Phentermine for Weight Loss: How Does It Work?

Jenna Hilton
March 7, 2024

Phentermine is a central nervous system (CNS) stimulant drug that works as an appetite suppressant and is available only on prescription. Medical weight loss experts may add it to a dieting and exercise regime to help their patients kickstart their weight loss journey.

Read about how phentermine for weight loss works, how to take it, and what to expect, and find out if it is suitable for you.

Phentermine for Weight Loss.

Does Phentermine Affect Weight?

Phentermine belongs to the class of drugs called anorectics or anorexigenics, also known as appetite suppressants. It increases the levels of the brain’s neurotransmitters, norepinephrine, dopamine, and serotonin. Higher amounts of these chemicals induce a feeling of fullness, thereby minimizing food cravings, reducing calorie intake, and gradually leading to weight loss.

The exact way phentermine works to suppress appetite remains unclear. Some patients develop a tolerance to it over time and may require a medication switch.

Some weight-reducing pills (e.g., Qsymia) contain phentermine and topiramate to speed up weight loss on calorie-restricted diets.

How Much Weight Can I Lose with Phentermine?

According to a study published in the Annals of Pharmacotherapy, you can lose 10% to 11% of total body mass after a year of taking Qsymia (15mg phentermine/92mg topiramate). Other studies found that the rate of weight loss is higher in the earlier months of therapy and that it decreases with prolonged use.

The speed of a patient’s weight speed will depend on the following factors:

  • Starting dose
  • Initial weight
  • Starting BMI
  • Metabolic rate
  • Activity level
  • Other medical conditions

Patients can expect additional weight-related improvements, such as reduced belly fat, lower blood pressure, blood sugar, and cholesterol.

Note: Discover the best appetite suppressants to manage cravings.

Benefits of Using Phentermine for Weight Loss

Phentermine is a short-term weight loss aid with multiple benefits, including the following:

  • Appetite suppression
  • Prolonged satiety
  • Reduced calorie intake
  • Metabolism boost
  • Increased energy levels

Phentermine Risks and Side Effects

Phentermine shows no significant side effects as a short-term weight loss medication. However, there is inconclusive evidence to confirm its long-term safety.

The FDA-approved combination of phentermine and topiramate in reduced dosages is a safer, long-term solution for losing weight. Nonetheless, some risks and side effects are involved.

Common side effects are mild and temporary, including the following:

  • Itching
  • Dry mouth
  • Dizziness
  • Headache
  • Nausea
  • Stomach pain
  • Diarrhea
  • Constipation
  • Vomiting
  • Insomnia
  • Fatigue
  • Reduced sex drive
  • Increased sex drive
  • Unpleasant taste

Serious complications occur rarely and require immediate medical assistance. They include the following:

  • Shortness of breath
  • Chest pain
  • Allergic reaction (e.g., hives)
  • Swollen ankles or feet
  • Tremors and restlessness
  • Pounding heartbeat
  • Unusual mood swings
  • Increased blood pressure
  • Suicidal thoughts
  • Metabolic acidosis
  • Fetal harm
  • Cognitive impairment
  • Vision problems
  • Increased creatinine levels

Phentermine Withdrawal Symptoms

Do not change the dosage without consulting the doctor. If you stop using this medication abruptly, you may experience some of the following withdrawal symptoms:

  • Fatigue
  • Weight gain
  • Mood swings
  • Depression
  • Drug cravings

Withdrawal is more likely if you have used phentermine in higher dosages or for an extended period. Patients with a history of substance abuse may develop addiction to phentermine, too.

Your doctor may lower the dosage or prescribe medication to alleviate the symptoms. Seek prompt medical help if you notice any of the mentioned symptoms.

Interactions Between Phentermine and Other Medication

Phentermine may interact with 174 prescription and over-the-counter drugs, including the following:

  • Diet medication (e.g., Phen-Fen, Redux)
  • Vitamin supplements
  • Herbal supplements
  • Birth control pills
  • Some sleep medication
  • Medicines for diabetes
  • Narcotic pain-relievers
  • Stimulant medicines (e.g., for attention disorders)
  • Stomach-pain medication (e.g., dicyclomine, hyoscyamine)
  • Diuretics
  • Some medication for Parkinson's disease (e.g., benztropine, trihexyphenidyl)

A dangerous drug interaction may occur with monoamine oxidase inhibitors (MAOIs):

  • Isocarboxazid
  • Linezolid
  • Methylene blue injection
  • Phenelzine
  • Rasagiline
  • Selegiline
  • Tranylcypromine

Do not self-medicate with phentermine. Seek medical advice to learn more about its safety, optimal dosage, and potential drug interactions.

Note: If you are considering other weight loss options, check out our list of the best weight loss products.

Combining phentermine for weight loss with an active lifestyle.

Who Can Use Phentermine for Weight Loss?

Doctors prescribe phentermine to adults and children aged 12 years or older with an initial BMI equal to or in excess of 30 kg/m2, which falls within the obesity range.

Phentermine is typically prescribed to individuals who have not been able to lose excess weight with other diet pills, or who have additional weight-related medical conditions (e.g., high cholesterol, type 2 diabetes, high blood pressure).

Who Should Not Use Phentermine?

Use of phentermine is contraindicated if the patient has any of the following conditions:

  • Pregnancy
  • Breastfeeding
  • Glaucoma
  • Overactive thyroid (i.e., hyperthyroidism)
  • Severe heart problems
  • Coronary artery disease
  • High blood pressure
  • A history of drug abuse
  • Anxiety or agitation

Safety Measures

A pregnancy test should be done before starting this medication and as needed while using this medication. Blood pressure and heart rate should be checked and monitored. Patients should also be monitored for suicide ideation and mood changes.

How to Take Phentermine for Weight Loss

The recommended FDA-approved dosage is one 8-mg pill taken three times a day, at least 30 minutes before meals. Doctors may prescribe a higher dosage (e.g., 15, 30, or 37.5 mg) that you can take once a day, either before breakfast or 1-2 hours after breakfast.

The phentermine and topiramate blend (i.e., Qsymia) is an extended-release capsule taken in the morning. It is available in four doses (3.75-15 mg of phentermine and 23-92 of topiramate).

Take phentermine by mouth with a glass of water, as directed by your doctor. Do not crush or chew, and do not take more than directed. In case of overdosage, contact an emergency room immediately.

Note: Struggling to reach your health goals? Try these weight loss tips.


Phentermine can help patients lose weight in combination with a reduced-calorie diet, exercise, and behavior changes. Doctors usually prescribe it to people who have had no significant improvements with other lifestyle modifications, dieting programs, and exercise plans.

Contact Vibrant Vitality Clinic’s weight loss experts for more information and advice.

Jenna Hilton
Jenna Hilton has been a practicing PA since 2009, specializing in Family, Internal Medicine and Medical Aesthetics. She attended Arizona State University where she received her Bachelor's Degree and graduated magna cum laude. She received her Master of Science degree in Physician Assistant Studies from A.T. Still University.

Jenna has been injecting neurotoxin and dermal filler since 2013. She received certification as a Master Injector in 2017 through Empire Medical in Los Angeles, California. She is currently working on a Fellowship Program in Anti-Aging and Regenerative Medicine through the American Academy of Anti-Aging Medicine. Her special interests include use of PLLA, Ablative/Non ablative skin resurfacing, PDO threads, hormone therapy and nutritional therapies to improve cellular regeneration and medically supervised weight loss.

Jenna Hilton believes in a multi-factorial approach, considering internal factors that accelerate aging and disease development. She always enjoys teaching. She co-founded Vibrant EDU courses at Vibrant Skin Bar and regularly performs one-on-one training with fellow injectors. She teaches Aesthetic and Advanced Injectable Courses at National Laser Institute. She has been named Preceptor of the Year and is an Adjunct Faculty Member at Midwestern University. She was born in Iowa, and lives with her husband and three children in Phoenix, AZ.

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