Eating a well-balanced, reduced-calorie diet and exercising regularly are the cornerstones of weight loss, but certain drugs can provide powerful support.
One such drug is phentermine — one of the most popular weight loss drugs in the world.
It has been proven effective for short-term weight loss when used alongside a reduced-calorie diet and exercise.
However, using phentermine for weight loss is not without risks and side effects.
This article explains everything you need to know about phentermine, including its benefits, dosage, and possible side effects.
In this article
What is phentermine?
Phentermine is a prescription weight loss medication.
The Food and Drug Administration (FDA) approved it in 1959 for short-term use of up to 12 weeks for people older than 16.
In the 1990s, drug makers combined phentermine with other weight loss drugs. This drug combination was commonly called fen-phen.
After reports of significant heart problems in users, the FDA pulled the other two drugs that were used in the treatment — fenfluramine, and dexfenfluramine — from the market.
Today, phentermine can be used on its own and goes by the brand names Adipex-P, Lomaira, and Suprenza. You can also find it in combination medications for weight loss, such as Qsymia, which combines phentermine and another drug called topiramate.
Phentermine is a stimulant and is available if you have a prescription. Because taking phentermine comes with a risk of dependence, it is considered a controlled substance. It’s chemically similar to the stimulant amphetamine, which is also a controlled substance.
A healthcare professional may prescribe phentermine if you have obesity, defined by a body mass index (BMI) of 30 or greater.
Healthcare professionals may also prescribe it if you have excess weight with a BMI greater than or equal to 27 and have at least one weight-related condition, such as high blood pressure, high cholesterol, or type 2 diabetes.
However, a healthcare professional will likely recommend other weight loss strategies first, such as exercise and a calorie-reduced diet.
Summary: Phentermine is an FDA-approved drug intended for weight loss. Its chemical structure is similar to that of the stimulant drug amphetamine, and it’s available only with a prescription.
How does phentermine work?
Phentermine belongs to a class of drugs called anorectics, also known as appetite suppressants.
Taking phentermine helps suppress your appetite, thereby limiting how many calories you eat. Over time, this can lead to weight loss.
While the exact way phentermine reduces appetite remains unclear, the drug is thought to act by increasing neurotransmitter levels in your brain.
Neurotransmitters — the chemicals norepinephrine, serotonin, and dopamine — are your body’s chemical messengers.
When your levels of these three chemicals increase, your feeling of hunger decreases.
However, you may build a tolerance to the appetite-suppressing effects of phentermine within a few weeks. If that happens, speak with the healthcare professional who prescribed it.
Summary: Phentermine is thought to decrease your appetite by increasing neurotransmitter levels in your brain.
What is phentermine used for?
Phentermine is most often used for weight loss, but may also be used in combination with the medication topiramate to help with certain eating disorders, such as binge eating disorder (BED).
Phentermine is effective for weight loss
Several clinical studies have shown that phentermine can support meaningful weight loss.
Though the FDA approved it only for short-term use of fewer than 12 weeks, healthcare professionals often prescribe phentermine off-label for longer use.
Doctors may prescribe it intermittently, meaning you take a break from the medication for a set period before resuming it.
The average weight loss when taking phentermine is 3% of your initial body weight after 3 months and 5–7% after 6 months. Some people report even more weight loss than this.
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To put this into perspective, a 5–7% weight loss is 10–14 pounds (4.5–6.4 kg) for a person who weighs 200 pounds (90.7 kg).
However, not everyone responds the same way to this medication. If you haven’t lost much weight after 3 months of using it, a healthcare professional may recommend discontinuing the medication.
Further, its effectiveness appears to gradually decrease after prolonged use of more than a year.
While phentermine is effective for weight loss, it may work better when combined with topiramate.
Topiramate is a drug that has been used on its own to treat seizures but, like phentermine, also has appetite-reducing properties.
Topiramate and phentermine is a combination medication sold under the brand name Qsymia. The FDA approved it in 2012 as an addition to healthy lifestyle behaviors such as increased exercise and a calorie-reduced diet.
Compared to three other commonly prescribed drugs for weight loss, the combination of phentermine and topiramate was associated with the highest odds of losing at least 5% of initial body weight.
What’s more, a recent analysis of studies found that people who took a medication that combined phentermine and topiramate lost 16 pounds (7.3 kg) on average compared with a placebo.
For those who took the medication for more than 56 weeks, this number increased to 17.8 pounds (8 kg).
Furthermore, the degree of weight loss increased with the dose. Average weight loss was:
- 7.8 pounds (3.55 kg) for a 3.75/23 mg (phentermine/topiramate) dose
- 16 pounds (7.27 kg) for a 7.5/46 mg dose
- 18 pounds (8.25 kg) for a 15/92 mg dose
In users of phentermine-topiramate, weight loss translated to a significant decrease in waist circumference as well as improved insulin sensitivity improved blood sugar management, and reduction in blood pressure.
Phentermine may be helpful for certain eating disorders
The combination of phentermine and topiramate may help reduce binge eating in people with BED and bulimia nervosa.
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BED is characterized by eating large amounts of food, often quickly and to the point of discomfort. It’s also associated with a feeling of losing control during the binge, as well as feelings of shame or guilt afterward.
Bulimia nervosa involves the same binge-eating behavior as with BED but also includes behaviors people use to compensate for the effects of binge eating, such as self-induced vomiting.
In a small 12-week study in people with BED who had excess weight, people who took a phentermine and topiramate drug combination experienced significant reductions in binge-eating episode frequency.
In another 12-week study, people with BED or bulimia nervosa received the drug combination or a placebo.
Over 28 days, treatment with the phentermine and topiramate combination decreased the participants’ number of binge-eating days from 16.2 to 4.2. The same results were not observed in the placebo group.
While these results are promising, larger studies are still needed.
If you think you have BED, bulimia nervosa, or disordered eating, it’s important to speak with a healthcare professional.
Summary: Phentermine combined with topiramate has been shown to promote significant weight loss and may help reduce the incidence of binge-eating episodes.
Phentermine dosage and forms
Dosages for phentermine vary depending on its form and concentration.
Before 2016, the only available doses of phentermine were 15, 30, and 37.5 mg.
However, since it’s a best practice for doctors to prescribe the lowest effective dose, the FDA approved an 8-mg formulation (Lomaira) in 2016, which can be taken up to three times daily at least 30 minutes before meals.
If you’re prescribed a higher dose (15, 30, or 37.5 mg), you can take it once per day in the morning before breakfast or 1–2 hours after breakfast.
To prevent insomnia or difficulty falling or staying asleep, you should avoid taking the last dose too late in the day.
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Phentermine and topiramate
The combination of phentermine and topiramate — sold under the brand name Qsymia — is a medication used for weight loss.
This medication is available in four doses, ranging in strength from 3.75–15 mg of phentermine and 23–92 mg of topiramate. It’s offered in an extended-release capsule that should be taken in the morning.
After you take the lowest dose for 14 days, your doctor can choose to progress you to a higher dose.
According to the FDA, you should stop using the medication if you don’t lose 3% of your body weight after 12 weeks on the highest daily dose.
Summary: The dosage of phentermine differs, depending on whether it’s used alone or alongside topiramate.
Phentermine side effects and precautions
Phentermine alone is designed for short-term use only, as there are few long-term studies on its safety.
One study observed that people experienced no significant side effects when they took phentermine for up to 24 months.
However, the FDA has approved phentermine in combination with topiramate for long-term use, since the doses of the two ingredients are lower than the maximum doses of the individual drugs.
The most commonly reported side effects of phentermine and combined phentermine and topiramate are:
- dry mouth
- sleep problems
- heart palpitations
- skin flushing
While rare, some severe side effects are possible, such as increased blood pressure, suicidal ideation, metabolic acidosis, increased creatinine levels, fetal harm, vision problems, cognitive impairment, and hypokalemia.
You shouldn’t take phentermine if you have heart disease, hyperthyroidism, glaucoma, severe kidney or liver disease, or if you’re pregnant or nursing.
Phentermine should also not be prescribed in combination with monoamine oxidase inhibitors (MAOIs), a class of medications used to treat depression.
Speak with a healthcare professional if you’re taking other medications, such as antihypertensive or antiepileptic medications.
Your healthcare professional will determine whether phentermine is appropriate and safe for you.
Summary: While some common side effects are associated with the use of phentermine and topiramate, most people tolerate it. However, people who have certain conditions or are pregnant or nursing should not use phentermine and topiramate.
Healthy ways to promote and maintain weight loss
Most often, healthcare professionals prescribe phentermine and topiramate after you’ve tried other lifestyle modifications for weight loss but haven’t seen significant results.
While phentermine can be a powerful weight loss aid, the only proven way to shed weight and keep it off in the long term is to engage in healthy lifestyle behaviors.
Without making these habit changes, you may gain back any weight you’ve lost — and possibly more — once you stop taking phentermine.
Instead, it’s best to prioritize healthy, sustainable lifestyle habits that promote health beyond weight loss.
Extensive research has shown that exercise, a nutritious diet, stress management, and proper sleep are crucial to promoting better health and reducing your risk of disease.
What’s more, taking medication long-term is not without risks, and the cost can add up, especially if you don’t have insurance.
For these reasons, a healthcare professional will likely recommend that you make healthy lifestyle changes before recommending a weight loss medication.
Some examples of healthy lifestyle changes you can make include:
- Eating a reduced-calorie diet. If you have excess weight to lose, you can try to eat 300–500 fewer calories per day. A registered dietitian can help you tailor this range based on your preferences and goals.
- Prioritizing nutrient-dense foods. Nutrient-dense foods such as fruits and vegetables are relatively low in calories and high in nutrients such as fiber, vitamins, and minerals.
- Increasing physical activity. The U.S. Department of Health and Human Services recommends getting a minimum of 150 minutes per week of moderate aerobic exercise such as brisk walking or running.
- Getting more sleep. Though individual needs vary, try to get 7–9 hours of sleep per night.
- Doing your best to manage your stress levels. Managing stress can help reduce emotional eating and inflammation. You can try strategies such as medication, going for walks, socializing with friends and family, or speaking with a trained professional.
- Trying behavioral strategies. Behavior changes that may be helpful include self-monitoring your food intake, physical activity, and weight and making adjustments as needed.
These lifestyle changes can be difficult to make, and you don’t have to do them all at once. Instead, slowly incorporate new healthy behaviors over time. This will help promote long-term, sustainable results.
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Summary: Lifestyle and behavior modifications are the cornerstones of successful weight loss and maintenance.
Phentermine is a prescription-only appetite suppressant and weight loss pill approved for short-term use.
The combination of phentermine and topiramate appears to be even more effective than phentermine alone, and it’s approved for long-term use. It may also help reduce bingeing episodes in people with BED.
Common side effects include dry mouth, dizziness, fatigue, irritability, and constipation. You should not use this medication if you have heart disease, hyperthyroidism, glaucoma, severe kidney or liver disease, or if you’re pregnant or nursing.
Also, be sure to consult a healthcare professional before trying phentermine if you’re taking certain medications, such as monoamine oxidase inhibitors (MAOIs).
While phentermine can be a useful weight-loss tool, it’s not recommended as a first-line treatment. Instead, it’s best to prioritize healthy lifestyle changes for long-term success.