Phentermine is a prescription medication used as a short-term part of a weight-reduction program for appropriately selected patients with obesity or overweight accompanied by certain risk factors.
It is intended to be used alongside measures such as caloric restriction, physical activity, and behavioral modification rather than as a replacement for lifestyle changes.
Phentermine is also a Schedule IV controlled substance in the United States, which means appropriate prescribing, dispensing, and controlled-substance requirements apply.
Because phentermine can affect the cardiovascular and central nervous systems and has risks related to misuse and dependence, treatment requires an individualized evaluation by an appropriately licensed healthcare professional.
This guide explains what phentermine is, how it is used, who may be considered for treatment, phentermine 37.5 mg formulations, possible side effects, important contraindications and warnings, and prescription requirements.
What Is Phentermine?
Phentermine is a sympathomimetic amine with pharmacologic activity similar to other medications in this class.
It is classified as an anorectic and can reduce appetite.
Current U.S. prescribing information indicates phentermine for short-term use—a few weeks—as an adjunct to a weight-reduction program based on exercise, behavioral modification, and caloric restriction in appropriately selected patients.
Phentermine is not intended to replace nutrition, physical activity, behavioral changes, or appropriate medical follow-up.
Who May Be Considered for Phentermine?
According to current U.S. prescribing information, phentermine may be considered for short-term treatment of exogenous obesity in adults with an initial:
- Body mass index (BMI) of 30 kg/m² or greater, or
- BMI of 27 kg/m² or greater when certain additional risk factors are present, such as controlled hypertension, diabetes, or hyperlipidemia
Meeting a BMI threshold does not automatically mean phentermine is appropriate.
A healthcare professional should consider the potential benefits and risks as well as the patient's medical history, medications, contraindications, and other relevant circumstances before prescribing treatment.
How Does Phentermine Work?
Phentermine is an anorectic medication that can decrease appetite.
Reducing appetite may help some appropriately selected patients follow a calorie-restricted weight-management program.
However, medication is only one component of treatment.
A comprehensive weight-management approach may also involve:
- Nutrition and caloric intake
- Physical activity
- Behavioral changes
- Management of weight-related health conditions
- Appropriate medical monitoring
Individual responses to weight-management treatment can vary.
Is Phentermine Intended for Long-Term Weight Management?
Current U.S. prescribing information describes phentermine as a short-term treatment, generally for a few weeks.
It should not be assumed to be a long-term weight-management medication simply because other prescription weight-management treatments may be intended for longer-term use.
The appropriate duration of treatment should be determined by the prescribing healthcare professional.
Patients should not extend treatment or increase their dose without medical guidance.
What Is Phentermine 37.5 mg?
Phentermine is available in different products and formulations.
37.5 mg is a commonly available phentermine hydrochloride strength and may be found in certain tablet and capsule formulations.
However, 37.5 mg should not be interpreted as the appropriate dose for every patient.
Current prescribing information states that dosing should be individualized to obtain an adequate response using the lowest effective dose.
The appropriate product, strength, and treatment plan should therefore be determined by the prescribing healthcare professional.
Phentermine Dosage and Proper Use
There is no single phentermine treatment plan that is appropriate for every patient.
Dosage should be individualized by the prescribing healthcare professional.
Patients who are prescribed phentermine should:
- Take it exactly as prescribed
- Follow the instructions supplied with their prescription
- Avoid taking more than prescribed
- Avoid increasing the frequency of use
- Avoid extending treatment without medical guidance
- Keep the medication secure
- Never share or sell the medication
- Discuss side effects or treatment concerns with their healthcare professional
Because different phentermine products and formulations may have different instructions, patients should follow the directions accompanying their prescription.
This article provides general educational information and does not provide individualized dosing instructions.
Why Medical Evaluation Is Important Before Taking Phentermine
Phentermine is not appropriate for everyone.
Before prescribing it, a healthcare professional may review:
- Current weight and BMI
- Weight-related medical conditions
- Cardiovascular history
- Blood pressure
- Current medications
- Previous weight-management treatments
- History of medication reactions
- Pregnancy status
- History relevant to substance misuse or dependence
- Other health conditions
- Potential medication interactions
The healthcare professional can then determine whether phentermine or another weight-management approach may be appropriate.
Who Should Not Take Phentermine?
Current U.S. prescribing information lists several contraindications.
Phentermine is contraindicated in people with certain conditions or circumstances, including:
- A history of cardiovascular disease, such as coronary artery disease, stroke, arrhythmias, congestive heart failure, or uncontrolled hypertension
- Use of monoamine oxidase inhibitors (MAOIs) during or within the preceding 14 days
- Hyperthyroidism
- Glaucoma
- Agitated states
- A history of drug abuse
- Pregnancy
- Nursing
- Known hypersensitivity or idiosyncrasy to sympathomimetic amines
This is one reason a complete health and medication history is important before treatment.
Patients should not attempt to determine eligibility based only on BMI or a desire to lose weight.
Phentermine and Cardiovascular Health
Phentermine has important cardiovascular considerations.
Current labeling contraindicates its use in patients with a history of certain cardiovascular diseases and advises caution even in patients with mild hypertension because of the potential for increases in blood pressure.
Reported cardiovascular adverse reactions can include:
- Palpitations
- Increased heart rate
- Elevated blood pressure
- Ischemic events
Rare but serious cardiovascular complications have also been reported.
Patients should discuss their cardiovascular history and blood pressure with their healthcare professional before treatment.
Serious Symptoms Requiring Medical Attention
Current prescribing information includes warnings concerning rare cases of primary pulmonary hypertension and serious valvular heart disease.
Patients should promptly seek appropriate medical evaluation if they develop concerning symptoms such as:
- New or unexplained shortness of breath
- Chest pain
- Fainting
- Swelling of the legs or ankles
- Significant decline in exercise tolerance
Patients should not ignore new or worsening symptoms while taking phentermine.
Common Phentermine Side Effects
Phentermine can cause side effects.
Reported adverse reactions include effects involving the cardiovascular, central nervous, and gastrointestinal systems.
These may include:
- Palpitations
- Increased heart rate
- Elevated blood pressure
- Restlessness
- Dizziness
- Insomnia
- Tremor
- Headache
- Dry mouth
- Constipation
- Diarrhea
- Unpleasant taste
Other side effects are possible.
Patients should contact their healthcare professional about persistent, severe, unexpected, or concerning symptoms.
Phentermine and Sleep
Phentermine can cause insomnia.
Current prescribing information advises against late-evening administration because of the possibility of sleep disturbance.
Patients should follow the timing instructions provided with their prescription rather than changing when or how they take the medication independently.
Phentermine, Tolerance, Dependence, and Misuse
Phentermine is a Schedule IV controlled substance.
Current prescribing information warns about the potential for abuse and dependence.
Tolerance to its appetite-suppressing effect may also develop.
Patients should:
- Take phentermine only as prescribed
- Never increase the dose to try to overcome reduced effectiveness
- Never share the medication
- Never sell or give the medication to another person
- Store it securely
- Discuss concerns about effectiveness, tolerance, or side effects with the prescribing healthcare professional
If tolerance develops, patients should contact their healthcare professional rather than increasing the dose themselves.
Can Phentermine Be Combined with Other Weight-Loss Medications?
Patients should not independently combine phentermine with other prescription or nonprescription weight-loss products.
Current prescribing information states that the safety and effectiveness of combining phentermine with other weight-loss drugs—including prescription medications, over-the-counter products, herbal preparations, and certain serotonergic medications—have not been established.
Patients should provide their healthcare professional with a complete list of medications and supplements before starting treatment.
Phentermine and MAO Inhibitors
Phentermine is contraindicated during or within 14 days following treatment with monoamine oxidase inhibitors (MAOIs).
Combining these medications can create a risk of hypertensive crisis.
Patients should tell their healthcare professional about all current and recently discontinued medications before beginning phentermine.
Phentermine and Pregnancy
Phentermine is contraindicated during pregnancy.
Weight loss offers no potential benefit to a pregnant patient and may result in fetal harm.
Patients who are pregnant, may be pregnant, or are planning pregnancy should tell their healthcare professional before considering phentermine.
Phentermine and Alcohol
Current prescribing information warns that using alcohol with phentermine may result in adverse drug reactions.
Patients should discuss alcohol use and other substances with their healthcare professional when being evaluated for treatment.
Phentermine and Driving
Phentermine may impair a patient's ability to perform potentially hazardous activities, including driving or operating machinery.
Patients should understand how the medication affects them and follow the safety instructions provided by their healthcare professional and prescription information.
Phentermine and Lifestyle Changes
Phentermine is intended to be used as one component of a broader weight-reduction program.
Depending on individual circumstances, a weight-management plan may include:
- Caloric restriction
- Nutrition changes
- Physical activity
- Behavioral modification
- Management of weight-related health conditions
- Regular follow-up
Medication should complement—not replace—these components.
Why Follow-Up Matters
Medical follow-up allows a healthcare professional to assess factors such as:
- Treatment response
- Blood pressure and cardiovascular considerations
- Side effects
- Medication tolerance
- Changes in other health conditions
- Whether continued treatment remains appropriate
Patients should discuss changes in symptoms, medication effectiveness, or side effects with the prescribing healthcare professional.
Does Phentermine Require a Prescription?
Yes.
Phentermine is a prescription medication and a Schedule IV controlled substance in the United States.
Appropriate prescribing and controlled-substance requirements apply.
Patients should be cautious of websites or sellers offering phentermine without appropriate prescription requirements or clinical evaluation.
Can Phentermine Be Evaluated Through Telehealth?
Whether phentermine can appropriately and legally be prescribed through telehealth depends on the patient's circumstances, the healthcare professional's evaluation, and applicable federal and state requirements.
An evaluation may include review of:
- Weight and BMI
- Weight-related medical conditions
- Cardiovascular history
- Blood pressure information
- Current medications
- Relevant medical history
- Previous weight-management treatments
- Contraindications
- Other information needed for safe prescribing
Because phentermine is a controlled substance, additional requirements may apply compared with non-controlled prescription medications.
An online evaluation should never guarantee that phentermine will be prescribed.
Understanding the A2ZRx Evaluation Process
Patients interested in an available weight-management treatment can begin by reviewing the relevant treatment information through A2ZRx.
The evaluation process may include:
1. Review Treatment Information
Review information about the available treatment, eligibility considerations, potential risks, and evaluation requirements.
2. Complete Required Health Information
Provide complete and accurate information about health history, medications, weight-management history, and other relevant factors.
3. Licensed Healthcare Provider Evaluation
An appropriately licensed healthcare professional evaluates the submitted information and any additional information required for clinical assessment.
4. Treatment Decision
The healthcare professional determines whether treatment is clinically appropriate and whether additional evaluation is required.
Prescription approval is not automatic.
5. Prescription and Fulfillment
If treatment is prescribed, the prescription proceeds through the applicable pharmacy fulfillment process in accordance with applicable requirements.
Clinical and prescribing decisions remain with the appropriately licensed healthcare professional.
Frequently Asked Questions About Phentermine
What is phentermine used for?
Phentermine is used as a short-term adjunct to a weight-reduction program involving caloric restriction, exercise, and behavioral modification in appropriately selected patients.
How does phentermine work?
Phentermine is an anorectic medication that can decrease appetite.
Who may qualify for phentermine?
Current U.S. prescribing information describes use in adults with an initial BMI of at least 30 kg/m² or at least 27 kg/m² in the presence of certain additional risk factors.
Meeting these criteria does not automatically make someone eligible. A healthcare professional must consider the individual's overall circumstances.
What is phentermine 37.5 mg?
Phentermine hydrochloride 37.5 mg is a strength available in certain tablet and capsule formulations.
The appropriate dose should be individualized by the prescribing healthcare professional.
Is phentermine intended for long-term use?
Current U.S. labeling describes phentermine as a short-term treatment—a few weeks—as part of an overall weight-reduction program.
Is phentermine a controlled substance?
Yes. Phentermine is classified as a Schedule IV controlled substance in the United States.
What are common side effects of phentermine?
Reported effects can include dry mouth, constipation, diarrhea, unpleasant taste, restlessness, dizziness, insomnia, headache, palpitations, increased heart rate, and elevated blood pressure.
Other reactions are possible.
Can phentermine affect blood pressure?
Yes. Phentermine can increase blood pressure, and current labeling advises caution even in patients with mild hypertension. Certain cardiovascular conditions are contraindications to treatment.
Can phentermine be used during pregnancy?
No. Phentermine is contraindicated during pregnancy.
Can phentermine be combined with another weight-loss medication?
Patients should not combine phentermine with other weight-loss drugs or supplements without appropriate medical guidance.
Current labeling states that the safety and efficacy of combination treatment with other weight-loss products have not been established.
Can I choose my own phentermine dose?
No. Phentermine dosing should be individualized by the prescribing healthcare professional.
Patients should not increase their dose if they believe the medication is becoming less effective.
Does phentermine require a prescription?
Yes. Phentermine requires a prescription and is a Schedule IV controlled substance.
Can phentermine be prescribed through telehealth?
Telehealth evaluation may be possible depending on individual circumstances and applicable federal and state requirements.
Because phentermine is a controlled substance, additional prescribing requirements may apply.
An online evaluation does not guarantee that a prescription will be issued.
Learn More About Phentermine and Weight Management
Phentermine may be an option for short-term weight management in appropriately selected patients, but it is not suitable for everyone.
Eligibility should be determined through appropriate clinical evaluation that considers BMI, weight-related risk factors, cardiovascular history, medications, contraindications, and other individual health information.
Patients interested in available treatment options can review the dedicated A2ZRx Phentermine treatment page and the A2ZRx Weight Management category.
About This Article
Written by: A2ZRx Editorial Team
Last updated: September 29, 2026
This article provides general educational information and does not replace individualized medical advice, diagnosis, or treatment.
Whether phentermine or another weight-management treatment is appropriate should be determined by an appropriately licensed healthcare professional based on the patient's individual circumstances.
Controlled-substance prescribing is subject to applicable federal and state requirements.
References
1. DailyMed — Phentermine Hydrochloride Prescribing Information
Official U.S. prescribing information covering indications and usage, contraindications, dosage and administration, warnings and precautions, adverse reactions, controlled-substance status, and use in specific populations.
2. MedlinePlus — Phentermine
U.S. National Library of Medicine patient information covering phentermine use, administration, precautions, potential side effects, and safe medication use.
Clinical and Regulatory Note
Phentermine is a Schedule IV controlled substance in the United States.
Medication information and controlled substance prescribing requirements should be checked against current official sources before publication because prescribing information, federal requirements, state requirements, and telehealth rules can change.
Treatment and prescribing decisions remain with the appropriately licensed healthcare professional.
