What is Phentermine Used For?
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Phentermine - Uses, Side Effects, Warnings, And Interactions

Published on Sep 29, 2022 and last reviewed on Nov 16, 2022   -  6 min read


Phentermine is used to suppress appetite and helps in weight loss. Learn about its uses, side effects, dosage, drug warnings, interactions, and more.


Phentermine is a short-term drug that helps to treat obesity, together with a low-calorie diet and exercise. It decreases the appetite and causes a feeling of fullness to last longer after eating. Phentermine is used with Topiramate as an extended-release capsule to help overweight adults with weight-related medical problems. It helps reduce weight by 3 % of the initial body weight in three to five months. Although Phentermine is the most commonly prescribed weight-loss medication, no prescription can replace a healthy diet and exercise.

Drug Group:

Phentermine is a sympathomimetic amine. It belongs to a medication called anorectics which is used for short-term therapy for obesity. It is classified as a Schedule IV drug with a mild potential for abuse and is available by prescription.

What Is Phentermine Used to Treat?

Phentermine is used to speed weight loss in overweight people on a low-calorie diet and exercise. It also aids in weight loss in obese people with certain health risk factors like high blood pressure, high cholesterol, and diabetes. The combination of Phentermine and Topiramate (used to treat seizures) has been approved by the FDA to be used in individuals with BMI over 27 kg/m² and having at least one obesity-related comorbidity. The drugs are to be used with a reduced caloric diet and exercise. It may help reduce binge eating in people with Bulimia nervosa, a life-threatening eating disorder.

How Does Phentermine Work?

Phentermine acts by increasing the neurotransmitter levels in the brain and suppressing hunger. Phentermine is an indirect-acting sympathomimetic agent that releases noradrenaline from the presynaptic vesicles. In addition, it stimulates the beta2-adrenergic receptors and reduces the hunger signal as this state is in immediate need of energy.

What Is the Onset of Action of Phentermine?

Phentermine is absorbed through the gastrointestinal tract and reaches peak concentration in the blood in three to four hours.

70 % to 80 % of Phentermine is excreted in the urine unchanged, while the rest undergoes AN-oxidation and N-hydroxylation to form metabolites in the liver. The drug remains effective for six to eight hours and may take more than 24 hours to be eliminated from the body.


Phentermine may be habit-forming and should not be taken for the long term. In addition, it can be addictive, and tolerance may develop, rendering it less effective with time.

Expiry Date:

It would be best not to take medicines after their expiry date. You can find the expiry date on the medicine packaging.

What Is the Dosage of Phentermine?

The dosage of Phentermine varies with age, the condition being treated, severity, other medical conditions, and how you react to the first dose. Phentermine is available in tablets and capsules and should be taken only with a doctor's prescription.


How to Use Phentermine?

Phentermine should be taken as directed by the doctor. The drug is available in capsules, disintegrating tablets, extended-release capsules, and tablets. The medicine is taken one hour before breakfast or one to two hours after breakfast. Avoid taking medication late at night as it may cause trouble sleeping.

The sustained-release capsules should be swallowed whole, and the tablets should be dissolved in the mouth to benefit from the drug. The dose of the medication should be gradually decreased to avoid withdrawal symptoms and should be stopped when advised by the doctor.

Missed Dose:

If you miss a dose of Phentermine, it should be taken as soon as possible. Do not take two doses at once, as it may cause adverse events.

What Are the Drug Warnings and Precautions?

Check with your doctor if you have any of the following:

  1. Pregnancy and Lactation: Phentermine should not be used in pregnancy as it may harm the fetus. It is not recommended in pregnancy as weight loss has no potential benefit during pregnancy. The drug may pass into breast milk and is not recommended.

  2. Allergy: The drug may cause allergic reactions in patients with a history of allergy to other sympathomimetic amines.

  3. Activities requiring Mental Alertness: Phentermine may cause dizziness or blurred vision. Do not involve in activities such as driving or the use of machinery.

  4. Elderly: The elderly may be at greater risk for dizziness and high blood pressure while using the drug.

  5. Drug Abuse and Misuse: Regular use of Phentermine may cause withdrawal symptoms such as depression and tiredness. Hence the drug is gradually reduced with the consultation of the doctor.

  6. Hyperthyroidism: Phentermine may increase thyroid activity and should be taken cautiously in people with hyperthyroidism.

  7. Children: The drug should not be used in children younger than 16.

  8. Kidney Disease: Phentermine may develop metabolic acidosis, which may cause kidney stones, and hence should be taken with caution in people with kidney disease.

What Are the Side Effects of Phentermine?

Some of the common side effects of Phentermine include -

  • Increased heart rate.

  • Shortness of breath.

  • Tingling in hands or feet.

  • Dry mouth.

  • Sleeplessness.

  • Constipation.

  • Nervousness.

Serious side effects may include -

  • Shortness of breath.

  • Chest pain.

  • Swelling in your ankles or feet.

  • Tremors.

  • Unusual change in behavior.

  • Increased blood pressure.

What Are the Interactions of Phentermine?

Phentermine can interact with other medicines, food, and medical conditions, preventing the drug from working well.

Consult your doctor about all your medications

Drug Interaction:

Monoamine Oxidase Inhibitors such as -

  1. Isocarboxazid.

  2. Phenelzine.

  3. Selegiline.

  4. Tranylcypromine.

Other drugs for weight loss may include -

  1. Amphetamine.

  2. Benzphetamine.

  3. Phendimetrazine.

  4. Dextroamphetamine.

  5. Diethylpropion.

Serotonin reuptake inhibitors such as -

  1. Fluoxetine.

  2. Citalopram.

  3. Sertraline.




Anti-diabetic drugs such as

  1. Nateglinide.

  2. Pioglitazone.

  3. Glimepiride.

  4. Glipizide.

Food Interactions:

  • Foods - Eating healthy food such as broccoli, carrots, cucumbers, bananas, and almonds increases Phentermine's absorption and should be taken. Avoid food rich in sugars and butter as they increase the excretion of Phentermine from the system.

  • Caffeine: Avoid drinking large amounts of beverages containing caffeine, like tea, coffee, or carbonated drinks, as it may increase the side effects of this medicine.

  • Alcohol: Using Phentermine with alcohol can increase the risk of cardiovascular side effects such as increased heart rate, chest pain, or blood pressure changes.

  • Smoking: Tobacco smoke interacts with Phentermine and influences its absorption, distribution, metabolism, or elimination. Smokers may require higher doses of medications.

With Diseases:

  • Cardiac Diseases - Phentermine is contraindicated in patients with cardiovascular impairments as it may cause a severe increase in blood pressure or heart valve problems. Patients with pre-existing high blood pressure must be monitored regularly.

  • Kidney Diseases - Phentermine should be administered with caution in patients with impaired renal function as there may be an altered rate of elimination in these patients.

  • Liver Diseases - The plasma clearance is significantly decreased in patients with impaired hepatic function. Phentermine should be cautiously administered in patients with moderate to severe liver disease.

  • Glaucoma - Using Phentermine in patients with narrow-angle glaucoma can induce transient mydriasis or increase intraocular pressure.

  • Anxiety - Phentermine aggravates symptoms of anxiety and agitation and should be administered cautiously in patients with a history of psychosis.

  • Diabetes - Patients with type 2 diabetes should be monitored and may require the dosages of the hypoglycemic medications to be adjusted.

What Are the Common Trade Names of Phentermine?

  1. Capsule Neuron.

  2. Capsule Phengold.

  3. Capsule Primeshred.

  4. Capsule Phen24.

  5. Capsule LeanBean.

  6. Tablet Adipex-P.

  7. Tablet Lomaira.

  8. Capsule Qsymia.

  9. Tablet Fastin.

  10. Tablet Ionamin.

Article Resources

Last reviewed at:
16 Nov 2022  -  6 min read




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