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> The Pill explained: Phentermine


What is phentermine?

Phentermine is a prescription appetite suppressant used for the short-term treatment of obesity (1,2). It is utilized in combination with behavioral changes such as eating a healthier diet and exercising regularly to promote weight loss.

Phentermine hydrochloride (phentermine HCL) is a schedule IV drug under the United States’ Controlled Substances Act and the international Convention on Psychotropic Substances (1).

A Brief History

Phentermine resin was first approved by the FDA in 1959 (3), and the medication’s current form (phentermine HCL) has been available since 1973 (4).

In the early 1980s, Dr. Michael Weintraub and his colleagues at the University of Rochester published a paper detailing the effectiveness of an off-label combination of fenfluramine and phentermine for weight loss (2). This cocktail was commonly known as “Fen-Phen” and doctors regularly prescribed the two medications together.

In 1997, doctors from the Mayo Clinic published findings that demonstrated that the popular combination increased patients’ risk of heart valve disease and fenfluramine, the culpable substance, was withdrawn soon thereafter (3).

However, phentermine has remained available for prescription and sale.

How It Works

Phentermine is a sympathomimetic amine anorectic. Its active ingredient, phentermine hydrochloride, is a central nervous system (CNS) stimulant that promotes a stress response similar to “fight or flight” (5). This biological change supports weight loss by suppressing appetite and boosting energy.

In addition, this medication stimulates the release of the neurotransmitter, adrenaline (epinephrine), which stimulates the breakdown of stored fat.

Therefore, phentermine promotes weight loss by reducing appetite, increasing energy and boosting fat breakdown (5).

However, while these pills can help you lose weight faster, but it’s still up to you to eat well, incorporate doctor-approved physical activity, and make healthy lifestyle choices. Committing to sustainable behavioral changes will not only maximize your weight loss while taking phentermine, but will also help you keep the weight off after your prescription comes to an end.

Active Ingredient

Phentermine diet pills have one active ingredient: phentermine. Most pills contain phentermine HCL and Ionamin (a name brand form) contains phentermine resin. However, different forms (capsules, tablets, etc.) and brands of phentermine contain distinct inactive ingredients. These inactive ingredients include fillers, dyes, and binding agents.

Phentermine has a chemical structure similar to amphetamines (6).


The only difference between the two substances is the methyl group (CH3) on the phentermine molecule, which takes the place of the single hydrogen (H) atom in the amphetamine. However, this small change means that phentermine is NOT an amphetamine.

Phentermine is highly effective at promoting weight loss, but it is also potentially-addictive and dangerous if not used as directed. However, phentermine is a Schedule IV drug while most amphetamines are Schedule II drugs, which indicates that this prescription medication has a lower addictive potential and is medically-recognized for treatment purposes.

Will phentermine show-up on a drug test?

Due to the structural similarities, phentermine may also show-up as a false-positive for amphetamines on standard drug screens (6).

Therefore, it is critical that you provide a valid doctor’s prescription if you are requested to undergo drug testing. Phentermine is legal when it is used for weight loss under a doctor’s supervision, but you should always be prepared to show a current and valid prescription.

If a false-positive result shows up on a drug test, more specific testing distinguishes between this prescription medication and other [illegal] drugs.

The half-life of phentermine HCL is 16-31 hours (average 20 hours), and this means that most people will positive for 4-7 days after the last dose of phentermine (7). However, the exact time period varies from person-to-person.

Factors that may influence how long the medication stays in your system include: urinary pH, the duration of use, and the daily dosage.


Phentermine is not appropriate or safe for everyone.

Inform your doctor of all current and past medical conditions, as well as any current or recent medications and supplements you may be taking. Consuming phentermine may lead to serious complications for people who have certain medical conditions (e.g. hyperthyroidism, glaucoma) or those who are taking other medications.

REMINDER: Only a medical doctor can evaluate if phentermine is appropriate and safe for you.

Side Effects

Phentermine produces a range of potential side effects. These reactions range from relatively mild to very severe.

Serious Side Effects

While most patients take this mediation without experiencing any issues, phentermine can cause serious side effects for some people. These reactions include potentially-fatal heart and lung problems.

Call emergency medical services or contact your doctor immediately if you experience any of the following reactions:

  • Chest pain

  • Trouble breathing

  • Fainting

  • Swelling of the feet or lower legs

  • Difficulty carrying out daily activities

Common Side Effects

Thankfully, most patients experience only minor to moderate side effects.

The most common side effects of phentermine are dry mouth (xerostomia), trouble sleeping (insomnia), constipation, and headache.

Additional reactions that may occur include: a change in sexual desire, diarrhea, dizziness, mood changes (e.g. “phen rage”), nausea or vomiting, rapid (racing) heartbeat, rash, stomach pain, tiredness, and an unpleasant taste in the mouth.

Why is phentermine sometimes prescribed with topiramate?

Phentermine is sometimes prescribed in combination with a low dose of topiramate as a generic substitute for the brand name medication Qsymia. I often use this particular option based on patient history and goals.

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