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Phentermine is an FDA-approved prescription weight-loss drug related to amphetamine. It was first approved in the United States in 1959, and it is a Schedule IV controlled substance. The FDA label approves it only as a short-term add-on, for “a few weeks”, to a weight-reduction plan of diet, exercise and behavior change, in adults with a body mass index (BMI) of 30 or more, or 27 or more with other risk factors such as controlled high blood pressure, diabetes or high cholesterol. It is sold as generic tablets and capsules and under brand names such as Adipex-P and Lomaira, and it is one of the two ingredients in Qsymia.
Peptide Labs does not sell phentermine or any prescription drug. This article is general information, not medical advice. Whether phentermine suits a person is a decision for a licensed prescriber. For the wider picture of prescription and nonprescription options, see our guide to appetite suppressants.
Key Takeaways
- Phentermine is a stimulant-like prescription drug related to amphetamine, approved in 1959 and controlled under federal law (Schedule IV).
- The label approves it only for short-term use of a few weeks, alongside diet, exercise and behavior change.
- Common phentermine side effects include dry mouth, trouble sleeping, restlessness, a fast or pounding heartbeat, raised blood pressure and constipation.
- People with heart disease, uncontrolled high blood pressure, an overactive thyroid, glaucoma or a history of drug misuse should not take it, nor should anyone pregnant or nursing.
- Rare lung and heart valve problems were linked mainly to the 1990s fen-phen combination; the label says a link with phentermine alone cannot be ruled out.
- There is no over-the-counter phentermine. Supplements sold as phentermine alternatives are a different product, and the FDA has found hidden drugs in many weight-loss pills.
What is phentermine?
The label describes phentermine hydrochloride as a “sympathomimetic amine anorectic”: a drug that mimics parts of the body’s fight-or-flight system and is used to reduce food intake. Its effects are similar to amphetamine, including stimulation of the central nervous system and a rise in blood pressure. The label is careful about how it works, saying it “has not been established” that appetite suppression is the main action, because other brain or metabolic effects may be involved.
| Phentermine at a glance | What the label and FDA records say |
|---|---|
| Drug type | Sympathomimetic amine, related to amphetamine |
| First U.S. approval | 1959 (Ionamin, since discontinued) |
| Federal control status | Schedule IV controlled substance, prescription only |
| Approved use | Short-term (a few weeks) add-on to diet, exercise and behavior change |
| Who it is labeled for | BMI 30 or more, or 27 or more with other risk factors |
| Current brand names | Adipex-P (Teva), Lomaira (KVK-Tech); also generics |
| Combination product | Qsymia (phentermine and topiramate extended-release) |
| Pregnancy | Must not be used (Pregnancy Category X) |
| Children | Not recommended for anyone 16 or younger |
Brands, generics and Qsymia
Drugs@FDA lists Adipex-P tablets from Teva as a current prescription product, while its capsule version is discontinued. Lomaira, a scored tablet from KVK-Tech, was approved in September 2016. Many companies make generic phentermine tablets and capsules. Older brands such as Ionamin and Fastin are no longer sold. Qsymia, approved in July 2012, pairs phentermine with topiramate in an extended-release capsule and, unlike phentermine alone, is approved for long-term weight management. It has its own label and its own birth-defect safety program.
How well does phentermine work?
The phentermine label is unusually modest. It says that in relatively short trials, adults taking drugs of this type lost more weight on average than those on placebo, but that the extra loss was “only a fraction of a pound a week”. Weight came off fastest in the first weeks for both groups. Because those studies lasted only a few weeks, the label says the overall impact compared with diet alone “must be considered clinically limited”.
The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) notes that some health care professionals prescribe phentermine for longer than the approved few weeks, an off-label use, and that not many studies have looked at how safe and effective it is long term. Across prescription weight medicines in general, NIDDK says adults lose on average 3% to 12% more of their starting weight after one year than with a lifestyle program alone. For comparison, newer drugs were tested for more than a year: Wegovy averaged 14.9% loss vs 2.4% on placebo over 68 weeks in STEP 1 (funded by Novo Nordisk), and Zepbound 15.0% to 20.9% vs 3.1% over 72 weeks in SURMOUNT-1 (funded by Eli Lilly). These were different trials, not head-to-head comparisons with phentermine.
Phentermine side effects
The label lists these side effects without percentages:
- Heart and circulation: palpitations, fast heartbeat, raised blood pressure, ischemic events (reduced blood flow, such as to the heart), pulmonary hypertension and heart valve disease.
- Brain and nerves: overstimulation, restlessness, dizziness, insomnia, euphoria, low or uneasy mood (dysphoria), tremor, headache and psychosis.
- Digestive: dry mouth, unpleasant taste, diarrhea, constipation and other stomach upset.
- Other: hives, impotence and changes in sex drive.
The label also warns that phentermine may affect the ability to drive or operate machinery.
Who should not take phentermine
The label lists these contraindications, meaning situations where it must not be used:
- A history of cardiovascular disease, including coronary artery disease, stroke, irregular heart rhythms, congestive heart failure or uncontrolled high blood pressure.
- Use of a monoamine oxidase inhibitor (MAOI) antidepressant now or within the past 14 days, because of the risk of a dangerous spike in blood pressure.
- Hyperthyroidism (an overactive thyroid) or glaucoma.
- Agitated states or a history of drug abuse.
- Pregnancy or breastfeeding.
- Known allergy or unusual reaction to stimulant drugs of this type.
The label also urges caution in people with even mild high blood pressure and in people with reduced kidney function, since phentermine leaves the body mainly through the urine.
Lung and heart valve warnings: the fen-phen story
In the 1990s, phentermine was widely prescribed together with fenfluramine or dexfenfluramine, a combination nicknamed fen-phen. On July 8, 1997, 24 cases of heart valve disease in women who had taken fenfluramine and phentermine were reported publicly, and on September 15, 1997, the manufacturers and the FDA announced the withdrawal of fenfluramine and dexfenfluramine, according to the CDC. None of those first cases involved phentermine alone, and phentermine stayed on the market.
The current label still carries two warnings from that era. Primary pulmonary hypertension, a rare and often fatal disease of the lung blood vessels, and serious leaking of the heart valves were reported mainly with the combinations, but there have been rare cases in people who took phentermine alone, so a link “cannot be ruled out”. The label says new, unexplained shortness of breath, chest pain, fainting or swelling of the legs should be reported right away and the drug stopped while the person is checked.
Tolerance, misuse and why it is a controlled drug
The label says tolerance has been shown with every drug of this class that has been studied. If the appetite effect fades, the drug should be stopped, not taken in larger amounts. Because phentermine is chemically and pharmacologically related to amphetamine, the label also warns about misuse and psychological dependence; stopping suddenly after long, heavy use can cause extreme tiredness and depression.
The Drug Enforcement Administration lists phentermine in Schedule IV, the same schedule as medicines such as alprazolam (Xanax) and zolpidem (Ambien). The DEA describes Schedule IV drugs as having a low potential for abuse and a low risk of dependence compared with higher schedules. The label tells patients to keep it somewhere safe and notes that selling or giving it away is against the law.
Interactions: alcohol, diabetes medicines and other weight-loss products
- Alcohol: the label says drinking alcohol with phentermine may cause an adverse reaction.
- Insulin and diabetes pills: needs may change as weight comes down, so the prescriber may need to adjust them.
- MAOIs: contraindicated during use or within 14 days.
- Some blood pressure medicines: phentermine may weaken adrenergic neuron blocking drugs.
- Other weight-loss products and SSRIs: the label says combining phentermine with any other weight-loss drug, including over-the-counter and herbal products, or with SSRI antidepressants such as fluoxetine or sertraline, has not been shown to be safe and is not recommended.
Is phentermine safe?
Is phentermine safe? The honest answer from the label is that it can be used safely by some adults for a short period under medical supervision, but it is not safe for everyone. It raises heart rate and blood pressure in some people, it is off-limits with heart disease and several other conditions, it carries a risk of misuse, and its long-term safety has not been well studied. A prescriber weighs these risks against the expected benefit, which the label itself describes as limited.
Phentermine over the counter does not exist
Phentermine is prescription-only, and as a controlled substance it cannot legally be sold without a prescription. Products sold online or in stores as phentermine alternatives are dietary supplements or unapproved products, not phentermine. The FDA warns that many products sold for weight loss, sometimes falsely labeled as supplements, are likely to contain dangerous hidden ingredients. A California study of FDA warnings from 2007 to 2016 found 317 adulterated weight-loss supplements, and 269 of them contained sibutramine, a prescription drug withdrawn from the U.S. market in 2010. Our guide to whether fat burners work covers what the ingredients in those products actually do. The one weight-loss medicine the FDA has approved for sale without a prescription is a nonprescription orlistat, explained in our alli guide.
Newer long-term options
Unlike phentermine alone, several prescription medicines are approved for long-term weight management. They include Qsymia, Contrave (naltrexone and bupropion) and the GLP-1 medicines Wegovy and Zepbound; what GLP-1 is explains how the last group works, and GLP-1 side effects covers their risks. Our guide to Ozempic alternatives lists all approved options side by side.
Frequently Asked Questions
What is phentermine?
Phentermine is a prescription stimulant-like drug related to amphetamine. The FDA approves it as a short-term add-on, for a few weeks, to diet, exercise and behavior change in adults with obesity, or overweight with other risk factors.
What are the most common phentermine side effects?
The label lists dry mouth, unpleasant taste, insomnia, restlessness, dizziness, headache, tremor, palpitations, fast heartbeat, raised blood pressure, diarrhea and constipation, among others.
Is phentermine a controlled substance?
Yes. It is a Schedule IV controlled substance under federal law, so it needs a prescription and selling or sharing it is illegal.
How long can you take phentermine?
The label approves it for short-term use of a few weeks. Some clinicians prescribe it for longer, an off-label practice, but NIDDK notes that few studies have tested long-term safety and effectiveness.
Can you drink alcohol on phentermine?
The label warns that alcohol with phentermine may cause an adverse drug reaction. Ask the prescriber or pharmacist before drinking.
Is there a phentermine pill over the counter?
No. Phentermine is prescription-only. Supplements marketed as phentermine alternatives do not contain it, and the FDA has found hidden drugs in many products sold for weight loss.
Peptide Labs does not sell these drugs. This article is general information, not medical advice. Read the Medication Guide or patient information that comes with a prescription, and ask a pharmacist or prescriber how this information applies to you.
Last reviewed September 2026.
Related guides: Ozempic Alternatives: FDA-Approved Options, OTC Facts and What to Avoid, Appetite Suppressants: Prescription Drugs, Natural Options and the Risks, Qsymia (Phentermine-Topiramate): Label Facts, Trials and Safety, Contrave: How Naltrexone Bupropion Works, Side Effects and Results.