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Dangers of Taking Phentermine

author image Karla Casco
Karla Casco began writing in 2010, focusing her work on diseases and treatments and their side effects. She has also created patient handouts, PowerPoint presentations and journal clubs. Karla has a doctorate in pharmacy from St. John's University and holds a pharmacy license for the state of New York.
Dangers of Taking Phentermine
Doctor taking patient's blood pressure. Photo Credit: Darrin Klimek/Photodisc/Getty Images

Phentermine is a prescription drug used for short-term weight reduction in obese patients. These patients must have a body mass index of over 30 and have other risk factors, such as diabetes or high blood pressure. The medication is used along with a health diet and regular exercise. Phentermine is effective in weight loss, but not all patients may be able to take the drug. The patient should discuss his entire medical history with a physician to avoid any dangerous affects from the medication.

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Abuse Potential

Phentermine has the potential for drug dependence and some patients may abuse the medication. The patient should use the medication exactly as directed by a health care professional and must never share medications with another person. The medication should not be discontinued suddenly in patients who have been on the drug for long periods of time; instead, the dose should be gradually decreased over time. According to “Pharmacotherapy: A Pathophysiologic Approach,” phentermine is contraindicated in patients with a history of cocaine, phencyclidine and methamphetamine use.

High Blood Pressure

Phentermine may cause an increase in the blood pressure and use is not recommended in patients with mild high blood pressure. It is contraindicated in patients with moderate and severe high blood pressure. The patient may experience a dangerous increase in blood pressure when phentermine is taken with a monoamine oxidase, or MAO, inhibitor, such as isocarboxazid, phenelzine, selegiline, rasagiline, furazolidone, or tranylcypromine. explains that the patient should not take phentermine if he has taken an MAO inhibitor in the last 14 days to avoid this effect. Other diet medications, such dexfenfluramine, should also be avoided when using phentermine to avoid a rare but fatal lung disorder called primary pulmonary hypertension, PPH. PPH is an increase in the pressure in the arteries that lead to the lungs. The patient should speak with a health care professional before using any other medication to avoid complications.

Heart Function

Phentermine may cause palpitations and irregular heartbeats. The medication is contraindicated in patients with heart disease, such as irregular heartbeats and congestive heart failure. A physician can determine if phentermine is safe for the patient.

Psychiatric Disorders

Patients with psychiatric disorders, such as schizophrenia and bipolar disease, may experience a worsening of symptoms with the use of phentermine. The “Drug Information Handbook” states that the medication should be used in caution in these individuals.


Phentermine may cause an excessive dilation of the pupils. “Pharmacotherapy” explains that the dilation can worsen glaucoma; therefore, patients diagnosed with glaucoma should not receive phentermine.

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