, phentermine alone typically produces a loss of about 5–10% of body weight over several months — a ceiling our Medical Director has attributed to the fact that any single medication targets only one of the several pathways regulating appetite. Many BeLite patients on phentermine combined with 5-HTP/carbidopa do better than that: in observational data from 91 patients published by Dr. Rothman in 2010, the combination produced an average loss of about 4.6 kg (roughly 10 pounds) in four weeks. Weight loss is usually fastest in the first three months and continues more slowly after that.
Phentermine plus 5-HTP/carbidopa
Phentermine works on norepinephrine. 5-hydroxytryptophan (5-HTP) is the natural precursor to serotonin, a second appetite pathway. Taken by mouth, most 5-HTP is broken down before reaching the brain; carbidopa, a prescription medication, blocks that breakdown so that a small dose of 5-HTP raises brain serotonin. BeLite has used this combination for years. Dr. Rothman described the approach, and reported outcomes from 91 patients treated with it, in a 2010 paper in the American Journal of Therapeutics. Those are observational results from clinical practice rather than a randomized trial, and the paper itself calls for controlled trials before combination treatment is widely adopted. Read more in Is 5-HTP the Right Weight Loss Medication for You?.
Who can benefit from phentermine — and who should not take it
Phentermine is approved for adults with a BMI of 30 or higher, or 27 or higher with a weight-related condition. It should not be taken by anyone with a history of heart attack or stroke, heart failure, uncontrolled high blood pressure, significant heart-rhythm problems, hyperthyroidism, glaucoma, or drug abuse, by anyone taking an MAOI antidepressant, or during pregnancy. This is exactly why experienced prescribers matter: BeLite checks blood pressure at every visit and performs an EKG at the first visit before prescribing.
How it is taken
- One dose in the morning, before or shortly after breakfast. Some patients are prescribed a second, smaller afternoon dose.
- Doses range from 8 mg to 37.5 mg; your provider chooses the dose that works best for you with the fewest side effects.
- Do not take it late in the day; it can interfere with sleep.
- If you forget a morning dose, skip it — never double up.
Side effects
The common ones are dry mouth, constipation, trouble sleeping, faster heart rate, and jitteriness, most of which fade within a couple of weeks or respond to a dose adjustment. Chest pain, shortness of breath, or a severe headache should be reported the same day. Phentermine is a Schedule IV controlled substance, so prescriptions require regular in-person follow-up.
Long-term use
Qsymia, a fixed combination of phentermine and topiramate, is FDA-approved for long-term treatment of obesity, and the 2019 cohort study above supports extended phentermine use under medical supervision. Many BeLite patients use phentermine for maintenance after reaching their goal.
Ready to find out which medication fits you? BeLite Medical Center’s medical providers have been prescribing weight-loss medication in Fairfax since 1995. Schedule online 24/7 at belitecom.simplybook.me or call or text (703) 359-9200. The consultation is free for new patients; the $150 new-patient fee applies only if you enroll.
Disclaimer
This page is general information, not medical advice. Whether any medication is right for you depends on your health history and can only be decided by a licensed medical provider after an evaluation. Prescription medications are prescribed only after an in-person visit at BeLite Medical Center, 3923 Blenheim Blvd, Suite 61A, Fairfax, VA 22030.
Sources (verified September 7, 2026)
- Lewis KH et al. Safety and Effectiveness of Longer-Term Phentermine Use: Clinical Outcomes from an Electronic Health Record Cohort. Obesity (Silver Spring). 2019;27(4):591-602. https://pubmed.ncbi.nlm.nih.gov/30900410/
- Rothman RB. Treatment of obesity with “combination” pharmacotherapy. Am J Ther. 2010;17(6):596-603. https://pubmed.ncbi.nlm.nih.gov/19352140/
Medications
Zepbound
The most effective injectable for patients who plateau or cannot take stimulants.
Foundayo
A non-stimulant daily pill with a different mechanism.
Wegovy
The weekly semaglutide injection, with proven cardiovascular benefit.
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