Drug information of phentermine


Drug group:

Phentermine is a stimulant similar to an amphetamine. It acts as an appetite suppressant by affecting the central nervous system.

Phentermine is used together with diet and exercise to treat obesity (overweight) in people with risk factors such as high blood pressure, high cholesterol, or diabetes.

Phentermine may also be used for purposes not listed in this medication guide.

Mechanism of effect

Phentermine is an amphetamine that stimulates neurons to release or maintain high levels of a particular group of neurotransmitters known as catecholamines; these include dopamine and norepinephrine. High levels of these catecholamines tend to suppress hunger signals and appetite.

The drug seems to inhibit reuptake of noradrenaline, dopamine, and seratonin through inhibition or reversal of the reuptake transporters. It may also inhibit MAO enzymes leaving more neurotransmitter available at the synapse. Phentermine (through catecholamine elevation) may also indirectly affect leptin levels in the brain.

It is theorized that phentermine can raise levels of leptin which signal satiety. It is also theorized that increased levels of the catecholamines are partially responsible for halting another chemical messenger known as neuropeptide Y. This peptide initiates eating, decreases energy expenditure, and increases fat storage.


  • Absorption: Phentermine is rapidly absorbed after oral ingestion.
  • Volume of distribution: Not Available
  • Protein binding: Approximately 96.3%
  • Metabolism: Hepatic.
  • Route of elimination: Not Available
  • Half life: 16 to 31 hours
  • Clearance: Not Available

Drug indications

Obesity management




Indicated as an adjunct to a reduced-calorie diet and exercise for chronic weight management in adults with an initial BMI ≥30 kg/m² (obese), or BMI ≥27 kg/m² (overweight) in the presence of at least 1 weight related comorbidity (eg, hypertension, type 2 diabetes mellitus, dyslipidemia)

8 mg orally 3 times a day 30 minutes before meals, OR
15 to 37.5 mg orally once a day before breakfast or 1 to 2 hours after breakfast.


Safety and efficacy not established

Drug contraindications

pregnancy , Hyperthyroidism , glaucoma



May cause fetal harm (see Contraindications; see Pregnancy & Lactation)

May increase resting heart rate up to 20 bpm; caution in patients with history of cardiac or cerebrovascular disease

Antiepileptic drugs (AEDs), including topiramate, increase the risk of suicidal thoughts or behavior

Acute myopia associated with secondary angle closure glaucoma has been reported with topiramate; monitor for increased IOP due to risk of permanent loss of vision

May cause mood disorders, including depression, and anxiety, as well as insomnia; patient with history of depression may be at increased risk of recurrent depression or other mood disorders

Cognitive dysfunction (eg, impairment of concentration/attention, difficulty with memory, and speech or language problems, particularly word-finding difficulties) reported; rapid titration or high initial doses may be associated with higher rates

Hyperchloremic, nonanion gap, and metabolic acidosis reported

Increased serum creatinine reported; peak increases were observed after 4-8 weeks of treatment and gradually declined but remained elevated over baseline values

Weight loss may increase the risk of hypoglycemia in patients with type 2 diabetes mellitus treated with insulin and/or insulin secretagogues; monitor for needed diabetic therapy adjustments

In patients treated for hypertension, weight loss may increase the risk of hypotension; monitor for needed antihypertensive dose adjustments

Coadministration with alcohol or CNS depressants drugs with phentermine or topiramate may potentiate CNS depression or other centrally mediated effects of these agents (eg, dizziness, cognitive adverse reactions, drowsiness, light-headedness, impaired coordination, somnolence)

Abrupt withdrawal of topiramate associated with seizures in individuals without a history of seizures or epilepsy; gradually discontinue if taking 15 mg/92 mg

Caution with renal or hepatic impairment and adjust dose

Kidney stone formation reported; topiramate inhibits carbonic anhydrase activity and promotes kidney stone formation by reducing urinary citrate excretion and increasing urine pH; avoid coadministration with other carbonic anhydrase inhibitors; use in patients on a ketogenic diet may also increase this risk; increase fluid intake to decrease risk

Oligohidrosis (resulting in hyperthermia) reported with topiramate

Can cause changes in laboratory values; can increase the risk of hypokalemia through its inhibition of carbonic anhydrase activity; caution with coadministration of drug that cause hypokalemia (eg, furosemide, hydrochlorothiazide)

Obtain blood chemistry profile that includes bicarbonate, creatinine, potassium, and glucose at baseline and periodically during treatment

Use with combination oral contraceptives may increase frequency of irregular bleeding/spotting; need not discontinue unless spotting is trouble to patient

Consider dose reduction in patients with hepatic impairment; use with caution

Points of recommendation

  • Administer before breakfast or one-to-two hours after breakfast. Avoid late evening administration as this may cause insomnia. Some people may benefit from a twice-daily dosage regimen.
  • Dosage must be tailored to different individuals. Take exactly as directed and for the duration intended. If you find yourself gaining weight while taking phentermine see your doctor.
  • Phentermine should be used alongside dietary changes, exercise, and behavioral modification as discussed with your doctor.
  • Do not drive or operate machinery if this medicine impairs your judgment.
  • Do not drink alcohol while taking phentermine as it may increase the risk of side effects and further impair your judgment.
  • Should not be used at the same time as other medications used for weight loss.
  • Recommended only for people with an initial BMI of equal or more than 30 kg/m2, or 27 kg/m2 with risk factors for a cardiovascular event (such as controlled high blood pressure, diabetes, or high cholesterol). Do not share your phentermine with anybody else (this is also illegal).
  • Report any new shortness of breath, angina pain, fluid retention in legs or feet, or a decrease in your ability to exercise (not attributable to any other cause) to your doctor immediately.
  • Phentermine may be sought out by drug users and one accidental dose may be fatal to children or pets. Keep supplies out of reach and sight of others.

Pregnancy level


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