Phentermine (Oral)

Phentermine is a drug that is used, together with a low calorie diet to achieve a reduction in weight for obese patients.


In the US, phentermine is known under the following brand names:

  • Pro-Fast
  • Phentride
  • Phentercot
  • Ionamin
  • Fastin
  • Atti-Plex P
  • Adipex-P
  • Adipex

The drug is generally used in conjunction with a calorie controlled diet to bring about weight loss in obese patients. Phentermine is used in patients where weight loss through exercise and diet alone has not taken place.

Phentermine is one of a group of medicines called appetite suppressants. The drug works by causing the patient's appetite to become greatly reduced. If the patient feels less hungry, they will eat less and hopefully lose weight. The medicine will not work in isolation as a weight reduction aid. It must be used as part of a holistic therapy program that will be devised for you by your GP.

The medication is only available from your GP on prescription, and it comes in a number of forms, including:

  • Extended-release capsules
  • Disintegrating tablets
  • Tablets
  • Capsules

Conditions treated

Type of medicine

  • Appetite suppressant
  • Tablet (including disintegrating)
  • Capsule (including extended release)

Side effects

In addition to the effects that it is intended to have, some medicines can cause unwanted side effects. You may not notice all or any of these effects, but if you do, you should consult your doctor, as further medical treatment may be required.

If you notice any of the following effects while taking phentermine, you should check with your GP:

  • Weakness
  • Problems with thinking, walking, or speaking
  • Trouble breathing
  • Trembling or shaking of the hands, arms, legs, or feet
  • Swelling of the lower legs or feet
  • Severe mental changes
  • Hallucinations
  • Tingling or numbness in the arms or legs
  • Headache
  • Rapid, irregular, pounding, or racing pulse or heartbeat
  • Fainting
  • Dizziness
  • Decreased ability to exercise
  • Chest pain

There are a few effects that are caused by phentermine that do not generally need more medical attention. These effects usually disappear, once your body gets used to the new drug. Your doctor may also be able to give you some advice on how to reduce or prevent a few of these effects. If any of the unwanted effects become especially bothersome or do not go away within a week or so of starting to use the drug, check with your GP:

  • Unpleasant taste or aftertaste
  • Reddening of the skin
  • Loss in sexual ability, drive, desire, or performance
  • Poor libido
  • Increase in sexual ability, drive, desire, or performance
  • Inability to have or maintain an erection
  • Skin rash, hives, welts, or itching
  • Unusual or false sense of well-being
  • Dry mouth
  • Constipation

Some patients report side effects that are not mentioned in this guide. If you notice any other effects, you should discuss them with your doctor.


This medication should only be taken as prescribed. Do not increase the dose, reduce the dose, or take it more frequently or for a longer period than you have been told to by your GP. Phentermine can be addictive, leading to mental or physical dependence.

Phentermine comes in four different forms: tablets, capsules, extended-release capsules, and disintegrating tablets. Be sure to follow the instructions for the form you have been given to use.

You should swallow the extended-release capsule in one piece. Do not break them open, crush them, or chew them.

You may take the disintegrating tablet with a meal or just with water if you prefer. Dry your hands thoroughly before you touch the tablet. Once you have removed the tablet from the bottle, put in on your tongue immediately and allow it to dissolve. Once the tablet has melted completely, take a sip or swallow of water.

You should swallow the tablet in one piece. Do not crush, suck, or chew it.

This medicine is for use as part of a holistic weight loss regimen, including exercise and a diet plan. Your doctor will go through the whole program with you so that you understand how it works.

This drug can interfere with sleep patterns. Try to take your last dose four to six hours before you go to bed, unless you are instructed to do otherwise by your GP.

The dose of phentermine that you are told to take will vary between patients. Take the drug in accordance with your doctor's directions or follow the instructions on the product label. The information shown herein is only based on the average. If your dose is at variance to this, do not change it unless you are told to by your GP.

The amount of your dose will depend on the potency of the product you are given. The number of daily doses, the time you leave between them, and the duration of time that you use the drug for will all depend on your medical condition and on how well your body reacts to the drug.


  • Adults and children aged over 17: Take 15 mg to 30 mg once daily, roughly two hours following your breakfast.

The use of phentermine in children under 16 years of age is not recommended.

Disintegrating tablets

  • Adults and children aged over 17: Take one tablet daily, preferably in the morning.

The use of phentermine in children under 16 years of age is not recommended.

Extended-release capsules

  • Adults and children aged over 16: Take one capsule daily, before breakfast or roughly 10 to 14 hours before bedtime.

The use of phentermine in children under 15 years of age is not recommended.

LomairaTM tablets

  • Adults and children aged over 17: Take one tablet thrice daily, approximately half an hour before meals.

The use of phentermine in children under 16 years of age is not recommended.

Phentermine tablets

  • Adults and children aged over 16: Take 18.75 mg to 37.5 mg before breakfast or one to two hours following breakfast.

The use of phentermine in children under 16 years of age is not recommended.

Do not share your phentermine tablets with anyone else.

If you forget to take a dose of your medicine, you should try to take it right away. If it is nearly time for you next scheduled dose, leave out the one you missed and go back to your usual dosing regimen. Do not take twice the amount.


There are some drugs that must never be used in combination, as doing so could cause a serious or unpleasant interaction between them. However, there may be occasions when two drugs could be used together safely, even if there is an interaction. In cases like this, your GP may decide to change the dose of one of your medications or suggest precautions that you could take.

Before you start taking phentermine, be sure to tell your GP or specialist if you are using any other medicines, especially those mentioned below, all of which may interact with phentermine.

It is not recommended that you take phentermine while you are using any of the following drugs. Your GP may change your medication or might decide against treating you with phentermine:

  • Tranylcypromine
  • Sibutramine
  • Selegiline
  • Safinamide
  • Rasagiline
  • Procarbazine
  • Phenelzine
  • Nialamide
  • Moclobemide
  • Methylene Blue
  • Linezolid
  • Isocarboxazid
  • Iproniazid
  • Furazolidone

Phentermine should not be taken with Fenfluramine, although it may be required in your case. If you are told to take both drugs together, your GP might change the dose or frequency of one or both of them.

Some medication should not be used when you are eating or eating some types of food. In addition, some drugs react badly with tobacco or alcohol. For this reasons, you should discuss these aspects of your treatment program with your GP, before you begin using phentermine.

Medical interactions

Pre-existing medical problems can affect how this medicine works. You must tell your doctor if you have any other health conditions.

You must not use phentermine if you have a history of any of the following medical conditions:

Phentermine can cause an allergic reaction in patients who have allergies to tartrazine (SuprenzaTM).

Phentermine should be used with caution by patients who have a history of any of the following conditions, as doing so could make these conditions even worse:

  • Pulmonary hypertension (elevated pressure in the lungs)
  • Hypertension (high blood pressure)
  • Heart valve disease

Patients with kidney disease should be closely monitored when taking phentermine. The effects of the drug may be increased, due to the slower removal of the drug from the patient's system.


Before you decide to use a particular drug, you should discuss the risk and benefits of doing so with your GP.

You must tell your GP if you have any allergic reactions to phentermine or to any other medicine, including non-prescription products. In addition, be sure to mention if you have any allergies to food colors, preservatives, or animal by-products. Tell your GP if you regularly use herbal products or vitamin supplements.


It is not recommended to use phentermine in children aged under 16 years of age. It is not clear if the medicine would be safe or effective to use.


There have been no studies carried out to date to show that there this drug can cause geriatric-specific problems. However, old people are more likely to have age-related health conditions such as heart, liver, or kidney problems. Phentermine should therefore be used with caution in these patients and the dose levels may have to be adjusted.

Pregnancy and breastfeeding

Phentermine causes birth defects and should therefore not be used to treat obese pregnant women. If you are pregnant or think you may be likely to become so during your treatment, you must tell your doctor, before you begin taking this drug. Discuss with your GP what methods of contraception you could use to ensure that you do not get pregnant.

Phentermine has been shown to pass into breast milk and can cause harm to nursing infants. Lactating women should ask their GP or midwife for advice on alternative feeding methods for their infant while they are using this drug. Do not express milk for later use.

Medical complications

You must attend your medical clinic regularly for check-ups while you are using this medication. These visits will be used to ensure that the drug is working correctly and to check your weight loss progress. You can also mention any unwanted side effects that you have noticed while using the drug.

If you are using other diet drugs, you should not use phentermine. Such products include:

  • Phendimetrazine
  • Mazindol
  • Diethylpropion
  • Didrex
  • Bontril
  • Benzphetamine

You must also not use phentermine if you are already using MAO inhibitor medication or have used MAOI in the previous two weeks, including:

  • Tranylcypromine (Parnate)
  • Selegiline (Eldepryl)
  • Phenelzine (Nardil)
  • Isocarboxazid (Marplan)

Phentermine can be addictive. If you think that the medication is not fully effective after a few weeks of taking it, do not be tempted to take more. Check with your doctor.

If you notice any of the symptoms listed below, you should check with your GP, as these could be indicative of serious lung or heart problems:

  • Decreased ability to exercise
  • Pains in the chest
  • Swelling of the lower legs or feet
  • Breathing difficulties

Some people notice that they feel less alert than they would do normally while taking this medication. Others report feeling lightheaded or dizzy. If you notice that you are reacting in this way to phentermine, do not drive, use machinery or undertake any other potentially hazardous activity.

Phentermine can affect your blood sugar levels. If you are diabetic, you may therefore note a change in your urine or blood sugar test results. If you are concerned about this, check with your GP.

You should not drink alcohol while you are taking phentermine.


You should keep your supply of phentermine in a sealed container at room temperature, not near a heat source, direct sunlight or where it could get wet. Do not freeze or refrigerate the drug.

Keep the medication well away from children and pets. If a pet does eat any of your phentermine capsules or tablets, you should seek veterinary advice promptly.

Do not retain any medicine that has gone past its use-by date. Do not take any phentermine capsules or tablets that appear damaged or if the packaging has been interfered with.

Do not throw any unused tablets or capsules into your trash can, or down the drain. Do not flush medication down the toilet. Ask your pharmacist or doctor to dispose of unwanted medicines for you.


Phentermine is a drug that is commonly given to obese patients who have failed to control their weight through exercise and diet alone. The drug is one of a family of diet aids called appetite suppressants. It works by reducing the patient's appetite so that they do not want to eat, making it easier for weight loss to occur. It is important for patients to note that this drug will not simply make them lose weight. It must be used to help control the weight as part of a carefully devised treatment program.

During the course of your treatment, you must attend your GP for regular update appointments. These appointments will allow your GP to check that your therapy program is working effectively and that the drug is not causing any dangerous side effects.

Phentermine can cause some side effects in patients who have certain existing medical conditions and if used together with some drugs. Be sure to discuss your full medical history with your GP, before you begin using this medicine.