Phentermine for weight loss: how it works, what to expect and who it's for.

What is phentermine?

Phentermine is a prescription weight loss medication in a class called sympathomimetic amines. It's chemically related to amphetamine, and it works on the brain's appetite signals so you feel less hungry and get full sooner.

It's one of the oldest weight loss medications still in use: the FDA first approved it in 1959. When people talk about "prescription diet pills," phentermine is usually what they mean.

Brand names and forms

• Generic phentermine hydrochloride: 37.5 mg tablets or capsules, usually taken once a day in the morning. Lower-strength generics exist too.
• Adipex-P: the best-known brand of the same medicine.
• Lomaira: a low-dose 8 mg tablet that can be taken up to three times a day before meals.
• Qsymia: phentermine combined with extended-release topiramate in one capsule (covered below).

How it works

Phentermine increases the release of norepinephrine and related brain chemicals that dial down hunger. Most people notice less appetite within the first days. That's also why its side effects look like those of other stimulants: a faster heartbeat, trouble sleeping, dry mouth and feeling jittery.

Does phentermine work for weight loss?

Yes, for many people, but its effect is modest and it isn't a stand-alone fix. Phentermine is approved as an "adjunct": something that makes a calorie-reduced diet, more activity and new habits easier to stick with.

Across prescription weight loss medications, the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) reports that adults who take a medication as part of a lifestyle program lose, on average, 3% to 12% more of their starting weight after a year than people in the same program without medication. Results vary a lot from person to person and from drug to drug.

What affects how well it works

How long can you take phentermine?

Phentermine's label calls it a short-term treatment, meaning "a few weeks." NIDDK notes that health care professionals sometimes prescribe weight loss medicines longer than the FDA-approved period, which is called off-label use. That's a decision for you and your prescriber to make with regular blood pressure and heart-rate checks. When long-term medication is the goal, prescribers more often turn to drugs approved for long-term use, including phentermine and topiramate (Qsymia) and the GLP-1 medications.

Who can take phentermine, and who shouldn't?

The FDA label limits phentermine to adults with a body mass index (BMI) of 30 or higher, or 27 or higher with at least one weight-related condition such as controlled high blood pressure, type 2 diabetes or high cholesterol.

Who should not take phentermine

The label lists these contraindications:

Use with caution

• Even mild high blood pressure: your prescriber should check it regularly.
• Diabetes: doses of insulin or other diabetes medicines may need to come down as you lose weight.
• Kidney disease: the label limits the dose in severe kidney impairment.
• Alcohol and other stimulants: they can add to phentermine's effects and side effects.

Tell your prescriber about every medication and supplement you take, including over-the-counter decongestants and anything sold for weight loss or energy.

Phentermine side effects

Most side effects are mild and come from phentermine's stimulant action. A few are rare but serious, so it's worth knowing which ones need a call to your prescriber.

The label also notes that phentermine can affect your ability to drive or operate machinery, especially at first, and that it shouldn't be stopped and restarted on your own. If side effects bother you, talk to your prescriber rather than adjusting the dose yourself.

Phentermine and topiramate (Qsymia)

Phentermine and topiramate is a combination of two medicines that work on appetite in different ways. As one extended-release capsule, it's sold as Qsymia, which the FDA approved in 2012 for chronic weight management, meaning long-term use. (The name is often misspelled "topimirate"; the correct spelling is topiramate.)

Topiramate is a seizure and migraine medicine. In Qsymia, it's paired with a lower dose of phentermine than the standard 37.5 mg tablet. The capsule comes in four strengths, from a starting dose of 3.75 mg/23 mg up to 15 mg/92 mg, taken once a day in the morning.

How well does it work?

In the two main year-long trials, adults on the combination lost considerably more weight than those on placebo, with everyone following a diet and lifestyle program:

Average loss of starting body weight. Individual results vary.

What's different about it

• Built-in check-ins: after 12 weeks on a dose, your prescriber checks your progress and either raises the dose or tapers you off if you haven't lost enough.
• Pregnancy precautions: topiramate can cause birth defects. Anyone who could become pregnant takes a pregnancy test before starting and every month, uses reliable birth control, and fills it through a pharmacy in the Qsymia REMS safety program.
• Different side effects: tingling in the hands, feet or face, changes in taste, trouble sleeping, dry mouth and constipation are the most common. Kidney stones, mood changes, trouble concentrating and eye problems are less common.
• Never stop suddenly: topiramate needs to be tapered, because stopping all at once can cause seizures.
• Teens: Qsymia is approved for children 12 and older who meet BMI criteria for their age.

Separate phentermine and topiramate prescriptions

Some prescribers write two separate prescriptions, for generic phentermine and generic topiramate, instead of Qsymia. That's off-label use: the separate pills aren't the same extended-release formulation that was studied in the trials above, and the dosing, monitoring and pregnancy precautions are up to the prescriber. If you're offered this approach, it's reasonable to ask why, how you'll be monitored, and how it compares with the approved combination.

How does a phentermine prescription work?

Because phentermine is a Schedule IV controlled substance, a licensed prescriber has to evaluate you before writing it, and refills are limited by law.

1. A medical evaluation.Expect your height, weight and BMI, blood pressure and heart rate, your medical history and a full list of your medications. Some prescribers also order lab work or an EKG.
2. A decision together.Your prescriber weighs the benefits against the risks for you, and may suggest a different medication, or none, depending on your health and goals.
3. Regular follow-up.Most prescribers recheck your weight, blood pressure and heart rate every few weeks, especially in the first months.
4. A refill plan.Federal law limits how many times a controlled-substance prescription can be refilled, so ongoing treatment means ongoing check-ins.
5. A plan for stopping.Ask early what happens when the medication ends, and how you'll keep the weight off afterward.

Can you get phentermine online?

Rules for prescribing controlled substances through telehealth are stricter than for most medications, and they've been changing, so many prescribers see patients in person for phentermine. Any website that sells phentermine without a prescription and an evaluation is operating illegally. The FDA's BeSafeRx program explains how to recognize a legitimate online pharmacy: it requires a valid prescription, is licensed in the U.S. and has a pharmacist you can reach.

Does insurance cover it?

Coverage for weight loss medication varies by plan. Generic phentermine is one of the lower-cost prescription options, while brand-name combinations like Qsymia usually cost more. Ask your pharmacy and your insurer before you fill a prescription.

Phentermine vs. over-the-counter diet pills

There is no over-the-counter phentermine. Supplements with "phen" in their names, or that promise "phentermine-like" results, don't contain phentermine, and their claims aren't reviewed by the FDA the way a prescription drug's are.

Some products are worse than ineffective. The FDA warns that many diet pills, fat burners and weight loss supplements and teas may be contaminated with hidden drug ingredients, even when they're advertised as natural, and it keeps a running list of products it has found. Hidden stimulants can raise heart rate and blood pressure, the same risks the phentermine label warns about, without anyone checking on you.

For a broader look at what the evidence says about weight loss pills, see Do weight loss pills work?

Phentermine vs. GLP-1 medications

GLP-1 medications such as semaglutide and tirzepatide work differently from phentermine. They mimic gut hormones that slow digestion and signal fullness, and in large trials they produced more weight loss than phentermine-based treatment. Phentermine is an oral, lower-cost option with a much longer track record.

Trial results are averages from different studies with different participants, so they aren't a head-to-head comparison. The right choice depends on your health history, other medications, budget and how you feel about pills versus injections. Learn more about semaglutide for weight loss.