what is phentermine?

what is phentermine?
what is phentermine?

What is Phentermine? A Comprehensive Guide to the Weight Loss Medication


In the ever-evolving landscape of weight loss solutions, phentermine stands out as one of the most widely discussed and prescribed medications. But what exactly is phentermine, and how does it work? This article delves into the details of phentermine, exploring its history, mechanism of action, effectiveness, side effects, and considerations for those considering its use. By the end of this comprehensive guide, you will have a thorough understanding of phentermine and whether it might be a suitable option for your weight loss journey.

History and Background of Phentermine

Phentermine, a prescription medication, has been used for decades as an appetite suppressant to aid weight loss. It was first approved by the U.S. Food and Drug Administration (FDA) in 1959. As an amphetamine-like prescription medication, phentermine belongs to a class of drugs known as anorectics, which work by reducing appetite.

Originally, phentermine gained popularity in the 1990s as part of the infamous weight loss combination known as "fen-phen," which paired phentermine with fenfluramine or dexfenfluramine. However, fen-phen was linked to serious heart-related side effects, leading to the withdrawal of fenfluramine and dexfenfluramine from the market in 1997. Despite this, phentermine remained available as a stand-alone prescription medication and continues to be a popular weight loss aid today.

Mechanism of Action: How Does Phentermine Work?

Phentermine works primarily by influencing the central nervous system. It stimulates the release of certain neurotransmitters in the brain, specifically norepinephrine, serotonin, and dopamine. These neurotransmitters are known to suppress hunger signals, leading to reduced appetite and, consequently, lower food intake.

Key Mechanisms:

  1. Norepinephrine Release: Phentermine increases the levels of norepinephrine, a neurotransmitter that activates the "fight or flight" response, making the body feel more energetic and less hungry.

  2. Serotonin and Dopamine Effects: By increasing the levels of serotonin and dopamine, phentermine enhances mood and contributes to a feeling of fullness, further reducing the desire to eat.

  3. Thermogenic Effect: Some studies suggest that phentermine may have a mild thermogenic effect, slightly increasing the body's metabolic rate and helping to burn more calories.

Effectiveness of Phentermine for Weight Loss

Phentermine is typically prescribed for short-term use, generally up to 12 weeks, due to its potential for dependence and side effects. During this period, it can be an effective tool for weight loss, especially when combined with lifestyle changes such as a reduced-calorie diet and increased physical activity.

Clinical Evidence:

Several studies have demonstrated the effectiveness of phentermine in promoting weight loss:

  • Short-term Studies: Research shows that phentermine can lead to an average weight loss of 5% to 10% of initial body weight over a 12-week period.

  • Long-term Observations: While long-term data is limited due to the short-term nature of phentermine prescriptions, some studies suggest that the weight loss achieved with phentermine can help kickstart a longer-term weight management plan when combined with ongoing lifestyle changes.

Who Should Consider Phentermine?

Phentermine is prescribed for individuals who are obese or overweight with weight-related medical conditions, such as hypertension, type 2 diabetes, or high cholesterol. It is intended for use as part of a comprehensive weight loss plan under the supervision of a healthcare provider.

Eligibility Criteria:

  • Body Mass Index (BMI): Phentermine is typically recommended for individuals with a BMI of 30 or higher, or a BMI of 27 or higher with associated obesity-related conditions.

  • Medical Supervision: A healthcare provider will assess your overall health, medical history, and potential risk factors before prescribing phentermine.

Potential Side Effects and Risks

As with any medication, phentermine carries the potential for side effects and risks. It's essential to be aware of these and discuss them with your healthcare provider before starting the medication.

Common Side Effects:

  • Dry Mouth: One of the most frequently reported side effects.

  • Insomnia: Difficulty sleeping is common due to the stimulant nature of phentermine.

  • Increased Heart Rate: Phentermine can cause an increase in heart rate and blood pressure.

  • Nervousness and Restlessness: Feelings of anxiety or restlessness may occur.

Serious Side Effects:

  • Cardiovascular Issues: Phentermine can increase the risk of cardiovascular problems, particularly in individuals with pre-existing heart conditions.

  • Psychiatric Symptoms: In rare cases, phentermine may cause mood swings, aggression, or other psychiatric symptoms.

  • Dependence and Withdrawal: There is a risk of developing dependence on phentermine, especially with prolonged use.

Contraindications: Who Should Avoid Phentermine?

Phentermine is not suitable for everyone. Certain medical conditions and medications can interact negatively with phentermine, making it unsafe for some individuals.

Conditions to Consider:

  • Heart Disease: Individuals with a history of heart disease, stroke, or uncontrolled high blood pressure should avoid phentermine.

  • Thyroid Problems: Those with hyperthyroidism should not take phentermine.

  • Glaucoma: Phentermine can worsen glaucoma symptoms.

  • Pregnancy and Breastfeeding: Phentermine is not recommended for use during pregnancy or while breastfeeding.

Drug Interactions:

  • MAO Inhibitors: Phentermine should not be taken with monoamine oxidase inhibitors (MAOIs) or within 14 days of stopping an MAOI.

  • Other Stimulants: Combining phentermine with other stimulant medications can increase the risk of adverse effects.

How to Use Phentermine Safely

If your healthcare provider determines that phentermine is appropriate for you, it's crucial to follow their instructions carefully to maximize benefits and minimize risks.

Dosage and Administration:

  • Dosage: Phentermine is typically taken once daily, usually before breakfast or 1-2 hours after breakfast.

  • Duration: The course of treatment is generally limited to 12 weeks.

Lifestyle Integration:

  • Diet: Adhere to a balanced, calorie-controlled diet as prescribed by your healthcare provider.

  • Exercise: Incorporate regular physical activity into your routine to enhance weight loss and overall health.

  • Behavioral Changes: Consider working with a dietitian or a weight loss counselor to develop sustainable habits and strategies for long-term success.

Alternatives to Phentermine

For those who may not be suitable candidates for phentermine or prefer not to use medication, there are several alternative weight loss options available.

Prescription Medications:

  • Orlistat (Alli, Xenical): A medication that reduces fat absorption in the intestines.

  • Liraglutide (Saxenda): An injectable medication that helps control appetite.

  • Naltrexone-Bupropion (Contrave): A combination medication that affects appetite and reward centers in the brain.

Non-Prescription Options:

  • Diet and Exercise: A personalized diet and exercise plan remains the cornerstone of effective weight loss.

  • Behavioral Therapy: Counseling and support groups can provide valuable tools and support for weight loss.

  • Surgery: Bariatric surgery is an option for those with severe obesity who have not succeeded with other methods.


Phentermine can be an effective weight loss aid for those who meet the eligibility criteria and use it under the guidance of a healthcare provider. By suppressing appetite and enhancing energy levels, it can help individuals achieve significant short-term weight loss. However, it's crucial to be aware of the potential side effects and risks associated with phentermine use. Always consult with a healthcare professional before starting any weight loss medication.

Incorporating phentermine into a broader weight loss strategy that includes diet, exercise, and behavioral changes can lead to more sustainable long-term results. For those who cannot use phentermine or prefer alternative methods, numerous other options are available to support weight loss and overall health.

Understanding phentermine, its benefits, risks, and proper usage is essential for anyone considering this medication as part of their weight loss journey. With informed decisions and professional guidance, phentermine can be a valuable tool in achieving and maintaining a healthier weight.