Phentermine: short-term weight-loss pill, risks and rules

Key takeaways

• Phentermine is FDA approved only as a short-term (a few weeks) aid to diet, exercise and behavior change for adults with a BMI of 30+, or 27+ with other risk factors.^[1]
• It is a Schedule IV controlled substance related to amphetamine, and it is not for people with heart disease, uncontrolled high blood pressure, glaucoma, hyperthyroidism, a history of drug abuse, or during pregnancy or breastfeeding.^[1]
• In a 28-week trial, 43% to 46% of people on phentermine alone lost at least 5% of their weight, compared with 15.5% on placebo.^[3]
• Rare but serious risks include pulmonary hypertension and heart-valve disease; new shortness of breath, chest pain, fainting or leg swelling need prompt evaluation.^[1]
• Pharmacies paid about $2 to $3 for 30 generic 37.5 mg tablets or capsules (NADAC, rates effective 2026-09-23); retail prices are higher and vary.^[6]

In short: Phentermine is a prescription appetite suppressant that has been FDA approved since 1959. Its label allows it only as a short-term aid (“a few weeks”) alongside diet, exercise and behavior change, for adults with a 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 a Schedule IV controlled substance, chemically related to amphetamine, and it is not for people with heart disease, uncontrolled high blood pressure, glaucoma, an overactive thyroid, a history of drug misuse, or during pregnancy or breastfeeding (label checked on DailyMed on 2026-10-06).

Phentermine is one of the most widely known weight-loss pills in the U.S., partly because generic versions are inexpensive. This page explains what the official prescribing information says about it: the short-term rule, how it works, what studies found, its side effects and serious risks, who should not take it, how it combines into Qsymia, and what the controlled-substance status means in practice. It is general education, not personal medical advice. Please research your options, talk with a licensed doctor and get a full evaluation, including a heart and blood pressure check, before considering it.

What phentermine is

Phentermine hydrochloride belongs to a group the label calls sympathomimetic amines, or “anorectics”: stimulant medicines similar in action to amphetamine. It is sold as generic 37.5 mg tablets and capsules (equal to 30 mg of phentermine base), as lower-strength 15 mg and 30 mg capsules, and as an 8 mg tablet under the brand name Lomaira. You may also see the older brand name Adipex-P. Phentermine is also one of the two ingredients in Qsymia, a long-term combination with topiramate.

The label is candid about how it works: drugs of this class are thought to reduce appetite, but “it has not been established” that appetite suppression is the main way they cause weight loss; other effects on the nervous system or metabolism may be involved. Typical actions of amphetamine-like drugs include stimulating the central nervous system and raising blood pressure. The label also notes that tolerance, a fading effect over time, has been shown with all drugs of this class.

FDA status: approved for short-term use only

The indication is narrow. Phentermine is approved as “a short-term (a few weeks) adjunct” in a weight-reduction regimen based on exercise, behavior change and calorie reduction, for people with an initial BMI of 30 or more, or 27 or more with other risk factors (the label’s examples are controlled hypertension, diabetes and high cholesterol). The label adds that the “limited usefulness” of this class should be weighed against its risks. It is not recommended for anyone 16 or younger, and the label says that because obesity in children is a long-term condition, a short-term medicine is not recommended for them.

In practice, some clinicians prescribe phentermine for longer than a few weeks. That is off-label use: a medical judgment outside what the FDA approved. The American Gastroenterological Association’s 2022 guideline, for example, suggested phentermine as one option for long-term management based on low-certainty evidence, while rating newer medicines on moderate-certainty evidence. If a longer course is proposed, ask your clinician why, how you will be monitored and what the plan is for stopping. Check your BMI with our BMI calculator to understand the label’s thresholds, but remember that only a clinician can decide whether a medicine fits you.

Who should not take phentermine

The label lists these contraindications, meaning situations where phentermine should not be used:

• A history of cardiovascular disease, such as coronary artery disease, stroke, heart rhythm problems, congestive heart failure or uncontrolled high blood pressure
• Taking, or having taken within the last 14 days, a monoamine oxidase inhibitor (MAOI), because of the risk of a dangerous rise in blood pressure
• Hyperthyroidism (an overactive thyroid)
• Glaucoma
• Agitated states
• A history of drug abuse
• Pregnancy and nursing (breastfeeding)
• Known allergy or sensitivity to sympathomimetic amines

The label also advises caution with even mild high blood pressure, avoiding use in people with an eGFR below 15 or on dialysis, a lower maximum dose with severe kidney impairment, and careful dosing in older adults because the drug is cleared by the kidneys. People with diabetes may need their insulin or other diabetes medicines adjusted as they lose weight.

How it is taken, in general

The label says the dose should be individualized to the lowest effective amount. The 37.5 mg products are usually taken once a day before breakfast or one to two hours after it, and some people may need only half a tablet; the 8 mg Lomaira tablet is labeled for up to three times a day, half an hour before meals. Phentermine can be taken with or without food. Late-evening doses should be avoided because it can cause insomnia. If the appetite effect fades, the label says the dose should not be increased; the medicine should be stopped. Your prescriber sets the dose and duration; never take more than prescribed, share it, or combine it with other weight-loss products.

What the studies showed

The label’s own summary is modest. In short-term trials, people taking anorectic drugs lost more weight on average than people on placebo, but “the magnitude of increased weight loss” was “only a fraction of a pound a week”, weight loss was fastest in the first weeks and then slowed, and the studies lasted only a few weeks. The label concludes that the overall impact over diet alone “must be considered clinically limited.”

Newer randomized trials give more concrete numbers. In a 28-week U.S. trial published in Obesity in 2013 (Aronne and colleagues), adults with obesity received placebo, phentermine alone, topiramate alone, or the phentermine-topiramate combination, all with lifestyle counseling. The share of people who lost at least 5% of their weight was 15.5% on placebo, 43.3% on phentermine 7.5 mg, 46.2% on phentermine 15 mg, and 62.1% to 66.0% on the two combination doses that later became Qsymia.

Placebo15.5%

Phentermine 7.5 mg43.3%

Phentermine 15 mg46.2%

Topiramate ER 92 mg48.6%

Phentermine/topiramate ER 7.5/46 mg62.1%

Phentermine/topiramate ER 15/92 mg66%

People who lost at least 5% of body weight in 28 weeks

Item	Value
Placebo	15.5%
Phentermine 7.5 mg	43.3%
Phentermine 15 mg	46.2%
Topiramate ER 92 mg	48.6%
Phentermine/topiramate ER 7.5/46 mg	62.1%
Phentermine/topiramate ER 15/92 mg	66%

All groups received lifestyle counseling. The combination doses correspond to Qsymia strengths. Topiramate 46 mg alone: 39.2%.

Source: Aronne LJ et al., Obesity 2013 (phentermine and topiramate vs combination) (checked on October 6, 2026)

A small 12-week trial in Korea (Kang and colleagues, Diabetes, Obesity and Metabolism, 2010) tested a controlled-release 30 mg phentermine formulation in 74 adults with obesity and a controlled weight-related condition. Average weight loss was 8.1 kg with phentermine and 1.7 kg with placebo, and 95.8% versus 20.8% lost at least 5% of their weight. Dry mouth and insomnia were the most common side effects. This was a short study of a formulation not sold in the U.S., so it shows the size of the early effect rather than what happens over years.

1.7 kgPlacebo

8.1 kgPhentermine 30 mg (controlled-release)

Average weight lost in 12 weeks, phentermine vs placebo (small trial)

Item	Value
Placebo	1.7 kg
Phentermine 30 mg (controlled-release)	8.1 kg

74 adults in Korea; a controlled-release formulation not sold in the U.S. Short trials do not show long-term results.

Source: Kang JG et al., Diabetes, Obesity and Metabolism 2010 (checked on October 6, 2026)

There is no long-term outcome trial showing that phentermine lowers heart attacks, strokes or deaths. Weight often returns after a short course ends unless eating, activity and sleep habits have changed, which is why our guide to keeping weight off matters as much as the medicine choice.

Side effects

Unlike newer medicines, the phentermine label does not give percentages for side effects. It lists these reported reactions:

• Heart and circulation: palpitations, a fast heartbeat, raised blood pressure, ischemic events, and rare pulmonary hypertension or heart-valve disease
• Nervous system: overstimulation, restlessness, dizziness, insomnia, euphoria or low mood, tremor, headache and, rarely, psychosis
• Digestive: dry mouth, unpleasant taste, diarrhea, constipation and other stomach upset
• Other: hives, impotence and changes in libido

Phentermine may affect your ability to drive or operate machinery, and alcohol can cause an adverse reaction when combined with it. For a broader look at appetite-suppressing medicines and why supplement “phentermine alternatives” are a different thing entirely, see appetite suppressants.

Serious risks

• Primary pulmonary hypertension, a rare and often fatal lung-blood-vessel disease, was reported with the old combination of phentermine and fenfluramine (“fen-phen”), and rare cases have been reported with phentermine alone.
• Heart-valve disease (regurgitant valvular disease) was reported with fen-phen; a link with phentermine alone “cannot be ruled out.”
• Abuse and dependence. Phentermine is related to amphetamine. Stopping suddenly after prolonged high doses can cause extreme fatigue and depression; chronic misuse can cause severe skin problems, insomnia, irritability, personality changes and psychosis. The label asks prescribers to dispense the smallest practical amount.
• Raised blood pressure, even in people with mild hypertension.
• Overdose can cause restlessness, tremor, rapid breathing, confusion, panic, irregular heartbeat and, in severe cases, seizures and coma.

What Schedule IV means for you

As a Schedule IV controlled substance, phentermine is regulated more tightly than an ordinary prescription: extra federal and state rules apply, for example to refills and to prescribing through telehealth, and they vary by state. Ask your prescriber and pharmacist how they apply to you. The label tells patients to keep it in a safe place to prevent theft or misuse, and notes that selling or giving it away is against the law. Buying phentermine without a prescription, from websites that do not require one or from overseas sellers, is unsafe and may be illegal; pills sold that way may not contain what the label says.

Interactions

The label lists these interactions: MAOIs (contraindicated, risk of hypertensive crisis), alcohol (possible adverse reaction), insulin and other diabetes medicines (needs may change), and adrenergic neuron-blocking blood pressure medicines, whose effect phentermine may reduce. It also says phentermine is labeled only as monotherapy: the safety of combining it with other weight-loss drugs, over-the-counter or herbal products, or with SSRI antidepressants such as fluoxetine, sertraline or paroxetine “has not been established,” so the combination is not recommended. Qsymia is the exception, because the combination with topiramate was tested and approved as its own product.

Pregnancy and breastfeeding

Phentermine is contraindicated in pregnancy. The label explains that weight loss offers no benefit during pregnancy and may harm the baby, and that a minimum weight gain, not weight loss, is recommended for all pregnant women. It is also contraindicated while breastfeeding, because related amphetamines pass into breast milk. Anyone who could become pregnant should discuss contraception and plans with their clinician before starting.

Cost and insurance

Generic phentermine is one of the least expensive weight-loss medicines, but there is no manufacturer list price for most generic versions, and pharmacy cash prices vary widely. The most neutral public benchmark is Medicaid’s National Average Drug Acquisition Cost (NADAC), the average price pharmacies pay to buy a drug; it is not what you pay at the counter, which adds pharmacy fees and markups. Using NADAC rates effective September 23, 2026, the pharmacy cost of a 30-day supply works out as shown below.

Generic 37.5 mg tablets, 30 tablets$2.18

Generic 37.5 mg capsules, 30 capsules$3.15

Phentermine 8 mg tablets, 90 tablets (three a day)$55.63

Generic phentermine/topiramate ER 7.5/46 mg, 30 capsules$91.69

What pharmacies pay for a 30-day supply of phentermine (NADAC)

Item	Value
Generic 37.5 mg tablets, 30 tablets	$2.18
Generic 37.5 mg capsules, 30 capsules	$3.15
Phentermine 8 mg tablets, 90 tablets (three a day)	$55.63
Generic phentermine/topiramate ER 7.5/46 mg, 30 capsules	$91.69

NADAC per-unit cost multiplied by the number of units. This is the pharmacy's acquisition cost, not a retail price; your price will be higher and varies by pharmacy.

Source: CMS data.medicaid.gov, NADAC 2026 (rates effective 2026-09-23), InstaTuck calculation (checked on October 6, 2026)

Many insurance plans do not cover weight-loss medicines, and Medicare is prohibited by law from covering drugs used specifically for weight loss. Because the medicine itself is cheap, the larger costs are often the clinic visits, blood pressure checks and any program fees. Ask for the total cost before you start. Our insurance guide and cost hub explain what is usually covered, and weight-loss clinics explains how to judge a clinic that prescribes it.

Phentermine compared with other weight-loss pills

	Phentermine	Qsymia	Contrave
Ingredients	Phentermine	Phentermine + topiramate ER	Naltrexone + bupropion ER
Label duration	Short term (a few weeks)	Long term	Long term
Controlled substance	Schedule IV	Schedule IV	No
Ages on the label	Not recommended 16 and under	Adults; children 12+ with obesity	Adults only
Key safety rule	Not with heart disease, uncontrolled high blood pressure, glaucoma, hyperthyroidism	Not in pregnancy; REMS with monthly pregnancy tests	Boxed warning on suicidal thoughts; not with seizures or chronic opioids

The table summarizes the FDA labels as checked on 2026-10-06; each medicine has more warnings than fit in a table, so read its own page before your appointment. A practical way to prepare is to write down your blood pressure readings, any heart symptoms, your sleep, your mood history and every medicine and supplement you take, and bring that list with you.

Alternatives to discuss with your clinician

Medicines approved for long-term weight management include Qsymia (phentermine with topiramate), Contrave, orlistat, and the GLP-1 family, such as Wegovy, Zepbound and Saxenda. Contrave vs Qsymia compares the two combination pills, our weight-loss pills guide ranks pills by evidence, and the medications hub lists every option with its FDA status. Whatever you choose, the everyday foundations in how to lose weight and a realistic calorie deficit are what keep results going once a short course ends.

FDA status and approved uses

Indication	Labeled population	For weight	Approved	Source
Short-term (a few weeks) adjunct in a regimen of weight reduction based on exercise, behavioral modification and caloric restriction in the management of exogenous obesity	Patients with an initial BMI of 30 kg/m2 or more, or 27 kg/m2 or more with other risk factors (e.g., controlled hypertension, diabetes, hyperlipidemia)	Yes	1959	Source

From the FDA label. Who a medicine is labeled for is not the same as whether it fits you; only a clinician can decide that.

How it is taken (in general)

Dose is individualized to the lowest effective dose. 37.5 mg products are usually taken once daily before breakfast or 1 to 2 hours after; some patients need half a tablet. Lomaira 8 mg is labeled up to three times daily before meals. Avoid late-evening doses (insomnia). If tolerance develops, do not exceed the dose; stop. Short-term (a few weeks) per label. Follow your prescriber.

This is what the label describes in general, never a personal dose or schedule. Follow your prescriber.

What studies showed

Trial	Population	Duration	Outcome as reported
Aronne 2013 (phentermine and topiramate monotherapies vs combination)	Adults with obesity, 7 arms including placebo, phentermine 7.5 and 15 mg	28 weeks	5% or more weight loss: placebo 15.5%, phentermine 7.5 mg 43.3%, phentermine 15 mg 46.2%, PHEN/TPM ER 7.5/46 62.1%, 15/92 66.0%.[3]
Kang 2010 (phentermine DCR 30 mg)	74 adults with obesity and a controlled comorbidity (Korea)	12 weeks	Weight -8.1 kg vs -1.7 kg with placebo; 5% or more loss 95.8% vs 20.8%.[4]

Average results from trials; individual results vary. Results from different trials cannot be compared directly.

Side effects

Common side effect	How often (label)
Palpitations, fast heartbeat, raised blood pressure	Reported; the label gives no percentages
Insomnia, restlessness, overstimulation, dizziness, tremor, headache	Reported; no percentages on the label
Dry mouth, unpleasant taste	Reported; no percentages on the label
Diarrhea or constipation	Reported; no percentages on the label
Changes in libido, impotence	Reported; no percentages on the label

Serious risks

• Primary pulmonary hypertension (rare; mainly reported with fenfluramine combinations, cannot be ruled out alone)
• Regurgitant cardiac valvular disease (rare)
• Abuse and dependence (Schedule IV; related to amphetamine)
• Increase in blood pressure
• Impaired ability to drive or operate machinery
• Tolerance to the appetite effect; discontinue rather than exceed the dose

Warnings and who should not use it

Who should not use it (per the label)

• History of cardiovascular disease (coronary artery disease, stroke, arrhythmias, congestive heart failure, uncontrolled hypertension)
• During or within 14 days after a monoamine oxidase inhibitor (MAOI)
• Hyperthyroidism
• Glaucoma
• Agitated states
• History of drug abuse
• Pregnancy
• Nursing (breastfeeding)
• Known hypersensitivity or idiosyncrasy to sympathomimetic amines

Interactions, pregnancy and breastfeeding

Drug or class	What the label says
MAOIs	Contraindicated during or within 14 days: risk of hypertensive crisis.
Alcohol	May result in an adverse drug reaction.
Insulin and oral diabetes medicines	Requirements may change; dose reduction may be needed.
Adrenergic neuron blocking drugs	Phentermine may decrease their blood-pressure-lowering effect.
Other weight-loss drugs, OTC or herbal products, SSRIs	Safety of combinations not established; not recommended (Qsymia is a separately approved combination).

Pregnancy and breastfeeding

Contraindicated in pregnancy: weight loss offers no benefit to a pregnant woman and may cause fetal harm. Contraindicated while nursing; other amphetamines are present in human milk.

Cost, insurance and savings

Price	Amount	Basis	Checked on
Other	2.18 USD	NADAC pharmacy acquisition cost, 30 generic 37.5 mg tablets (rate effective 2026-09-23)	October 6, 2026
Other	3.15 USD	NADAC pharmacy acquisition cost, 30 generic 37.5 mg capsules (rate effective 2026-09-23)	October 6, 2026
Other	55.63 USD	NADAC pharmacy acquisition cost, 90 phentermine 8 mg tablets (rate effective 2026-09-23)	October 6, 2026

Prices change often and depend on pharmacy, plan and location. Insurance can change what you pay. Every price shows the date we checked it.

Questions to ask your doctor or pharmacist

• Given my heart health and blood pressure, is phentermine a safe option for me, and how will we monitor it?
• How long would I take it, and what is the plan for keeping weight off when I stop?
• Do any of my medicines, such as antidepressants, MAOIs or diabetes medicines, interact with it?
• Would a long-term medicine such as Qsymia, Contrave or a GLP-1 be more appropriate for me?
• What is the total cost, including visits and checks, and how are refills handled for a controlled substance in my state?

Frequently asked questions

How long can you take phentermine?

The FDA label approves phentermine only for short-term use, described as "a few weeks." Some clinicians prescribe it longer, which is off-label; that decision, and how you are monitored, belongs to you and your clinician.

How much weight do people lose on phentermine?

Results vary. In a 28-week trial, 43% to 46% of people on phentermine alone lost at least 5% of their weight, vs 15.5% on placebo. The label itself describes the extra loss over diet alone as only a fraction of a pound a week in older short trials.

Is phentermine a controlled substance?

Yes. It is Schedule IV because it is chemically related to amphetamine and has a potential for abuse and dependence. Extra federal and state rules apply to how it is prescribed and refilled.

Who should not take phentermine?

The label lists heart disease (including stroke, arrhythmias, heart failure and uncontrolled high blood pressure), use of an MAOI in the past 14 days, hyperthyroidism, glaucoma, agitated states, a history of drug abuse, pregnancy, breastfeeding and allergy to stimulant amines.

What are the common side effects of phentermine?

The label lists a fast or pounding heartbeat, raised blood pressure, insomnia, restlessness, dizziness, tremor, headache, dry mouth, unpleasant taste, diarrhea or constipation, and changes in libido. It does not give percentages.

Is phentermine the same as fen-phen?

No. Fen-phen was phentermine combined with fenfluramine, which was linked to heart-valve disease and pulmonary hypertension. Rare cases have also been reported with phentermine alone, so the label carries warnings about both.

Can phentermine be combined with other weight-loss medicines?

The label says the safety of combining it with other weight-loss drugs, OTC or herbal products, or SSRI antidepressants has not been established and is not recommended. Qsymia (phentermine with topiramate) is a separately tested and approved combination.

Why should phentermine not be taken in the evening?

It is a stimulant, and the label warns that late-evening doses can cause insomnia.

Are "phentermine alternatives" sold online the same thing?

No. Over-the-counter products marketed as phentermine alternatives are dietary supplements, not FDA-approved medicines, and some have been found to contain hidden drugs. Phentermine itself requires a prescription.

Official sources

• Prescribing information (FDA label)

References

1. Phentermine hydrochloride tablets: prescribing information. Via DailyMed (generic labeler), 2026. (accessed October 6, 2026) Drug label
2. LOMAIRA (phentermine hydrochloride) tablets: prescribing information. via DailyMed. (accessed October 6, 2026) Drug label
3. Aronne LJ, Wadden TA, Peterson C, Winslow D, Odeh S, Gadde KM. Evaluation of phentermine and topiramate versus phentermine/topiramate extended-release in obese adults. Obesity (Silver Spring), 2013. doi:10.1002/oby.20584 · PMID 24136928 (accessed October 6, 2026) Randomized trial
4. Kang JG, Park CY, Kang JH, Park YW, Park SW. Randomized controlled trial to investigate the effects of a newly developed formulation of phentermine diffuse-controlled release for obesity. Diabetes, Obesity and Metabolism, 2010. doi:10.1111/j.1463-1326.2010.01242.x · PMID 20920040 (accessed October 6, 2026) Randomized trial
5. Grunvald E, Shah R, Hernaez R, et al.. AGA Clinical Practice Guideline on Pharmacological Interventions for Adults With Obesity. Gastroenterology, 2022. doi:10.1053/j.gastro.2022.08.045 · PMID 36273831 (accessed October 6, 2026) Guideline
6. NADAC (National Average Drug Acquisition Cost) 2026. Centers for Medicare and Medicaid Services (data.medicaid.gov). (accessed October 6, 2026) Government page
7. A New Use for Wegovy Opens the Door to Medicare Coverage for Millions of People with Obesity. KFF, 2024. (accessed October 6, 2026) Other
8. Prescription Medications to Treat Overweight & Obesity. National Institute of Diabetes and Digestive and Kidney Diseases (NIH). (accessed October 7, 2026) Government page

Facts checked on October 6, 2026

Educational information, not medical advice. Talk with a qualified healthcare professional about your own situation. In an emergency call 911.