Does phentermine burn fat or just suppress appetite?

Phentermine is one of the most commonly prescribed weight-loss medications in the U.S. Still, many people wonder exactly how it works. Does phentermine burn fat or just suppress appetite?

In this article, we answer that question. We also explain how phentermine works, the pros and cons of taking it, and what you can expect in terms of weight loss. By the end, you’ll have a clear understanding of whether phentermine may be the right choice for your weight loss journey — or if another option might be better suited for your needs.

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What is phentermine?

Phentermine was approved by the U.S. Food and Drug Administration (FDA) in 1959 as a prescription appetite suppressant for short-term weight management, in combination with diet and exercise. It’s prescribed to adults with a body mass index (BMI) of 30 or higher, or a BMI of 27 or higher if they have weight-related conditions, such as high cholesterol, high blood pressure (treated or untreated), heart disease, or type 2 diabetes. Phentermine is available under various brand names, including Adipex-Pand Lomaira, and as a generic medication.

How quickly can you lose weight on phentermine?

In 1 study on phentermine use, a majority of people (62%) lost 3% of their starting body weight in 12 weeks. How much weight you lose on phentermine can vary based on individual response and lifestyle factors, including metabolism, diet, and exercise level and frequency.

Phentermine is classified as a Schedule IV controlled substance due to its potential for misuse and dependency. However, research on the role of phentermine in obesity suggests that there’s no evidence that phentermine is addictive. Other research supports that and that phentermine isn’t addictive even at higher doses than typically recommended.

So, does phentermine burn fat or just suppress appetite? 

While phentermine primarily works as an appetite suppressant, it indirectly supports fat burning. Let’s explain. 

Phentermine is both an appetite suppressant and a central nervous system (CNS) stimulant. It works by increasing the levels of neurotransmitters (chemical messengers) in the brain, including serotonin, noradrenaline, and dopamine. 

By enhancing these neurotransmitters, phentermine creates a cascade of effects that help with weight loss:

• It suppresses appetite by stimulating feelings of fullness, so you feel less hungry.
• It increases your energy levels, which makes it easier for you to work out — resulting in additional calorie burn. 
• Even if you’re eating less, you may feel less fatigued, so you have more energy to stay active. 
• While phentermine doesn’t directly “burn” fat, its stimulating effects can encourage your body to break down stored fat for energy.

What to expect if you take phentermine

By now you know that phentermine causes significant changes in appetite and weight. But what’s it like to take phentermine on a day-to-day basis? Here’s what you need to know.

Phentermine dosage

Phentermine is an oral medication you take by mouth. It comes in tablet and capsule forms. The dosage depends on the generic or brand name you use, your weight loss goals, and your tolerance for side effects.

• Adipex-P and generic phentermine come in 37.5-milligram tablets or capsules. You take either 1 tablet or a half-tablet once daily, either right before breakfast or 1 to 2 hours afterward. Some people may take 2 half-tablets daily, if their healthcare provider advises it.
• Lomaira comes in 8-milligram tablets. You take 1 tablet 3 times a day about 30 minutes before meals.

You shouldn’t take phentermine at night because it can cause insomnia.

How phentermine might make you feel

Phentermine stimulates the central nervous system, so it can cause feelings of increased energy and alertness, much like a stimulant. 

Some people experience side effects such as jitteriness, restlessness, and difficulty sleeping. It’s also common to experience dry mouth and constipation, which can be relieved by drinking plenty of water and eating fiber-rich foods.

How fast can you lose weight on phentermine?

Retrospective studies of phentermine show you can lose at least 3% of your body weight in 12 weeks on phentermine. Compared to placebo, people taking phentermine lose 4.4% more weight after 28 weeks, according to a review of antiobesity medications. The same review found that after 36 weeks, people taking phentermine lost 13% of their body weight, compared to only 5% for those on a placebo. 

Benefits of taking phentermine

Phentermine offers several benefits, especially when used as part of a comprehensive weight loss treatment plan.

• Appetite suppression: The main benefit of phentermine is appetite control. In fact, studies on hunger and restraints show that people who feel hungrier and have less control over their eating tend to experience greater weight loss on phentermine. 
• More energy: Many people feel more energetic when taking phentermine. That extra boost can make it easier for you to keep up with your exercise routine.
• Rapid initial weight loss: A majority of people lose at least 3% of their body weight in 12 weeks. When you see the pounds melting away, you may feel more motivated to keep up with other healthy lifestyle habits — such as diet and exercise.

Cons of taking phentermine

While phentermine offers significant benefits for those looking to lose weight, there are also drawbacks to consider.

• Short-term use only: Phentermine isn’t intended for long-term use, and weight regain is common after stopping it.
• Side effects: As a stimulant, phentermine can cause insomnia, jitteriness, and increased heart rate. It can also cause gastrointestinal side effects like dry mouth and constipation.
• Risk of dependency: As a controlled substance, there’s a risk of dependency and abuse with phentermine.
• Tolerance: Phentermine’s effectiveness may decrease over time as your body builds up a tolerance to it. In 1 study of phentermine in people with obesity, those taking phentermine for 6 months regained about 10% of the body weight they’d lost 3 months after stopping it. You may also experience withdrawal symptoms when stopping phentermine.

Side effects and warnings of phentermine

Phentermine has a range of potential side effects, though not everyone will experience them. The most common side effects of phentermine in females and males include dry mouth and insomnia. Some people may experience more serious side effects like chest pain, heart palpitations, swelling in the legs and ankles, pulmonary hypertension (high blood pressure in the blood vessels of the lungs), valvular heart disease, or impaired functioning while operating heavy machinery or driving a car. These should be reported to a healthcare provider immediately.

When not to take phentermine

Phentermine may not be suitable for everyone. It should be avoided in people with certain medical conditions, including:

• Heart disease: People with a history of heart disease, high blood pressure (hypertension), or arrhythmias should generally avoid phentermine.
• Agitated states: Those with a history of anxiety or nervousness may find that phentermine worsens symptoms.
• Pregnancy and breast-feeding: Phentermine is not safe for use during pregnancy or breastfeeding.
• Hyperthyroidism: Individuals with an overactive thyroid should avoid phentermine as it can exacerbate symptoms.
• Glaucoma: Phentermine may increase pressure in the eyes, making it unsuitable for people with glaucoma.
• History of drug abuse: Due to its stimulant properties and potential for dependency, phentermine is not recommended for people with a history of substance abuse.

Always discuss your full medical history with a healthcare provider before starting phentermine. You can find a provider to discuss if phentermine is a good option for you on Klarity Health.

Phentermine can interact with alcohol and some medications, including certain antidepressants. Share all current medications with your healthcare provider to make sure this weight loss pill is safe for you. Phentermine’s drug interactions include: 

• Monoamine oxidase inhibitors (MAOIs) such as isocarboxazid or phenelzine
• Insulin
• Oral hypoglycemics, such as metformin
• Adrenergic neuron-blocking drugs, such as tamsulosin or prazosin 
• Caffeine

Alternative fat burners and appetite suppressants

If you and your provider decide that phentermine doesn’t seem like the right option, there are other appetite suppressants and weight loss pills you can ask your doctor about. We’ve included phentermine in the table below, along with phentermine’s cost, so you can compare and contrast.

Like phentermine, Qsymia and Contrave are also controlled substances, so there’s a risk of abuse and dependence. Costs are from drugs.com and reflect its discount. 

Key takeaways

• Phentermine primarily supports weight loss by suppressing your appetite. This makes it easier to consume fewer calories and maintain a calorie deficit.
• While phentermine doesn’t directly burn fat, it promotes fat loss by reducing hunger and increasing your energy levels. This may make you more likely to follow a healthy diet and increase your physical activity.
• Phentermine is FDA-approved for short-term weight loss in people with a BMI of 30 or higher, or 27 or higher if they have weight-related health conditions.
• For longer-term weight loss, the FDA has approved other medications that also suppress appetite or burn fat, including Wegovy and Xenical.
• Phentermine isn’t suitable for everyone and should be avoided by people with certain health conditions. Common side effects include dry mouth, insomnia, and jitteriness.
• Phentermine can be an effective tool for kickstarting your weight loss program. But long-term success requires sustainable lifestyle changes, including a balanced diet and regular exercise.

Find out if weight loss medication is right for you

Weight loss can be challenging, but medication can make it easier. If you’re curious about how phentermine might fit into your weight loss journey, connecting with a healthcare provider is the first step. They can answer your questions and recommend a weight loss treatment plan that aligns with your goals and lifestyle and that may or may not include medication. Support is just a click away on Klarity Health. Make an appointment with a weight loss provider today.