How much weight do people lose on phentermine?

In a 12-week randomised trial, people taking 37.5 mg once a day lost 6.7 kg on average — significantly more than placebo — and over 80% of them lost at least 5% of their starting weight. A second 12-week trial of a 30 mg controlled-release version recorded 8.1 kg against 1.7 kg on placebo. In a health-record study of 13,972 people, those using it for a few weeks as the label allows had lost 2.7% of their body weight by 6 months, while the 144 who stayed on it for more than a year were 7.5% down at 24 months. Those last two figures come from health records rather than a trial, and individual results in all of these studies varied widely.

Is phentermine a controlled substance?

Yes. Both the Adipex-P and Lomaira labels state that phentermine is a Schedule IV controlled substance. The labels also note that amphetamines and other stimulant drugs have been extensively abused, and say the possibility of abuse should be kept in mind when deciding whether to include the drug in a weight-reduction programme.

Why is it only approved for a few weeks?

Phentermine was approved for weight loss in 1959, and the approval was granted for short-term use — the label says "a few weeks". The 2015 Endocrine Society guideline records the approval as "Approved in 1960s for short-term use (3 mo)" and devotes a section to the resulting problem: weight tends to come back when the drug stops, yet the approval covers only a few weeks. It sets out conditions under which prescribing beyond that period could be reasonable and says state medical board guidance takes precedence. Whether to continue beyond the label is a decision for a prescribing clinician, not a rule you can read off a website.

What does the evidence say about taking it for longer than a few weeks?

One large study followed 13,972 adults through their health records. The 144 of them who took phentermine continuously for more than 12 months had lost 7.4% more of their body weight at 24 months than people who used it short-term, and the combined rate of heart attack, stroke, angina, bypass surgery, carotid procedure or death across the whole cohort was 0.3% (41 events) with no significant difference between the duration groups. Two limits matter: it was an observational study, not a randomised trial, so people who chose to stay on the drug may have differed from those who stopped; and 41 events spread across five groups is too few to rule out a difference in risk.

Who does the label say should not take it?

The label lists a history of cardiovascular disease (including coronary artery disease, stroke, arrhythmias, congestive heart failure and uncontrolled hypertension), use during or within 14 days of a monoamine oxidase inhibitor, hyperthyroidism, glaucoma, agitated states, a history of drug abuse, pregnancy, nursing, and known hypersensitivity to sympathomimetic amines. It is also not recommended for anyone aged 16 or under.

How does it compare with the newer weight-loss drugs?

There are no head-to-head trials, so the honest answer is that the two sets of numbers come from different studies. Phentermine's randomised trials ran 12 to 28 weeks and reported kilograms; the trials behind tirzepatide and semaglutide ran 68 to 72 weeks and reported average weight loss of 20.9% and 14.9% of body weight. Phentermine costs a small fraction of what those drugs cost.

Phentermine (Adipex-P, Lomaira)

Schedule IV controlled substance

Both phentermine labels state that phentermine is a Schedule IV controlled substance, and that the possibility of abuse should be kept in mind when deciding whether to include it in a weight-reduction programme. The labels also warn that it may impair the ability to drive or operate machinery.

What phentermine is and how it works

The oldest and cheapest prescription weight-loss drug still widely used in the United States, first approved for weight loss in 1959. Adipex-P is a 37.5 mg tablet or capsule taken once a day; Lomaira is an 8 mg tablet taken three times a day before meals. Both labels describe it as a short-term treatment, and it is the phentermine half of the combination pill Qsymia.

A sympathomimetic amine anorectic — a stimulant-type appetite suppressant related to the amphetamines. It acts on the brain's appetite centres to reduce hunger; the labels also record stimulant effects on blood pressure and heart rate.

Two versions are on the market. The 37.5 mg tablet or capsule is taken once a day, before breakfast or one to two hours after it. The 8 mg tablet is taken three times a day, half an hour before meals, and is scored so half a tablet can be given to people who do not need the full dose. The labels say the dose should be individualised to the lowest amount that works.

What the studies found

Phentermine has been on the market since long before the era of large, 72-week obesity trials, so the evidence behind it is thinner and shorter than the evidence behind the newer drugs. Four studies carry most of the weight.

A 12-week randomised trial in Seoul

68 adults were randomly assigned to phentermine 37.5 mg once a day or placebo, with behavioural therapy for everyone. On average, the phentermine group lost 6.7 kg — significantly more than placebo. The paper reports that over 80% of them lost at least 5% of their starting weight and more than half lost at least 10%. Dry mouth and insomnia were the only side effects significantly more common than on placebo. Two things to hold alongside those numbers: entry needed a BMI of 25 or above, the obesity threshold used in the Asia-Pacific region rather than the 30 or 27 on the US label, and only 12 of the 33 placebo participants finished the study.

A 12-week trial of a controlled-release version

74 adults with obesity and controlled diabetes, high blood pressure or high cholesterol took either a 30 mg controlled-release phentermine or placebo once a day. Average weight loss was 8.1 kg on the drug against 1.7 kg on placebo. At least 5% of body weight was lost by 95.8% of the treated group against 20.8% on placebo, and at least 10% by 62.5% against 4.7%. Dry mouth and insomnia were again the most common side effects. This was a newly developed controlled-release formulation tested in Korea, not one of the phentermine products sold in the United States.

A 28-week trial that tested phentermine on its own

756 adults were randomised across seven groups to test a combination pill against each of its two ingredients alone and against placebo. At least 5% weight loss was reached by 46.2% of the phentermine 15 mg group and 43.3% of the phentermine 7.5 mg group, against 15.5% on placebo. Placebo participants lost 1.7% of their body weight. The combination groups lost more than either ingredient alone.

A health-record study of 13,972 people

Researchers followed adults across eight US health systems from their first phentermine prescription. People who used it short-term, as the label allows, had lost 2.7% of their body weight by 6 months, 1.4% by 12 months, and by 24 months their weight was no different from where they started. The 144 people who took it continuously for more than a year were 7.5% down at 24 months. This was a records-based study rather than a randomised trial, and that long-term group was a small fraction of the whole cohort.

Who the label says it is for — and who it is not for

The label describes phentermine as a short-term (a few weeks) addition to a weight-reduction programme of exercise, behaviour change and reduced calories, for adults with a starting BMI of 30 or above, or 27 or above alongside another risk factor such as controlled high blood pressure, diabetes or raised cholesterol. Phentermine is a Schedule IV controlled substance. It is not recommended for people aged 16 or under.

The label lists these as contraindications

A history of cardiovascular disease — coronary artery disease, stroke, arrhythmias, congestive heart failure, uncontrolled high blood pressure
Use during or within 14 days of a monoamine oxidase inhibitor
An overactive thyroid (hyperthyroidism)
Glaucoma
Agitated states
A history of drug abuse
Pregnancy
Nursing
Known hypersensitivity or idiosyncrasy to the sympathomimetic amines

The 2015 Endocrine Society guideline goes further on one point: in people with uncontrolled high blood pressure or a history of heart disease, it recommends against using phentermine or diethylpropion at all. Whether any of this applies to you is a question for your own clinician, who can see your blood pressure, your history and your other medicines.

Side effects the label lists

The phentermine labels predate the modern format, so they list side effects by body system without percentages. That is a gap in the evidence, not a sign that side effects are rare: in the two 12-week trials, dry mouth and insomnia were common enough to separate clearly from placebo.

Dry mouth
Insomnia
Restlessness or overstimulation
Raised blood pressure, fast heartbeat or palpitations
Headache, dizziness or tremor
Constipation, diarrhoea or an unpleasant taste
Changes in libido or impotence
Pulmonary hypertension and heart-valve disease (rare)

On the two rare but serious entries: the label reports that primary pulmonary hypertension — a rare, frequently fatal lung disease — and serious heart-valve disease have been reported in people who took phentermine combined with fenfluramine or dexfenfluramine, a combination the label does not sanction and which is not prescribed today. The label does not stop there, though. For both conditions it adds that an association with phentermine on its own cannot be ruled out, and that there have been rare cases in people who reportedly took phentermine alone. It instructs that treatment be stopped in anyone who develops new, unexplained breathlessness, chest pain, fainting or swelling of the lower legs. It also states that the safety and effectiveness of combining phentermine with any other weight-loss product — prescription, over-the-counter or herbal — has not been established.

How long it is meant to be taken

Both phentermine labels say short-term — 'a few weeks'. The 2015 Endocrine Society guideline records the approval as 'Approved in 1960s for short-term use (3 mo)', discusses whether phentermine can reasonably be prescribed off-label long term for what is a long-term condition, and defers to state medical board guidance. In the health-record cohort, the 144 people who stayed on it continuously for more than 12 months had lost 7.4% more weight than short-term users at 24 months, and the combined rate of cardiovascular events or death across the whole cohort was 0.3% (41 events) with no significant difference between duration groups — too few events to rule out a difference. The labels also say that if tolerance to the appetite effect develops, the dose should not be increased and the drug should be stopped instead.

So the label and the largest study point in different directions, and both facts are worth knowing. The label is the regulator's position on what has been proven; the health-record study is the largest body of real-world experience, with the weaknesses that come with not being randomised. Clinicians weigh those against your blood pressure, heart rate, history and how much weight you are actually losing.

What it costs

$10–$51/mo

Average retail price $50.51 for 30 tablets of 37.5 mg — a month at one tablet a day — falling to $9.91 with a discount card, per singlecare.com on 11 September 2026.

That puts phentermine at the cheap end of the prescription market, in the same bracket as generic phentermine-topiramate and the other older appetite suppressants, and far below the newer weight-loss drugs. Prices shown here are US cash prices and change often; a discount card price is not an insurance price, and your pharmacy may quote something different.

How it compares in the evidence

No trial has compared phentermine head-to-head with the newer weight-loss drugs, so the numbers below sit side by side rather than against each other. They come from different trials, in different populations, over different lengths of time.

Trial length. Phentermine's randomised trials ran 12 to 28 weeks. The trials behind tirzepatide, semaglutide and orforglipron ran 68 to 72 weeks, long enough to show where weight loss levels off.
What was measured. The phentermine trials reported kilograms lost; the newer trials reported percentage of body weight, which is why a like-for-like percentage for phentermine is hard to quote at all. The percentages on this page come from a health-record study, not a trial.
Size of effect. Average weight loss was 20.9% of body weight in the tirzepatide trial and 14.9% in the semaglutide trial. Phentermine's 12-week trials recorded 6.7 kg and 8.1 kg.
Approved duration. The newer drugs are approved for long-term use. Phentermine is approved for a few weeks, and is a controlled substance.

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Written and maintained by one person · Built with agentic AI tools · Every number checked against the cited trial · Not medical advice

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Last reviewed: September 2026

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Medical disclaimer: This website is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any treatment. Prices shown are United States self-pay or list prices and vary by insurance and country.