phentermine weight loss

Phentermine: Usages, Dosing, Side Effects

Teresa Hong

Teresa Hong


Teresa Hong is a dietician who is actively engaged in food education. Develops nutritional methods available to as many people as possible. Offers specific dietary modification tools for people with endocrine disorders, food allergies, atherosclerosis, malnutrition, irritable bowel syndrome and other diseases. She will help to get rid of stress and enjoy every meal again!

, reviewed by
– Updated August 9, 2022


Generic name: Phentermine  [ˈntəmɪ:n]
Brand names:
Adipex-P, Lomaira, Suprenza
Drug class:
S6 – Stimulants
iNova Pharmaceuticals
Molecular Formula:
Create Date:
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Consumer Medicine Information (CMI) summary

The full CMI on the next page has more details. If you are worried about using this medicine, speak to your doctor or pharmacist.


What is phentermine? What is it used for?

Phentermine is similar to amphetamine. It stimulates the central nervous system (nerves and brain), which increases your heart rate and blood pressure and decreases your appetite.

Phentermine is used together with diet and exercise to treat obesity, especially in people with risk factors such as high blood pressure, high cholesterol, or diabetes.

Phentermine may also be used for purposes not listed in this medication guide.

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 [1], [2].

TGA Pharmacology Summary

Phentermine is an approved weight loss medication by the Therapeutic Goods Administration (TGA) in Australia that is known for its efficacy in reducing weight. Taking phentermine alone or in combination with other drugs may assist in increasing weight loss.

Phentermine belongs to the class of Sympathomimetic Amine Anorectics. The drug works by suppressing appetite and increasing sympathetic activity, resulting in a physiological effect.

Medicines that contain phentermine

Phentermine (GH), Supremine ER, Alenami, Phentodur, Duromine, Metermine.


Before taking this medicine

You should not use phentermine if you are allergic to it, or if you have:

✕ a history of heart disease (coronary artery disease, heart rhythm problems, congestive heart failure, stroke);
severe or uncontrolled high blood pressure;
overactive thyroid;
extreme agitation or nervousness;
a history of drug abuse; or
if you take other diet pills. 

Do not use phentermine if you have used an MAO inhibitor in the past 14 days. A dangerous drug interaction could occur. MAO inhibitors include isocarboxazid, linezolid, methylene blue injection, phenelzine, rasagiline, selegiline, tranylcypromine, and others.

Weight loss during pregnancy can harm an unborn baby, even if you are overweight. Do not use phentermine if you are pregnant. Tell your doctor right away if you become pregnant during treatment.

You should not breastfeed while using this medicine.

To make sure this medicine is safe for you, tell your doctor if you have:

✕ heart disease or coronary artery disease;
a heart valve disorder;
high blood pressure;
diabetes (your diabetes medication dose may need to be adjusted); or
kidney disease.

Phentermine is not approved for use by anyone younger than 16 years old.


Phentermine is contraindicated for use if you have or you know of the following diseases or health conditions:

Known hypersensitivity or idiosyncratic reaction to sympathomimetic amines
History of cardiovascular disease (coronary artery disease, uncontrolled hypertension, arrhythmias, stroke, congestive heart failure)


Agitated states


History of drug abuse

Use of monoamine oxidase inhibitors (MAOIs) within the preceding 14 days; concomitant administration of other CNS stimulants

Pregnancy and breastfeeding

Phentermine side effects

Get emergency medical help if you have signs of an allergic reaction to phentermine: hives, difficult breathing, swelling of your face, lips, tongue, or throat.

Phentermine may cause serious side effects. Call your doctor at once if you have:

Common side effects of phentermine may include:

Some of the adverse effects of phentermine may include:

This is not a complete list of side effects, and others may occur. Call your doctor for medical advice about side effects. You may report side effects to TGA at 1300 134 237.

How does it 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 [3], [4].

Neurotransmitters — the chemicals norepinephrine, serotonin, and dopamine — are your body’s chemical messengers [5], [6].

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.

How should I take phentermine?

Dosing information

Usual Adult Dose for Obesity:

8 mg orally 3 times a day 30 minutes before meals, OR

15 to 37.5 mg orally once a day before breakfast or 1 to 2 hours after breakfast.

Use: Short-term (a few weeks) adjunct in a regimen of weight reduction based on exercise, behavioural modification, and caloric restriction in the management of exogenous obesity in patients with an initial body mass index (BMI) of 30 kg/m2 or greater, or BMI of 27 kg/m2 or greater in the presence of other risk factors (e.g., controlled hypertension, diabetes, hyperlipidemia).

What happens if I miss a dose?

Take the medicine as soon as possible, but skip the missed dose if it is late in the day. Do not take two doses at one time.

What happens if I overdose?

Overdose symptoms may include confusion, panic, hallucinations, extreme restlessness, nausea, vomiting, diarrhoea, stomach cramps, feeling tired or depressed, irregular heartbeats, weak pulse, seizure, or slow breathing (breathing may stop).

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222. An overdose of phentermine can be fatal.

What to avoid

Avoid driving or hazardous activity until you know how phentermine will affect you. Your reactions could be impaired.

Drinking alcohol with this medicine can cause side effects.

Phentermine forms and appearance

Before 2016, the only available doses of phentermine were 15, 30, and 37.5 mg [7], [8].

Phentermine 37.5 mg
Color: pale yellow capsules
Phentermine 30mg

If you’re prescribed a higher dose (15, 30, or 37.5 mg), you can take it once daily in the morning or 1–2 hours after breakfast.

What other drugs will affect phentermine?

Taking phentermine and other diet medications such as fenfluramine (Phen-Fen) or dexfenfluramine (Redux) can cause a rare fatal lung disorder called pulmonary hypertension. Do not take phentermine with any other diet medications without your doctor’s advice.

Many drugs can interact with phentermine. This includes prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible interactions are listed here. Tell your doctor about all your current medicines and any medicine you start or stop using.

What other information should I know?

Keep all appointments with your doctor and the laboratory. Your doctor will order certain lab tests to check your body’s response to phentermine and topiramate.

Do not let anyone else take your medication. Giving or selling phentermine and topiramate may harm others and is against the law. Phentermine and topiramate are controlled substances. Prescriptions may be refilled only a limited number of times; ask your pharmacist if you have any questions.

You must keep a written list of all the prescription and nonprescription (over-the-counter) medicines you are taking and any products such as vitamins, minerals, or other dietary supplements. You should bring this list with you each time you visit a doctor or if you are admitted to a hospital. It is also important information to carry with you in case of emergencies.


  1. Is Phentermine a good option for weight loss? Available from: [URL Link]
  2. Phentermine HCL – Uses, Side Effects, and More Available from: [URL Link]
  3. Phentermine. Available from: [URL Link]
  4. Side effects and precautions. Healthline. Available from: [URL Link]
  5. Phentermine and Topiramate. Trusted Health Information for you. Available from: [URL Link]
  6. Phentermine. Related Compounds with Annotation. Pubchem. Available from: [URL Link]
  7. Phentermine. When and how to use. MedicalNewsToday. Available from: [URL Link]
  8. Active ingredient: Phentermine. Healthdirect. Available from: [URL Link]
  9. Phentermine Hydrochloride. PeaceHealth. Available from [URL Link]

Popular FAQ

Exhaustion may happen from restless nights or overstimulation, but phentermine can also cause sleepiness, fatigue, and low energy. If you feel tired or sluggish while taking phentermine, see your doctor. You may have an undetected medical issue.

When prescribed, phentermine is safe. Make sure your doctor knows about all your medications and health issues. It shouldn’t be used with other diet pills. Rarely people have high blood pressure.Phentermine should not be taken if:
You have cardiovascular disease
You are taking or recently stopped taking drugs known as monoamine oxidase inhibitors
You have hyperthyroidism
You have glaucoma
You have a history of drug abuse or agitation
You are pregnant or nursing
You are sensitive to or had a reaction to other stimulant drugs

Phentermine is part of a weight-loss programme that typically involves exercise, behaviour modification, and calorie reduction. Phentermine treatment normally lasts 12 weeks. Diet, exercise, and other variables affect phentermine’s weight-loss results.

Phentermine stays 4 days in your system. Half the medicine is gone after 20 hours. Drug elimination depends on several factors. Age, diet, renal and liver health are all contributors.

Doctor-prescribed phentermine use is the best one. It is usually taken first thing in the morning, 1 to 2 hours after breakfast. Phentermine might induce sleeplessness if used at night.

Phentermine affects the brain’s neurotransmitters. It is thought to cause weight loss by suppressing appetite. The exact way phentermine works to cause weight loss is not known. Phentermine belongs to the class of drugs known as sympathomimetic amines. These are more commonly known as stimulant drugs.

Phentermine is a drug approved for weight loss as part of a regimen of exercise, calorie restriction and behaviour modification. Phentermine may be prescribed for short-term treatment in people who have a body mass index (BMI) of 30 kg/m2 or greater. It may also be prescribed to people with a BMI of 27 kg/m2 or greater with other risk factors, such as high blood pressure, diabetes or high cholesterol.

6 thoughts on “Phentermine: Usages, Dosing, Side Effects”
  1. Be ready for severe discomfort during the first weeks. I barely survived the side effects. Anyway, no regrets now. I’ve shed 30 pounds!

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