Generic Name: phentermine
Brand Name(s): Adipex, Qsymia, Lomaira, Suprenza, Ionamin, Bontril
Medically reviewed by Ahmed Zayed, MD on October 8, 2021

Phentermine is a popular prescription drug that is often used to treat obesity. The drug is classified under a certain chemical class known as amphetamine as it has a similar pharmacology value to other drugs classified in the class. Phentermine is most commonly prescribed to patients who experience negative health effects to excessive amounts of body weight.

The drug is always accompanied by increased physical activity and a diet that includes a lower amount of calories. The drug has two primary functions in the human body, which is the reason why it is always accompanied by these lifestyle changes. The drug is mainly an appetite suppressant, thus making a patient feel fuller after (and between) meals and making the patient more likely to consume foods that have lower amounts of calories. In addition to its function as an appetite suppressant, Phentermine drugs also act as energy boosters. They help the body produce a larger amount of energy from fat cells, which further reduces body fat percentage. The additional boost in energy will also increase the physical activity levels, thereby providing an additional method for them to lose weight and gain overall health benefits.

Different Brands of Phentermine

Different brands manufacture, distribute and sell Phentermine drugs. Each brand has its own way of manufacturing the drug and, in some cases, combines it with other chemicals to make it more effective or to add additional features to its use. The most common brand of Phentermine is a product known as Adipex, but officially called Adipex-P. This brand is manufactured by Teva Pharmaceuticals (formerly GATE Pharmaceuticals). The promotion of this brand as a diet pill started in 1990.

Other brands of this drug include Suprenza, Phentercot, Acxion, Elvenir, Terflamex, Panbesy, Duromine, Fentermina, Phentermin, PHentermine and Lomaira. These are all promoted as weight loss pills that assist obese individuals. Apart from these brand names, other brands that contain the drug as part of their multi-ingredient formula also exist and the most popular one is Qsymia.

History of Phentermine

The first use of Phentermine as a drug to help treat obesity dates back to 1938. Doctors used it to help patients who experience certain health issues caused by obesity to lose weight and improve the symptoms they are experiencing. Later on, between 1940 and 1960, Phentermine was combined with other medications to improve its effectiveness. Doctors often combined the use of Phentermine with diuretics, thyroid hormone medicines and digitalis. This was often referred to as “Rainbow Pills” as all four drugs were often provided to a patient at the same time. Unfortunately, patients experienced a variety of side effects, which included severe heart toxicity and hypertension. In some cases, the side effects were life-threatening and led to the death of a patient.

The FDA in the United States first approved the use of Amphetamine as a drug that can be used to suppress a patient’s appetite in 1959. Six years after the approval, a similar drug, called Aminorex, were released in Europe with the same purpose of helping obese individuals. However, it caused a chronic condition known as pulmonary hypertension, which led to the death of many patients. In 1968, three years after its release, Aminorex was withdrawn.

Phentermine was later provided to patients in combination with dexfenfluramine and fenfluramine as a more powerful appetite suppressant that could also act as a fat burner. This combination of ingredients was branded as Fen-Phen. A total of 24 patients developed heart valve disease while taking this drug, which led the FDA to withdraw the product from the market in 1997.

Today, Phentermine can still be obtained through the doctor’s prescription in the majority of countries around the world. The medication has been classified as a controlled substance in most countries and is also classified as a schedule IV prescription drug. Some preparations of the formula, however, have been classified as schedule II drugs.

What You Should Know Before Taking Phentermine

Since Phentermine is a drug that contains chemical compounds, it is important to know about all of the risks that are involved with its use and precautions a person should be aware of before the consumption. One of the most important things to consider is the fact that the drug should never be accompanied by an MAO inhibitor. It should also not be taken within 14 days after taking an MAO inhibitor. The MAO inhibitors (Monoamine Oxidase Inhibitors) are used to treat depression, especially a specific type called atypical depression. They inhibit the activity of an entire enzyme family known as monoamine oxidase.

Apart from not combining Phentermine with the MAO inhibitors, a patient should also first ensure they are not allergic to Phentermine before they take it. If you are allergic to other stimulants, amphetamines, cold medications or diet pills, you have a higher risk of being allergic to these pills too.

Precaution should be taken if a patient has any existing health concerns as the drug may cause further harm to their body. Existing health conditions that may act as a problem include diabetes, thyroid disorders, hypertension and kidney disease. While suffering from any of these health concerns may be a cause for concern, it does not mean a patient will not be able to use Phentermine at all. In some cases, especially when the health concern is not too serious, the doctor may prescribe the drug at lower doses spread throughout the day, instead of advising the patient to take one single dose every day.

Another important fact to consider before taking Phentermine is that it should never be used by women who are pregnant or who are currently breastfeeding. The FDA has categorized this drug as “X” for pregnancy, which means it should not be used during pregnancy under any circumstances. Not only can losing weight harm an unborn baby during pregnancy, but the drug may also cause adverse effects. When breastfeeding, the drug may enter the breast milk and travel into the nursing baby’s body, which may cause harm to them.

Obtaining A Prescription

In order to use Phentermine, the doctor needs to prescribe the drug for a specific patient. It isn’t prescribed on request, but rather when a healthcare provider notices it may be beneficial for a specific patient to lose weight in order to improve their overall health – or to decrease the risk of certain health concerns. A physician will usually start with a physical examination to determine if the patient’s health is in a state that will not be negatively affected when taking this drug. The doctor also looks at the patient’s medical history for any pre-existing medical conditions that the medication should not be taken with. Patients are advised to report any symptoms or diseases they may have before the doctor prescribes the drug – this may include high blood pressure and kidney problems, as well as any type of diabetes and thyroid disorders. If the doctor finds that the patient is a good candidate for Phentermine, they may prescribe a certain dose and provide specific details on dosage and lifestyle choices that should accompany the use of this medicine.

How To Take Phentermine

Taking Phentermine depends on the brand and preparation of the drug. The dosage varies from 15mg to 37.5mg per day. Of course, it is taken orally. Some patients need to take the drug an hour before they eat breakfast, while others are advised to take it approximately one to two hours after breakfast. In some cases, the doctor may prescribe Phentermine in smaller doses and advise the patient to take it up to three times in a single day.

What Happens If I Miss A Dose?

Taking more than the prescribed amount of Phentermine can lead to an overdose, which means if a patient misses a dose, they should never try to make up for it by taking more than the prescribed amount at the next dose. Should a patient realize he/she has missed a dose within a reasonable timeframe after the dose’s timing, it is acceptable to take it once they remember it. If the time is too close to the next dose, it is recommended to skip the dose they have missed.

What Happens If I Overdose?

Taking too much Phentermine can be harmful to the patient’s body. It is advised to stay in strict adherence with the instructions and dosage information a doctor prescribes to the patient in order to avoid dangerous side effects. The overdose symptoms include:

  • Confusion and hallucinations
  • Feeling aggressive and hostile
  • Panic
  • Nausea and vomiting
  • Stomach cramps and diarrhea
  • Rapid breathing and irregular heartbeat
  • A light-headed feeling or even fainting
  • Overactive reflexes
  • Convulsions (seizures)

What Should I Avoid?

The side effects of Phentermine can lead to several symptoms that may impair certain functions in the patient’s body. It is important not to perform activities that require a patient to be alert while experiencing mental impairment as an adverse effect. When mental impairment is present while taking Phentermine, a patient is also advised not to drive to avoid the risk of being involved in an accident. Moreover, it is strongly recommended to avoid or limit the alcohol intake.

Side Effects of Phentermine

Numerous side effects have been linked to the use of Phentermine. Before starting with consumption of this medication, patients should get informed about potential adverse effects to be able to determine whether a newly-developed symptom is a sign of a health issue or just a side effect associated with Phentermine. The most common side effects that patients experience include tremors, headaches, dizziness, dry mouth, diarrhea, constipation, upset stomach, feeling restless and being hyperactive.

It is not uncommon for patients to experience a change in their sexual interest. Some patients experience an increase in sexual interest, while others notice that they have a lower libido. In some cases, the use of Phentermine has also led to the development of symptoms that are related to erectile dysfunction. If the drug is taken too late in the day, it may also cause insomnia.

While these are the most common side effects of the drug, they are not the only consequences associated with the intake of this medication. Doctors often advise patients that in the case of more serious side effects, they should immediately call for assistance. Some of these include:

  • Shortness of breath
  • Feeling like they are going to faint, often accompanied by chest pain
  • Feet and ankles swell up
  • Fluttering in the patient’s chest, as well as pounding heartbeats
  • Symptoms of extremely high blood pressure, such as uneven heartbeats, chest pain, blurred vision, anxiety and a severe headache.
  • Being extremely irritable and confused, or experienced unusual behavior or thoughts.
  • Being extremely happy or sad.

Drug Interactions With Phentermine

As with many other prescription drugs, Phentermine can also interact with certain medications. In some cases, the use of this drug in combination with other pills may cause the effects of other drugs to increase. Also, it may cause certain side effects to become much worse. It is vital to talk with the doctor before using Phentermine with other products. This does not only include prescription drugs but also:

  • Over-the-counter drugs and medication
  • Protein powders and nutritional shakes
  • Vitamins and herbal supplements
  • Recreational and illegal drugs
  • Alternative medicines

Common drugs that are known to interact with Phentermine include Dopamine, Venlafaxine, Linezolid, Ephedrine, Isoproterenol, Epinephrine, Norepinephrine and a Phenylephrine injection. In some cases, patients have also found that Phentermine interacts with drugs that contain Tranylcypromine, Henelzine, Isocarboxazid, Diethylpropion, Phendimetrazine, Amphetamine, and Benzphetamine. For a more comprehensive list of drug and nutraceuticals which interact with phentermine hydrochloride, refer to our phentermine drug interactions data.

Pregnancy Warnings

Phentermine is not to be used while pregnant or breastfeeding under any circumstances. Even patients who are overweight and pregnant at the same time are not allowed to use this product. Losing weight during pregnancy can harm both a mother and her baby. As mentioned above, new mothers should avoid using this medication when breastfeeding.


The strength and dosage of Phentermine depend on the brand that is prescribed and the patient’s specific condition. The drug is also available in regular capsules that are taken orally with a glass of water. Some brands use a dissolving tablet that is placed under the tongue, which allows the body to absorb the chemicals in the drug faster. The drug is usually prescribed at the lowest possible dose to new users, which is 15mg per day. Patients with a larger body weight may need to take a larger dose, depending on whether they have any health concerns that may be worsened by the usage of a larger dose.

Phentermine should never be taken after 6 pm in the afternoon as it causes insomnia. It is often prescribed to be taken in the morning, before or after breakfast. Ideally, patients should take it on an empty stomach. In most cases, the drug is prescribed to users in periods of 12 weeks. After 12 weeks, the patient needs to take a break from the product as it may lead to physical dependence.

Making The Most Of the Phentermine

Phentermine is not a miracle drug taken daily and the body simply starts losing weight all by itself. It is an appetite suppressant with the added ability to help the body produce more energy from fat cells. Thus, in order to make the most of this drug, patients need make some effort too. While discussing the use of Phentermine with the doctor, a patient is provided a list of lifestyle changes to implement while using this product. It is important to follow these instructions religiously.

These lifestyle changes often include participating in regular exercise and physical activities, as well as eating a well-balanced diet that does not contain large amounts of calories or harmful substances. One should bear in mind that Phentermine is not only a drug to help reduce the baseline body weight of patients, but it also reduces a patient’s risk of developing certain health diseases. In fact, one study conducted at the Duke University School of Medicine found that apart from the Phentermine’s ability to effectively reduce body weight, it can also help improve a patient’s health-related quality of life.

How To Stop Using Phentermine

Patients who have been on a Phentermine program should not suddenly stop using the drug. This medication has been linked to physical dependence, which means a patient may become addicted to it. Since it may lead to such symptoms, it also has the potential to cause severe withdrawal symptoms, especially when a patient goes “cold turkey”. Instead of simply quitting the program, a patient should rather consult a healthcare provider about quitting. The doctor may advise the patient to complete the remaining period of the program or help him/her end the program safely. Even after the period that the drug has been prescribed for, the doctor will often recommend to gradually stop using the Phentermine to avoid the withdrawal symptoms.

Some of the most common withdrawal symptoms include weight gain, fatigue, insomnia and a bad mood. It is obvious that these symptoms can make the entire program useless. The goal of the drug is to help patients lose weight and increase energy – since the withdrawal symptoms of include fatigue and weight gain through an increase in appetite, the product is rendered a waste of the patient’s time and money.

Many patients find that if their doctor gradually lowers their dose, for example from 37.5mg per day down to 15mg per day for the last few days, they experience fewer withdrawal symptoms and are able to keep the weight off later.

Scientific Evidence Behind Phentermine

In order to determine the effectiveness of certain pharmaceuticals, supplements and other substances, clinical trials and scientific research are often conducted among a group of volunteers. Phentermine is one of the substances that have been researched thoroughly, thus its effects have been proven effective and the claims that are made about this drug are backed by scientific evidence. Some of the studies that have linked Phentermine to positive results in humans include:

  • A study by the Loma Linda University in California studied the effects of Phentermine on food cravings and weight loss among a group of 77 adults. The adult participants were split into two groups. One group received a daily dose of Phentermine, while the other group received a placebo. The study lasted for 12 weeks in total. After the study’s conclusion, the group that was asked to take a daily dose of Phentermine lost an average of 12.1% of their baseline body weight. The placebo group lost an average of 8.8%. The Phentermine group also had reduced cravings for sweets and fats.
  • A clinical trial conducted by the University of Colorado School of Medicine in 2016 reports that while Phentermine is used to reduce hunger, they found patients who were affected in such a way that their hunger ratings went higher had better success with the drug. A total of 27 participants were involved in the study. After eight weeks, participants lost an average of 5.4 kilograms. The participants that lost more than 5% of their baseline body weight reported a higher level of hunger pre-breakfast.
  • The Lindner Center of HOPE reports that the combination drug that uses Phentermine and Topiramate as their main active ingredients has shown promising results when it comes to treating patients with binge eating disorder. This disorder can cause excessive weight gain, which means the use of Phentermine in this combination drug can help patients with this disorder control their eating habits and may even contribute to additional weight management benefits.
  • While carrying out a clinical study in August 2015, the School of Medical Sciences department of the University of Adelaide discovered another benefit of Phentermine. A woman, aged 49, frequently experienced paradoxical hyperphagia, a relatively newly discovered process that is said to contribute significantly to binge and uncontrollable eating. She also frequently experienced migraines and intrusive thoughts about food. After she was put on a daily dose of Phentermine, she started experiencing a significant reduction in paradoxical hyperphagia and did not experience migraines as frequently as before. The woman also found that the intrusive thoughts about food started fading away.

Frequently Asked Questions About Phentermine

What are the main ingredients of Phentermine?

The main active ingredient is known as Phentermine Hydrochloride.

Can I buy Phentermine without first seeing my doctor?

Unfortunately, the answer is no in most cases. Phentermine is a schedule IV drug that requires a consultation with the doctor and a prescription.

How old should I be to use Phentermine?

While the drug is considered to be relatively safe for individuals who are older than 16, the recommended age is 18.

My doctor prescribed Topamax with my Phentermine prescription. Why is this?

The use of Topamax and Phentermine is actually an FDA-approved combination. The Topamax is added to Phentermine in order to intensify its effects and to help patients lose weight faster. Your doctor may prescribe both of these drugs separately, or choose to prescribe Qsymia, which is another medication that combines Phentermine and Topamax.

Is it safe to take supplements with this drug?

It depends on supplements a patient would like to take. Doctors often prescribe a dose of 5-HTP and vitamin B12 with a Phentermine prescription in order to enhance its effects and to relieve some side effects. If a patient would like to take other supplements, they should consult their doctor first.

For more FAQs, please see our full archive of Frequently Asked Questions about phentermine and related prescription appetite suppressants.

Final Words

As much as 33% of the U.S. adult population are considered to be obese. Obesity has been linked to many different diseases, including type 2 diabetes and cardiovascular disease. In order to help patients reduce their risk of developing these diseases when they are obese, doctors usually prescribe Phentermine. It helps patients lose weight by suppressing their appetite and boosting their energy levels, thus leading to a more active lifestyle and greater calorie burn. While this drug may sound like a “wonder pill”, patients should ensure they know are familiar with the precautions, interactions, side effects and other important details before they start taking it. Phentermine has been linked to many complications; knowing these details will help patients minimize their risk of experiencing such unwanted scenarios.


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