Phentermine

Phentermine is a prescription appetite suppressant taken by mouth for short-term weight reduction in adults with obesity. It is a controlled substance in the United States. It acts on the brain, not on fat tissue, and the label pairs it with a reduced-calorie diet, more physical activity and changes to eating habits.

This page explains, in everyday language, what the medicine is approved to do, how it feels to take, which side effects show up most often, who should avoid it, and what drives the monthly cost when you pay yourself. It is a reference page, not a prescription and not a treatment plan.

What this medicine is, and who it is meant for

Phentermine hydrochloride is an oral prescription drug in a class called sympathomimetic amine anorectics. Anorectic simply means appetite-reducing. It has been on the US market for decades and is sold as generic tablets and capsules, and under brand names including Adipex-P and Lomaira.

Because it is chemically related to amphetamine and can be misused, the Drug Enforcement Administration lists it as a Schedule IV controlled substance. That classification shapes almost everything practical about getting it: how many refills a prescription can carry, how often you must be seen, and whether a telemedicine visit is enough in your state. The federal scheduling framework is set out in the DEA drug scheduling overview.

The approved patient group is defined by body mass index, or BMI, which compares your weight to your height. Labeling covers adults with a BMI of 30 or higher, and adults with a BMI of 27 or higher who also have a weight-related risk factor such as controlled high blood pressure, type 2 diabetes or high cholesterol. It is not approved for cosmetic weight loss, and it is not a first step before diet and activity changes; it is meant to sit alongside them.

If you are unsure where your own numbers fall, work it out before your consult rather than guessing.

Knowing your BMI and your blood pressure ahead of time makes a first appointment much shorter, because both are screening gates for this drug class.

How it works, and why it does not burn fat by itself

The medicine triggers the release of norepinephrine, a signalling chemical, in parts of the brain that regulate hunger. The practical effect is that hunger signals get quieter and meals feel finishing sooner. Many people also notice a mild stimulant effect: more alertness, sometimes a faster heartbeat, sometimes trouble sleeping.

So does it burn fat, or only suppress appetite? Mostly the second. There may be a small rise in the energy your body uses at rest, but the weight change comes from eating less over time. Nothing about the drug removes fat directly. That distinction matters, because if appetite does not drop, or if calorie intake stays the same, the scale usually does not move. A plain patient-facing summary of the appetite mechanism is available from Mayo Clinic on weight-loss medicine.

How fast do results come? Published studies of short courses generally report average weight reductions in the mid single digits as a percentage of starting weight over roughly three months, with wide variation between individuals. One example of this kind of short-course data appears in a peer-reviewed efficacy and safety analysis. Averages are not promises. Some people lose little, and some cannot tolerate the stimulant effects long enough to find out.

The label also flags something patients rarely hear about in advance: tolerance to the appetite-suppressing effect usually develops within a few weeks. When that happens, the answer is not a bigger dose. The label directs that treatment be stopped instead of escalated beyond the recommended amount.

What the label approves, and how long a course usually runs

Approved use is narrow and specific: a short-term adjunct, described in labeling as a few weeks, within a weight-reduction program built on exercise, behavioral change and calorie restriction. It is not approved as a standalone treatment, and single-ingredient products are not approved for years of continuous chronic weight management.

In real practice, some clinicians prescribe longer courses or repeated courses, and researchers have studied use beyond twelve weeks. That is off-label prescribing. It can be reasonable and it can also be poorly monitored, so it is fair to ask a prescriber directly how long they intend to continue treatment, what they will monitor, and how they plan to stop. Current approved labeling for each product, including indications and contraindications, is published on DailyMed, the NIH label library.

One more label point that changes expectations: combining this drug with other weight-loss medicines is not recommended, because the safety and effectiveness of those combinations have not been established. If a program suggests stacking products, that is a question worth pressing on before you agree.

Side effects people actually report

Most side effects come from the stimulant action and from reduced fluid and food intake. Common complaints include:

• Dry mouth, sometimes with an unpleasant taste
• Trouble falling asleep, especially with later dosing
• Constipation, and sometimes nausea or diarrhea
• Faster heart rate and palpitations
• Higher blood pressure readings
• Restlessness, jitteriness, irritability or anxiety
• Headache, dizziness or tremor

Serious reactions are rare but they are the reason this drug is monitored. Labeling describes rare cases of primary pulmonary hypertension, which is high blood pressure in the lung arteries, and rare cases of serious heart valve disease. New or unexplained shortness of breath, chest pain, fainting or swelling in the legs are treated as stop-and-call-now symptoms. Psychosis has been reported at high doses. Dependence can develop, and abrupt withdrawal after long use at high doses can bring fatigue and low mood.

Searches often split by sex, so it is worth being precise. The label lists changes in libido and impotence among reported effects, which is where much of the interest from men comes from. For women, the important label facts are different: use during pregnancy is not appropriate, and the medicine is not recommended while breastfeeding. Weight loss offers no known benefit during pregnancy and may cause harm. Beyond that, there is no separate, established female side effect profile; the common effects listed above apply to everyone. A patient-level summary of adverse effects is maintained at MedlinePlus, the NIH consumer drug resource.

If something unexpected happens, side effects can be reported to the FDA MedWatch adverse event program, which feeds the national safety signal system.

Who should not take it, and which drugs clash with it

Contraindications are the strongest safety language a label carries. They mean do not use, not use with care. For this medicine they include a history of cardiovascular disease such as coronary artery disease, stroke, arrhythmias, heart failure or uncontrolled high blood pressure; overactive thyroid; glaucoma; agitated states; a history of drug misuse; pregnancy and breastfeeding; and known sensitivity to sympathomimetic amines.

The interaction that gets the most attention is with monoamine oxidase inhibitors, a class of older antidepressants and one Parkinson’s drug. Taking this appetite suppressant during or within fourteen days of an MAO inhibitor can cause a dangerous spike in blood pressure. That fourteen-day gap is spelled out in approved labeling and is not a judgement call.

Other combinations that need a prescriber’s attention:

• Other stimulants, including some decongestants and ADHD medicines, which can stack cardiovascular effects
• Alcohol, which can worsen side effects
• Blood pressure medicines, because effects can be blunted or amplified
• Insulin and oral diabetes medicines, because eating less can change how much you need
• Serotonergic medicines, where clinicians watch for signs of serotonin syndrome

Two practical points rarely covered elsewhere. First, this drug can impair the ability to drive or operate machinery, particularly early on. Second, it can produce a false positive for amphetamines on a urine drug screen, so tell an employer’s testing provider or a clinic that you take a prescribed appetite suppressant.

Strengths, brands and the topiramate combination

Generic phentermine hydrochloride is sold as capsules and tablets in more than one strength, and the 37.5 mg tablet is the version most people picture when they hear the name. Lomaira is a lower-strength 8 mg tablet designed to be taken more than once daily. Adipex-P is a long-standing brand. Which strength and schedule fits you is a prescriber decision made inside the limits the label sets; this page deliberately gives no dosing instructions.

There is also a separate approved product that combines phentermine with extended-release topiramate, an anti-seizure medicine that reduces appetite through a different route. Unlike the single-ingredient tablets, that combination capsule is approved for chronic weight management, and it carries its own risks, most notably a risk of birth defects such as cleft lip and palate, which is why pregnancy testing and contraception counselling are part of the program. Cleveland Clinic publishes a clear patient overview of the phentermine and topiramate combination capsule.

How it compares with GLP-1 medicines and with ADHD stimulants

Two comparisons dominate patient questions. The first is against the GLP-1 medicines. These are different drug classes with different label scopes, and neither this page nor the labels support a blanket claim that one is better for everyone.

The everyday differences matter as much as the pharmacology. One is a pill you take daily with a short approved course and stimulant side effects. The others are weekly injections designed for long-term use, with gastrointestinal side effects and, for most people paying cash, much higher monthly outlay. If you are weighing that route, our How Semaglutide Works page and Tirzepatide Explained cover their labels and side effect profiles in the same plain-English format.

The second comparison is with Adderall. Both are stimulants and both are controlled, but they sit in different places. Adderall is a Schedule II amphetamine product approved for ADHD and narcolepsy. The appetite suppressant discussed here is Schedule IV and approved only for weight reduction. They are not interchangeable, and appetite loss on an ADHD medicine is a side effect, not an approved use.

What a month costs when you pay yourself

Cost is the reason many readers land here, and this is one of the few weight medicines where generics change the picture. Across the self-pay programs tracked in our directory, the lowest recurring monthly price is and the median sits at across tracked programs. Advertised prices move often, so treat any figure as a snapshot rather than a fixed rate.

What actually moves your number:

• What the fee buys. Some programs quote a membership that includes the consult and follow-ups but not the medicine. Others bundle the prescription. The two are not comparable until you add them up.
• Brand versus generic. Generic tablets and brand-name products are priced very differently, and your prescription may specify one.
• Pharmacy route. Controlled substances are usually sent to a local pharmacy rather than shipped, so the pharmacy’s own cash price becomes part of your total.
• Intro versus renewal. A discounted first month tells you little about month four. Ask what the renewal charge is before you start.
• Extras. Lab work, blood pressure monitoring devices and coaching add-ons sometimes sit outside the headline fee.

Run the twelve-month figure rather than the monthly one, because that is the number that decides whether you can sustain treatment.

Insurance is a separate track from cash pay and should never be blended with it. Many plans exclude weight-loss drugs entirely, some cover generic appetite suppressants at a low generic copay, and prior authorization is common. Nothing on this page can tell you what your plan does; only your plan document and a call to your insurer can. For broader coverage-policy context, KFF publishes health coverage research on how insurers treat obesity treatment. If you want to see how a self-pay comparison is laid out before spending anything, the Sample Savings Report shows the format, and the Free Health Calculators cover the arithmetic side.

How telemedicine programs handle a controlled substance

Prescribing rules are the biggest difference between this medicine and the injectables sold online. Because it is federally controlled, telemedicine prescribing is governed by both federal rules and your state’s own requirements, and those requirements have changed repeatedly in recent years. Some states require an in-person examination before or during treatment. Some limit how long a course can run or how many refills a single prescription can carry.

Programs differ in ways that are worth comparing before you sign up:

1. State coverage. A program may serve most states for GLP-1 medicines but far fewer for controlled ones.
2. Screening depth. Some require a recent blood pressure reading, a heart history review or labs; others ask very little. Thin screening is a warning sign for a drug with cardiovascular contraindications.
3. Follow-up cadence. Ask how often you will be reassessed and who reviews the results.
4. Prescriber access. Can you message the clinician who wrote the prescription, or only a support inbox?
5. Exit plan. Because tolerance is expected, ask what happens when appetite suppression fades.

You can browse programs and patient reviews in the Telehealth Provider Directory, see how weight programs stack up in Comparing GLP-1 Telehealth Programs, and read how the comparison scores are constructed on the How Scores Are Built page. Related compounds are indexed across Every Medication Guide, and ongoing analysis sits in our Editorial Weight Loss Coverage.

Questions to ask, and symptoms that mean stop

Take these to a first consult. They are the questions that separate a careful program from a form-filling one:

• Given my blood pressure and heart history, is a stimulant appetite suppressant appropriate for me at all?
• How long do you plan to treat, and is that within or beyond approved labeling?
• What will you monitor, and how often will you check it?
• Which of my current medicines and supplements interact with this one?
• What is the plan when the appetite effect fades?
• What happens if I need to stop, and will I be tapered?
• What does the monthly charge include, and what does it exclude?

Some symptoms are not wait-and-see. Get urgent medical help for chest pain, new or worsening shortness of breath, fainting or near-fainting, swelling in the legs or feet, a very fast or irregular heartbeat, severe headache with blurred vision, or signs of a severe allergic reaction. Also contact your clinician promptly about severe insomnia, marked agitation, or mood changes. These are general warning signs, not a diagnosis, and reporting them early is how rare serious problems get caught.

Where these facts come from

The clinical statements above track approved US labeling and mainstream patient references rather than marketing material. Further primary sources:

• Approval records and label history via Drugs@FDA, the FDA approvals database
• Treatment context from NIDDK on prescription weight medicines

Educational content, not medical advice. Always consult a qualified clinician before starting, stopping or switching treatment.