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Side Effects of Adipex-P (phentermine): Interactions & Warnings

Does Adipex-P (phentermine) cause side effects?

Adipex-P (phentermine) is an appetite suppressant prescribed, combined with diet and exercise, for the short-term treatment of obesity in individuals with a body mass index greater than or equal to 30 kg/m2 or greater than or equal to 27 kg/m2 with risk factors for heart disease.

Adipex-P should not be combined with other weight loss agents. The combination of phentermine and fenfluramine (FEN-PHEN) is associated with serious heart problems. Fenfluramine is no longer available in the U.S.

Common side effects of Adipex-P include

Serious side effects of Adipex-P include

  • high pressures in the pulmonary arteries (pulmonary hypertension) and
  • defects in heart valves (especially when combined with fenfluramine).

Physical and psychological dependence may occur with the long-term use of Adipex-P. Withdrawal reaction may occur after prolonged uses, including

Drug interactions of Adipex-P include tricyclic antidepressants and monoamine oxidase inhibitors (MAOIs) may result in high blood pressure and other serious reactions because of elevated serotonin levels in the blood.

Adipex-P should not be used within 14 days after stopping a monoamine oxidase inhibitor.

People with diabetes being treated with Adipex-P may require a reduction in their dose of insulin or oral hypoglycemic drug.

Combining alcohol with Adipex-P may result in an adverse drug reaction.

Use of Adipex-P during pregnancy has not been adequately evaluated. It is unknown if Adipex-P is excreted in human breast milk. Since other amphetamines are excreted in breast milk, Adipex-P should not be used while breastfeeding.

What are the important side effects of Adipex-P (phentermine)?

Side effects of phentermine include:

Central nervous system
side effects include:

Phentermine, especially when combined with fenfluramine, has been
associated with high pressures in the pulmonary arteries (pulmonary hypertension) and defects in heart valves, both serious complications.

and psychological dependence may occur with the long-term use of phentermine. A
withdrawal reaction, which includes excessive drowsiness, fatigue, tremors and
depression may occur after prolonged use.

Adipex-P (phentermine) side effects list for healthcare professionals

The following adverse reactions are described, or described in greater detail, in other sections:

  • Primary pulmonary hypertension
  • Valvular heart disease
  • Effect on the ability to engage in potentially hazardous tasks
  • Withdrawal effects following prolonged high dosage administration

The following adverse reactions to phentermine have been identified:


Primary pulmonary hypertension and/or regurgitant cardiac valvular disease, palpitation, tachycardia,
elevation of blood pressure, ischemic events.

Central Nervous System

Overstimulation, restlessness, dizziness, insomnia, euphoria, dysphoria, tremor, headache, psychosis.


Dryness of the mouth, unpleasant taste, diarrhea, constipation, other gastrointestinal disturbances.




Impotence, changes in libido.

What drugs interact with Adipex-P (phentermine)?

Monoamine Oxidase Inhibitors

Use of Adipex-P is contraindicated during or within 14 days following the administration of
monoamine oxidase inhibitors because of the risk of hypertensive crisis.


Concomitant use of alcohol with Adipex-P may result in an adverse drug reaction.

Insulin And Oral Hypoglycemic Medications

Requirements may be altered.

Adrenergic Neuron Blocking Drugs

Adipex-P may decrease the hypotensive effect of adrenergic neuron blocking drugs.

Does Adipex-P (phentermine) cause addiction or withdrawal symptoms?

Drug Abuse And Dependence

Controlled Substance

Phentermine is a Schedule IV controlled substance.


Phentermine is related chemically and pharmacologically to the amphetamines. Amphetamines and other stimulant drugs have been extensively abused and the possibility of abuse of phentermine should be kept in mind when evaluating the desirability of including a drug as part of a weight reduction program.


Abuse of amphetamines and related drugs may be associated with intense psychological dependence and severe social dysfunction.

There are reports of patients who have increased the dosage of these drugs to many times that recommended. Abrupt cessation following prolonged high dosage administration results in extreme fatigue and mental depression; changes are also noted on the sleep EEG.

Manifestations of chronic intoxication with anorectic drugs include severe dermatoses, marked insomnia, irritability, hyperactivity and personality changes. A severe manifestation of chronic intoxication is psychosis, often clinically indistinguishable from schizophrenia.

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