Phentermine: The Complete Patient’s Guide

Phentermine is a schedule IV prescription drug commonly used for weight loss, since it is an appetite suppressant – which, as a psychostimulant drug, belongs to the sympathomimetic drug family. It works by stimulating the central nervous system, thereby increasing one’s heart rate and blood pressure. However, phentermine should be used in conjunction with exercise, a healthy diet, and behavioural therapy. Although it is not exactly known how it helps for weight loss, phentermine can also be used for various other reasons.

Phentermine brands and differences

There are several brands of phentermine available, each with their own differences to the other brands, however there are some that are more commonly used due to their popularity among frequent users and their well-established brand names. Furthermore, the effectiveness of some might be greater than others or certain generics. The following are examples of popular phentermine brands:

  • Adipex-P is available in capsule and tablet form, being the product of two pharmaceutical companies. The companies Gate and Teva are the leaders of generic drug production and marketing. The quality of this brand is of a high standard, being an effective appetite suppressant and energy booster, thus allowing you to incorporate more activity in your lifestyle for optimal weight loss.
  • Suprenza is a relatively new phentermine brand produced by Akrimax pharmaceuticals. It is not necessary to take this pill with water, since it dissolves on the tongue. It tastes like peppermint and can be taken before, during, and after meals, unlike other diet pills that require that the users take them 2 hours before a meal. However, it is not recommended for use by children younger than or equal to the age of 16.
  • Ionamin, a short-term appetite suppressant for the excessively obese, is available in 15 and 30 mg capsules. It is produced by Medeva pharmaceuticals. Ionamin should be taken before breakfast or 10 to 14 hours before going to bed. It should not be used by those with uncontrolled high blood pressure.

History of phentermine

Phentermine initially received approval for sale and distribution in 1959 as an appetite suppressant to aid in weight loss. Phentermine resin was the first to become available to the United States in 1959, followed by phentermine hydrochloride in the 1970s. Since phentermine has been on the market for decades, and has been approved by the US Food and Drug Administration (FDA), there have not been any new efficacy trials conducted.

Previously, however, phentermine was marketed under the name Fastin®, which was withdrawn from the market by the producing company SmithKline Beecham. It is also sold as a combination drug, such as fenflaurmine – which was withdrawn from the market due to it being suspected to cause heart valve disease. Phentermine is still widely used in many countries, including the US, as a common prescription weight loss drug.

Before taking phentermine

Phentermine is similar to an amphetamine, and therefore can produce a false-positive in a urinalysis test. Furthermore, you should not use phentermine if you are allergic to it or any substances with which it is combined. Phentermine should not be taken if you have any of the following conditions:

  •  Uncontrolled high blood pressure
  •  If you are pregnant or breastfeeding
  •  If you have a history of drug or alcohol abuse
  •  Glaucoma
  •  Hyperthyroidism
  •  Cardiovascular disease
  •  Using other medications

Should you experience any of the aforementioned conditions, it is important to inform your doctor and/or seek advice from a medical professional. It is also important to note that phentermine can be habit forming, thus it should only be used as prescribed.

How to take phentermine

Phentermine should be taken exactly how it is prescribed by your doctor, usually once a day, by mouth and on an empty stomach. Tablets should be swallowed whole or cut in half. Do not crush or chew the tablets. Similarly, phentermine capsules must be swallowed whole – not cut, broken or crushed. Orally disintegrating Suprenza tablets should be removed, using dry hands, from the container and placed onto the tongue. Wait until the tablet dissolves on the tongue before swallowing or drinking with water. Do not swallow whole or chew the dissolving tablet.

Avoid taking a late dose of phentermine, since it might cause insomnia. Therefore, it is advisable to take your daily dose an hour before or after breakfast, or 4-6 hours before going to bed. Follow the dosage and instructions on the label of the package. Do not change the dosage unless directed to do so by your doctor. Avoid overdose or taking phentermine with other appetite suppressants. Phentermine should only be taken a few weeks at a time, otherwise it may become habit-forming. Do not suddenly stop use of phentermine without first asking your doctor, since this may cause withdrawal symptoms.

What happens if I missed a dose?

If a dose is missed, it should be taken as soon as possible. If it is time for the next dose, after having missed the previous dose, it is better to skip the missed dose. Never double your dosage. However, it is best to prevent missing doses by taking phentermine the same time every day, and setting a reminder for it.


In the event of overdose, seek medical attention immediately. Although unlikely, phentermine overdose can be fatal. Common symptoms of overdose include: irregular heartbeat, hallucinations, chest pain, rapid breathing, confusion, seizures, loss of consciousness, nausea, vomiting.

What should I avoid?

The use of alcohol with phentermine should be avoided, as it may increase potential side effects. Phentermine and alcohol both impair your reactions, therefore you should avoid driving, and be alert when performing an activity that might cause harm. Do not use monoamine oxidase inhibitors (MAOIs), such as phenelzine, methylene blue, linezolid, and procarbazine, while taking phentermine. One should also avoid using products that may increase blood pressure or that can increase the side effects of phentermine.

Side effects

Taking this medicine may cause numerous side effects, ranging from common to more severe. The following are some of the common symptoms you can experience when taking phentermine:

  • Dizziness
  • Insomnia
  • Nausea
  • Impotence
  • Dry mouth
  • Vomiting
  • Constipation or diarrhea

Some rarer, more serious side effects include:

  • Heart or lung problems
  • Allergic reaction
  • Depression
  • Psychosis
  • High blood pressure

Drug interactions

Phentermine can interact with numerous drugs, substances, and diseases. Hence, it is important to inform your doctor about other drugs and substances you are using. Never use any medicines that can interact negatively with phentermine. Do not use phentermine with the following medications:

  • Asthma medications
  •  Aspirin
  •  Cocaine
  •  Nabilone
  •  Caffeine
  •  Amphetamines
  •  Cold medications
  •  Amantadine
  •  Other diet pills or Selective Serotonin Reuptake Inhibitors (SSRIs), including paroxetine, citalopram, and sertraline
  •  Pemoline
  •  Monoamine Oxidase Inhibitors, including furazolidone, selegiline, and tranylcypromine
  •  Tricyclic antidepressants, such as amoxapine, doxepin, imipramine, clomipramine and desipramine
  •  Ritalin
  •  Codeine

The above is not a full list of medications that interact with phentermine. For more information, see the label provided with your phentermine product, or seek advice from a medical professional.

It is also vital that your doctor is aware of any diseases or illnesses that might be affecting your health, which can result in certain interactions when using phentermine. Please notify your doctor if you suffer, or suffered, from any of the following:

  •  Alcohol abuse
  •  Atherosclerosis
  •  Allergies
  •  Glaucoma
  •  Kidney disease
  •  Diabetes mellitus
  •  Mental illness
  •  Hyperthyroidism
  •  Drug abuse

It is possible for phentermine to interfere with laboratory tests, such as scans for Parkinson’s disease, which may produce false or inaccurate test results. It is important that your doctor and pharmacist are aware that you are using phentermine before prescribing other medication or performing medical tests.

Pregnancy and breastfeeding warnings

There is currently no information on the effects of phentermine on pregnancy in humans, thus it is not known if using phentermine may result in birth defects. Therefore, it is paramount to talk to your doctor about your pregnancy or if you are planning on becoming pregnant while using phentermine. It is not recommended to use phentermine while breastfeeding, since it is possible for the drug to diffuse into the milk of lactating mothers, resulting in potential harm to the nursing babies.

Dosage information

The adult dose for obesity is 15-37.5 mg orally once per day at least 1-2 hours before or after breakfast. If more than one daily dose is prescribed, take the last dose 4 to 6 hours before going to bed. For tablets, your doctor may also instruct you to take divided doses 30 minutes before meals. Phentermine doses should be indicated by your doctor’s prescription or pharmacy label and should not be adjusted without prior advice from a medical professional.

Children and those over the age of 60

Phentermine should not be used by children under the age of 16. Appetite suppressant studies have only been done on adults, therefore there remains limited information on the effects of phentermine for use by children. Similarly, there is no specific information on the effectiveness of phentermine on the elderly. Persons over the age of 60 should take caution when using phentermine, as side effects may be more severe or different entirely than for a younger adult.

Patient tips

  •         The use of phentermine is mainly for weight loss. However, use is limited by an individual’s state of health and tolerance.
  •         It should be used in addition to exercise and a calorie restriction diet.
  •         Avoid late evening administration of doses.
  •         Do not drive or operate machinery if phentermine impairs your judgement.
  •         Report shortness of breath, fluid retention in legs or feet, or more severe, unusual symptoms of use to a medical practitioner immediately.

Stopping phentermine

Do not suddenly stop using phentermine without first informing your doctor. If you have been using phentermine in large doses or for an extended period of time, it may be more dangerous to one’s health to stop phentermine without gradually decreasing doses under a medical professional’s instructions. If you have developed a dependence on phentermine, with signs such as the need to increase your daily dose or a strong desire to continue using phentermine, you should speak to your doctor as soon as possible. Withdrawal symptoms after dependence may be apparent, and your body will require time to adjust in absence of the medicine.

Frequently Asked Questions (FAQs)

What are the active ingredients in phentermine pills?
The active ingredient in phentermine pills is phentermine hydrochloride.

Is phentermine safe?
Phentermine is safe as long as it is followed as prescribed by your doctor. It is approved by the FDA and is a common weight loss drug.

How long can I take phentermine?
Phentermine is not recommended for long-term use. A doctor will prescribe phentermine for each individual’s specific needs. Do not exceed this duration without talking to your doctor.

Is phentermine an amphetamine?
No, it is not. However, phentermine is similar to amphetamine in its chemical structure and classification as a controlled substance.

Who can take phentermine?
Phentermine can only be taken by those who have seen a doctor and have received a prescription for the medication.