Phentermine Contraindications and Precautions

The use of phentermine is contraindicated for any patient who has a prior history of sympathomimetic amine hypersensitivity or an idiosyncratic reaction to sympathomimetic amines.

Major Phentermine Contraindications

Cardiac disease, coronary artery disease, heart failure, angina, cardiac arrhythmias, stroke, valvular heart disease, hypertension, pulmonary hypertension.

For patients who have a history of cardiac disease, including heart failure, cardiac arrhythmias, stroke, coronary artery disease, or uncontrolled hypertension, phentermine is contraindicated for use, while those patients with controlled hypertension can receive phentermine. 1

This must be done with caution, while closely monitoring the patient’s blood pressure.

Women who have received a combination of phentermine and fenfluramine have been reported to have developed valvular heart disease.2

Combination therapy of phentermine and any other weight loss drug products has not yet been firmly established with regards to safety or efficacy.

This includes combinations with selective serotonin reuptake inhibitors such as sertraline, fluoxetine, paroxetine, and fluvoxamine. For this reason, administering any weight loss drug products with phentermine is not recommended. 3

In cases where patients have received phentermine alongside fenfluramine or dexfenfluramine, they have reported resulting in primary pulmonary hypertension (PPH). 4

It is not yet clear if this condition can be the result of phentermine being administered on its own, therefore this possibility cannot be ruled out. Typically, PPH’s initial symptom is dyspnea, with other potential symptoms such as syncope, angina perctoris, or lower extremity edema also possible.

The patient should always be advised to notify their provider of any deterioration in exercise tolerance. Any patients who develop new, unexplained symptoms of dyspnea, syncope, angina pectoris, or lower extremity edema, should be immediately advised to discontinue treatment. 5 6 7

Hyperthyroidism, Thyroid Disease

Being a sympathomimetic agent, phentermine use is contraindicated in patients who experience hyperthyroidism. Great caution should be exercised with any patients who have thyroid disease with this regard. 8

MAOI Therapy

Due to the risk of hypertensive crisis, phentermine use is contraindicated during, or within 14 days of MAOI therapy. 9

Anxiety, Mania, Psychosis, Insomnia, Schizophrenia

The use of phentermine is contraindicated for patients with any type of agitated state. Such a state is associated with, but not limited to, anxiety disorders, mania, schizophrenia, and psychosis.10

With patients who are using phentermine, monitor closely for mood changes, abnormal behavior, or insomnia. 11

Symptoms including irritability, insomnia, changes in personality and more severe psychotic symptoms could be indistinguishable clinically from psychotic disorders such as schizophrenia.12 13

Driving or Operating Machinery, Ethanol Ingestion

Phentermine use can result in dizziness, prevent signs from the need of rest, mask fatigue, or impair the patient’s ability to be fully mentally alert. 14

Patients should be advised of extra caution when driving, operating machinery, or performing any other tasks which require additional mental alertness until they have sufficiently adjusted to and are aware of how phentermine therapy will affect their mental and motor performance. 15

Additionally, the ingestion of ethanol may aggravate the above-stated effects. Patients should be advised to avoid consumption of alcohol while taking phentermine. 16

Diabetes Mellitus

The use of phentermine should be approached with caution in patients with diabetes mellitus. Due to weight loss and altered dietary regimens, insulin or other antidiabetic medication requirements may be altered for these patients. 17 18

Patients should practice the monitoring of their glucose levels and follow strict recommendations of the health care provider.

Anorexia Nervosa, Substance Abuse

Any type of appetite suppression therapy is not recommended in patients with a history of eating disorders such as anorexia nervosa. 19

Phentermine use is additionally contraindicated for those patients who have a history of substance abuse.

The reason for this is that the chemical makeup of phentermine is similar to that of amphetamine, which is a commonly abused substance, so the possibility of phentermine increases in such a patient.

For patients who part of this weight reduction treatment, the least amount reasonable of phentermine should be prescribed or dispensed at one time to limit the potential for drug diversion or overdose.

Abrupt Discontinuation

Withdrawal from phentermine is highly recommended to be gradual, as abrupt discontinuation of use after extensive use in high doses can lead to extreme fatigue or severe mental depressions. 20

If abrupt discontinuation is a medical necessity, the patient should be carefully monitored, and any symptoms should be readily managed. 21 22

Closed-angle Glaucoma

Phentermine use is contraindicated for patients who experience closed-angle glaucoma because there is an occurrence of sympathetic stimulation that can block aqueous overflow and raise intraocular pressure. 23

An ophthalmic evaluation may be required, so a patient should be advised to report any new visual disturbances.

Dialysis, Geriatric, Renal Disease, Renal Failure, Renal Impairment

Geriatric or debilitated patients could be more prone to CNS and sympathomimetic side effects when using phentermine, so caution should be exercised during use. 24

Phentermine using patients with renal impairments or renal disease are also more susceptible to side effects. While no dosage reduction is required, caution should be exercised when administering phentermine to patients with mild to moderate renal impairments. 25

Some manufacturers do recommend the reduction from the daily maximum for those patients with severe renal impairment, and to avoid phentermine entirely for those with renal failure, including those patients on dialysis.


Safe usage of phentermine during pregnancy has not been established, so phentermine products, as well as many anorexiants used for weight loss, are contraindicated for use during such time. 26

Women who are expecting to become pregnant should also avoid taking phentermine unless the physician weighs the benefits out to be greater than the risks. 27


Whether the metabolites from phentermine are excreted into breast milk is not known, so there are potential side effects on nursing infants. For this reason, the use of phentermine is contraindicated during breast-feeding. 28 29

Children, Infants

Phentermine is contraindicated in infants as well as children under 16 years of age, as there is no medical necessity for infants, and the safety and efficacy of use in children have not been established. 30 31


The use of phentermine should be discontinued within several days of surgery because the use of inhalational anesthetics during surgery sensitizes the myocardium of sympathomimetic drugs. This can have negative effects on blood pressure. 32 33 34 35

Related Pages:


  1. Cardiovascular effects of phentermine and topiramate: a new drug combination for the treatment of obesity
  2. Effects of phentermine and fenfluramine on extracellular dopamine and serotonin in rat nucleus accumbens: therapeutic implications.
  3. Fluoxetine increases the anorectic and long-term dopamine-depleting effects of phentermine.
  4. Pulmonary Hypertension Associated with Use of Phentermine
  5. Fatal Pulmonary Hypertension Associated with Short-Term Use of Fenfluramine and Phentermine
  6. Spontaneous Pneumomediastinum and “Diet Pills”: An Association?
  7. Changes in Cardiovascular Risk Associated With Phentermine and Topiramate Extended-Release in Participants With Comorbidities and a Body Mass Index ≥27 kg/m2
  8. Influence of thyroid hormones on the sensitivity of cardiac and smooth muscle to biogenic amines and other drugs.
  9. Does phentermine inhibit monoamine oxidase?
  10. Recurrent Psychosis after Phentermine Administration in a Young Female: A Case Report
  11. A Randomized, Double-Blind, Placebo-Controlled Study of an Oral, Extended-Release Formulation of Phentermine/Topiramate for the Treatment of Obstructive Sleep Apnea in Obese Adults
  12. Appetite Suppressor Induced Psychosis
  13. Schizophreniform-like psychotic disorder induced by phentermine: a case report.
  14. U.S National LIbrary of Medicine: Phentermine Side Effects
  15. A comparison of tyrosine against placebo, phentermine, caffeine, and D-amphetamine during sleep deprivation.
  16. Effects of phentermine and fenfluramine on alcohol consumption and alcohol withdrawal seizures in rats.
  17. Weight-loss therapy in type 2 diabetes: effects of phentermine and topiramate extended release.
  18. Weight-Loss Therapy in Type 2 Diabetes: Effects of Phentermine and Topiramate Extended Release
  19. The Connection between Anorexia and Substance Abuse.
  20. PB Institute: Guide to Phentermine Withdrawal
  21. A study of abrupt phentermine cessation in patients in a weight management program.
  22. Addiction potential of phentermine prescribed during long-term treatment of obesity.
  23. Bilateral angle closure following use of a weight loss combination agent containing topiramate.
  24. What weight loss treatment options do geriatric patients with overweight and obesity want to consider?
  25. Peripheral Vasculopathy and Nephropathy in Association with Phentermine
  26. Pregnancy outcomes after first trimester exposure to phentermine/fenfluramine.
  27. Phentermine Pregnancy and Breastfeeding Warnings
  28. Transfer of methylamphetamine and amphetamine into breast milk following recreational use of methylamphetamine
  29. MotherToBaby: Phentermine
  30. Effect of Phentermine on Weight Reduction in a Pediatric Weight Management Clinic
  31. Obesity in Children Medication
  32. Unusual Hypotension and Bradycardia in a Patient Receiving Fenfluramine, Phentermine, and Fluoxetime
  33. Phentermine and Anaesthesia
  34. Phentermine: A Systematic Review for Plastic and Reconstructive Surgeons.
  35. Anesthetic Considerations for the New Anti-Obesity Medications

Last updated: April 4th, 2020 by Sarah Gonzales. Bookmark the permalink.

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