Phentermine: The Complete Patient’s Guide

by Vivian Okirie, MD on July 12, 2024
Published on July 12, 2024

Phentermine, a magic pill available only if the doctor says so and tagged as a Schedule IV drug, has this awesome trick of tricking your stomach into thinking it’s full, all while helping you shed those extra pounds.

It works as a psychostimulant drug that stimulates the central nervous system to release norepinephrine from the hypothalamus.

By doing this, the metabolism is revved up along with an increase in heart rate and blood pressure.

The overall goal is that phentermine supplements weight loss in an individual that is also exercising, dieting appropriately, and engaging in an overall behavioral modification in relation to their relationship with food.

Phentermine brand names and differences between them

Phentermine can be packaged as different brand names, all of which utilize the same main ingredient. Just like with brand names, there are some that are more frequently used based on popularity or name recognition by frequent consumers. Any differences in effectiveness (in regards to weight loss) vary based on brands. The following are popular phentermine brands and additional information about their formulation.

  • Adipex-P is available in both tablet and capsule form and was the byproduct of two pharmaceutical companies (Gate and Teva). The quality of the brand is considered to be of a high standard while being extremely effective at suppressing hunger and increasing energy. The typical dose is 37.5mg daily. This brand aims to allow its users to seamlessly add lifestyle modifications such as exercise for optimal results.
  • Suprenza a newly designed phentermine brand produced by Akrimax Pharmaceuticals that works by simply dissolving on the tongue. With its peppermint taste, it can easily be taken before, during, and after meals; this greatly contrasts typical appetite suppressants that require consumption 2 hours before meals. It comes in 3 doses: 15mg, 30mg, and 37.5mg, which can be finalized with your physician. This drug is no longer available in the United States.
  • Ionamin serves as a short-term appetite suppressant for the morbidly obese (body mass index of greater than 35%) that is produced by Medeva Pharmaceuticals. Instructions recommend taking it before breakfast or at least 10-14 hours before bedtime. It comes in 2 doses: 15mg and 30mg capsules. One contraindication is its use in users with uncontrolled blood pressure, because of its potential to worsen symptoms.

History of phentermine

Phentermine initially received approval for sale and distribution in 1959 as an appetite suppressant to supplement additional weight loss in patients. Originally, phentermine resin was utilized, and this was followed by phentermine hydrochloride in the 1970s. It has been approved by the US Food and Drug Administration (FDA) without any additional efficacy trials being performed and continues to remain on the market.

Previously, phentermine was marked as Fastin® but was later withdrawn by its producer SmithKline Beecham. It was also marketed as a combination drug (Fen-Phen) which contained fenfluramine, and would eventually be recalled for its suspected association with heart valve disease. Nevertheless, it continues to be widely referenced in numerous countries, including the US, for its weight loss properties.

Before taking phentermine

Phentermine does possess similar biochemical properties to amphetamine and can illicit false-positives on urinalysis testing. It is highly recommended that users who are allergic to phentermine or any of its components should not consume it. Contraindication of its use is recommended in those with the following conditions:

  •  Uncontrolled high blood pressure
  •  Pulmonary hypertension
  •  If you are pregnant or breastfeeding
  •  If you have a history of drug or alcohol abuse
  •  Glaucoma
  •  Hyperthyroidism
  •  Cardiovascular disease (arrhythmias, heart failure, coronary artery disease, stroke)
  •  Using other stimulant medication
  •  If you are using or are within 14 days of using MAO inhibitor drugs

If you later develop these medical conditions after beginning phentermine treatment, it is extremely important that you notify your primary care provider immediately to avoid any adverse reactions. Keep in mind that phentermine can become habit forming and should only be used as prescribed (dosage and time period). Phentermine is not indicated for long-term use and the smallest effective dose should be utilized.

How to take phentermine

Phentermine requires strict adherence to the instructions when consuming it. So please take it exactly as it is prescribed by your healthcare provider. It will usually be taken once a day by mouth on an empty stomach. Tablets are allowed to be taken whole or cut in half; however, do not crush or chew tablets. Capsules must be swallowed whole and no manipulation must be done – not broken, crushed, or chewed. Disintegrating Suprenza tablets should be handled with dry hands. Wait for the tablet to fully dissolve before swallowing or consuming with fluid. Avoid swallowing whole, chewing, or crushing Suprenza.

To avoid unintended insomnia, phentermine should be consumed early in the day; this is why most instructions advice medication being taken 1-2 hours before breakfast or 4-6 hours before bedtime. It is strongly advised that no adjustments of dosage be made without the prior consent of your physician in order to avoid potential overdose. Phentermine should only be prescribed and taken for a few weeks at a time with pausing of treatment in between since it can become habit forming. Due to the stimulant effects of phentermine, discuss with your primary care provider about your desire to stop phentermine treatment appropriately; otherwise, withdrawal symptoms might ensue.

How is it monitored?

Within 1 month of starting phentermine, users should be scheduled to return back to see their primary care provider for effectiveness. At the visit, the patient’s weight, waist circumference, and blood pressure will be measured. If the individual is found to have extremely high blood pressure, the phentermine will likely be stopped. These visits will be scheduled every month for the first 3 months. If the patient and their doctor agree to continue phentermine, the visits will spread out to 3-month intervals.

What happens if I missed a dose?

If a dose is missed and phentermine can be taken without causing insomnia, take the dose. If too much time has elapsed, wait to take it the next day. You should never double your dosage because of potential overdose. It is best to take your dose at the same time every day. So if you find yourself missing doses, consider setting a reminder on your phone.


If you think you could have overdosed, seek medical attention immediately.  Though unlikely, phentermine overdose can be life-threatening and even fatal. Symptoms of overdose can include nausea, vomiting, irregular heartbeat, hallucinations, chest pain, rapid or shallow breathing, confusion, seizure, and loss of consciousness.

What should I avoid?

The most common item to avoid with phentermine use is alcohol, which can worsen potential side effects. If combined, patients should avoid operating motorized vehicles and/or equipment. Another commonly used item is a monoamine oxidase inhibitor (MAOI) such as rasagiline, selegiline, isocarboxazid, phenelzine, or tranylcypromine. Lastly, avoid taking phentermine while consuming any products that may increase blood pressure or magnify the effects of phentermine.

Side effects

As with any medication, phentermine has numerous side effects, ranging from common to severe. Listed below are some of the side effects to be aware of before a patient considers starting phentermine.

  • Dizziness
  • Insomnia
  • Restlessness
  • Nausea
  • Vomiting
  • Constipation or diarrhea
  • Dry mouth or unpleasant taste
  • Impotence

Some rarer, more serious side effects include:

  • Heart or lung problems (hypertension, ischemia, palpitations, tachycardia, primary pulmonary hypertension)
  • Allergic reaction
  • Depression
  • Psychosis
  • High blood pressure
  • Rash
  • Tremor

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 consuming. Never combine any medicines that can interact negatively with phentermine (especially without notifying your primary care provider). Do not use phentermine with the following medications:

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

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 critical that your primary care provider is informed of any prior or current medical conditions that could affect your health or interfere with your use of phentermine. Please consult with your physician before starting phentermine if you suffer from any of the following:

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

Phentermine does have the ability to interfere and distort results of laboratory test such as scanning for Parkinson’s disease, leading to false or inaccurate test results. This is another reason why other care providers should be aware of your phentermine use before prescribing medications or performing medical tests.

Pregnancy and breastfeeding warnings

With phentermine being a Class C medication, it means that potential harm could come to a fetus if used by the mother during pregnancy. No research has shown that phentermine will result in birth defects, but that does not mean it should be used in pregnant women.  Therefore, it is paramount that consumers discuss their pregnancy or plans to become pregnant with their primary care provider before using phentermine. Other methods are available for weight loss in pregnancy. Recommendations insist that phentermine is avoided while breastfeeding in order to avoid drug transference from mother to infant, leading to possible harm.

Dosage information

The typical dose for obese adults is 15-37.5 mg daily by mouth at least 1-2 hours before or after breakfast. Some patients will be prescribed 2 doses a day, which should be taken before breakfast and no later than 4-6 hours before bedtime to avoid insomnia. Physicians might use their discretion to split doses up that would be taken 30 minutes before each meal. Doses should not be altered without the prior consent of your primary care provider, and instructions on the pharmacy label should be followed. Should any questions arise, please contact your healthcare provider.

Children and those over the age of 60

Anyone under the age of 16 should not be prescribed phentermine, because all the appetite suppressant studies have only been performed in adults. This means we have limited to no data on the effects of phentermine-like medications in children. Do not be surprised if most physicians will refuse to prescribe phentermine to anyone less than 18. Just as in children, limited research has been done to show its effects in the elderly; therefore, it is cautioned that people greater than 60 should proceed with extreme caution in regards to phentermine. The elderly are more likely to have deleterious side effects than younger adults.

Patient tips

  • Phentermine use is mainly for weight loss, but this is impacted by a patient’s current health conditions and tolerance.
  • It is intended to be used in conjunction with regular exercise and a caloric restrictive diet.
  • Avoid taking it less than 4-6 hours before bedtime.
  •  If phentermine makes you impaired, avoid operating vehicles and/or machinery.
  •  If fluid retention, shortness of breath, or sudden, unusual symptoms present, go see a medical professional immediately.
  •  Only take phentermine for the prescribed length of time.

Stopping phentermine

It is important to not stop the use of phentermine suddenly, especially if you have been consuming large doses or for a lengthy period of time. Stopping phentermine suddenly (without the guidance of a medical professional) can not only precipitate withdrawal symptoms, but it can be quite dangerous for your blood pressure. If you begin to notice signs of dependence such as needing to increase your dose more than recommended or a strong urge to continue treatment despite the prescription being over, report this to your physician. As these are signs that your body may soon enter withdrawals.

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. Usually, phentermine will only be prescribed for weeks (6-10) at a time.

Is phentermine an amphetamine?
No, it is not. However, phentermine is similar to amphetamine in its chemical structure and classifies 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.