Short answer: Phentermine (Adipex-P) is an appetite suppressant related to amphetamine, used for a few weeks as part of a weight-loss program in people with obesity. It is the most prescribed weight-loss medicine in the US and a Schedule IV controlled substance. Combined with topiramate (Qsymia), it is approved for long-term use.
Key takeaways
- What it is: Phentermine is a sympathomimetic appetite suppressant and a Schedule IV controlled substance in the US.
- Main uses: Short-term obesity treatment.
- Timing: effects start in about 1 hour and last 12 to 14 hours.
- Most dangerous mix: MAOIs, which can cause high blood pressure crisis.
- Naloxone: it does not reverse this drug, but give it whenever opioids could be involved.
- Dependence risk: Low to moderate.
- Legal route: only with a prescription from a DEA-registered clinician, filled by a licensed pharmacy.
What is phentermine?
Phentermine is approved as a short-term (a few weeks) add-on to diet, exercise, and behavior change for adults with a BMI of 30 or more, or 27 or more with conditions such as high blood pressure or diabetes.
Phentermine/topiramate (Qsymia) is approved for long-term weight management. Newer GLP-1 medicines such as semaglutide and tirzepatide produce more weight loss but cost much more.
Quick facts
| Fact | Phentermine |
|---|---|
| Generic name | Phentermine hydrochloride |
| Brand names | Adipex-P, Lomaira; with topiramate: Qsymia |
| Drug class | Sympathomimetic amine (anorectic) |
| US legal status | Schedule IV, prescription only |
| Approved uses | Short-term treatment of obesity (BMI 30 or more, or 27 or more with a risk factor) |
| Forms | Tablets, capsules |
| Onset | About 1 hour |
| Half-life | About 20 hours |
| Duration of use | A few weeks (phentermine alone) |
How long does phentermine last?
- Starts working: About 1 hour
- Effects last: 12 to 14 hours
- Half-life: About 20 hours
Alone, it is approved for a few weeks. Some clinicians prescribe it longer off-label. Qsymia, which includes phentermine, is approved for long-term use.
Timing varies with the dose, the form, how it is taken, liver and kidney function, age, and other drugs taken at the same time.
What are the biggest risks of phentermine?
Common side effects include dry mouth, insomnia, restlessness, constipation, fast heartbeat, and raised blood pressure.
Serious risks include primary pulmonary hypertension and heart valve disease (reported mainly with the older fen-phen combination), and worsening of heart disease. It should not be used with uncontrolled high blood pressure, heart disease, hyperthyroidism, glaucoma, or during pregnancy.
What should you never mix with phentermine?
| Combined with | What happens | Label guidance |
|---|---|---|
| MAOIs | High blood pressure crisis | Contraindicated within 14 days |
| Other weight-loss drugs and stimulants | Heart and blood pressure risks | Avoid |
| Alcohol | Adverse reactions | Avoid |
| Insulin and diabetes medicines | Lower glucose as weight drops | Dose may need lowering |
Check any combination with the DoseUses combination checker or ask a pharmacist.
What are the signs of a phentermine overdose, and what should you do?
Overdose causes restlessness, tremor, rapid breathing, confusion, aggression, hallucinations, panic, high or low blood pressure, irregular heartbeat, and seizures.
If someone may be overdosing
- Call 911 for chest pain, a seizure, trouble breathing, signs of a stroke (face drooping, weakness on one side, slurred speech), a very high body temperature, or if the person collapses.
- Move them somewhere cool and calm, loosen tight clothing, and cool the skin with water. Do not try to hold them down.
- If they become unresponsive or their breathing slows, give naloxone. Street stimulants are often contaminated with fentanyl.
- For advice when the person is awake and stable, call Poison Help at 1-800-222-1222.
What happens when you stop phentermine?
Stopping phentermine after regular prescribed use does not cause dangerous physical withdrawal. ADHD symptoms return, and some people feel tired or low for a few days. After heavy misuse, a "crash" with exhaustion, increased sleep and appetite, and depression is common.
- Days 1 to 3CrashFatigue, long sleep, increased appetite, low mood, irritability.
- Weeks 1 to 2Low mood and cravingsLow energy, poor concentration, and cravings after heavy use.
- Weeks to monthsRecoveryMood and energy normalize; depression should be watched for.
See the withdrawal timelines and the full phentermine withdrawal guide.
Is phentermine legal?
In the United States, phentermine is a Schedule IV controlled substance. The only legal way to get it is with a prescription from a clinician registered with the Drug Enforcement Administration (DEA), filled by a licensed pharmacy. Depending on state law, that can be a physician, nurse practitioner, or physician assistant.
- Refills: a Schedule IV prescription can be refilled up to five times within six months of the date it was written. After that, a new prescription is needed.
- Telehealth: federal flexibilities let DEA-registered clinicians prescribe Schedule II to V medicines after a telemedicine visit, without a prior in-person exam, through December 31, 2026. A permanent DEA rule was under final review in late 2026. State rules can be stricter.
- Monitoring: prescribers and pharmacists in most states check a prescription drug monitoring program, a database of controlled-substance prescriptions, before prescribing or dispensing.
Outside the United States
| Country | Status |
|---|---|
| United Kingdom | Class C; not licensed for weight loss |
| European Union | Marketing authorizations withdrawn in 2000 |
| Australia | Schedule 4 prescription medicine |
| International | Schedule IV of the 1971 Convention on Psychotropic Substances |
Frequently asked questions about phentermine
How long can you take phentermine?
Alone, it is approved for a few weeks. Some clinicians prescribe it longer off-label. Qsymia, which includes phentermine, is approved for long-term use.
Is phentermine like Adderall?
It is chemically related to amphetamine but has much weaker effects on dopamine and a lower risk of misuse.
How much weight do people lose on phentermine?
On average about 3 to 5% of body weight beyond diet alone over 12 weeks; results vary.
Was phentermine part of fen-phen?
Yes. Fen-phen combined fenfluramine with phentermine. Fenfluramine was withdrawn in 1997 because of heart valve damage; phentermine stayed on the market.
Sources
- Adipex-P (phentermine) prescribing information, DailyMed, US National Library of Medicine.
- Controlled Substance Schedules, DEA Diversion Control Division.
- 21 CFR 1306.22, Refilling of prescriptions (Schedules III and IV), Electronic Code of Federal Regulations.
- Prescribing controlled substances via telehealth, US Department of Health and Human Services.
- BeSafeRx: Your Source for Online Pharmacy Information, US Food and Drug Administration.
- Safe Pharmacy, National Association of Boards of Pharmacy.
- One Pill Can Kill, US Drug Enforcement Administration.
- International drug control conventions, United Nations Office on Drugs and Crime.
This guide summarizes the sourced DoseUses Phentermine profile, written from FDA labels, DEA rules, and federal health agencies. It is educational and is not medical advice. Last updated October 5, 2026.
