Phentermine is an appetite suppressant of the amphetamine and phenethylamine class.

It is approved as an appetite suppressant to help reduce weight in obese patients when used short-term and combined with exercise, diet, and behavioral modification. It is typically prescribed for individuals who are at increased medical risk because of their weight and works by helping to release certain chemicals in the brain that control appetite.

Commercial trade names

• Adipex P (Immediate release)
• Anoxine-AM®
• Fastin®
• Ionamin® (Slow Release Resin, Australia)
• Duromine® (Slow Realease Resin, New Zealand)
• Obephen®
• Obermine®
• Obestin-30®
• Phentrol®
• Pro-Fast SA
• Redusa
• Panbesy
• Phentermine Trenker
• Obenix
• Oby-Trim

Mechanism of action

Phentermine, like many other prescription drugs, works in the brain. It stimulates nerve bundles to release a particular group of neurotransmitters known as catecholamines. The anorectic activity seen with these compounds would thus seem likely due to this effect on the central nervous system.  As a result, it causes a loss in appetite because the brain does not receive the hunger message.

Contraindications and warnings

• Patients with the following should not use Phentermine:
  • A allergy to any ingredient in Phentermine or other sympathomimetics 
  • Are also takingdexfenfluramine, fenfluramine, furazolidone, quanadrel, guanethidine, or have taken a monamine oxidase inhibitor (MAOI) (eg, phenelzine) in the last 14 days
  • Have moderate to severe high blood pressure, an overactive thyroid, glaucoma, heart or blood vessel disease, or severe narrowing of the blood vessels
  • Are in an agitated state, or have a history ofsubstance abuse. 
• Some medical conditions may interact with Phentermine, patients with the following should consult with their doctor before using phentermine:
  • Are pregnant, planning to become pregnant, or are breast feeding
  • Are taking any prescription or nonprescription medicine, herbal preparation, or dietary supplement
  • Have allergies to medicines, foods, or other substances
  • Have a brain or spinal cord disorder, hardening of the arteries, high blood pressure, diabetes, or hig cholesterol or lipid levels
• Some medicines may interact with Phentermine, such as the following:
  • Dexfenfluramine, fenfluramine, furazolidone, or MAOIs (eg, phenelzine) because the risk of serious side effects, such as increasing headache, high blood pressure, slow heart rate, elevated temperature, or possibly fatal lung problems, may be increased
  • Serotonin specific reuptake inhibitors (eg, fluoxetine) because the risk of their side effects may be increased by Phentermine
  • Guanadrel or guanethidine because their effectiveness may be decreased by Phentermine

Side effects

Generally, phentermine appears to be relatively well tolerated. It can produce side effects consistent with its catecholamine-releasing properties, e.g., tachycardia (increased heart rate) and elevated blood pressure, but the incidence and magnitude of these appear to be less than with the amphetamines. Because phentermine acts throughsympathomimetic pathways, the drug may increaseblood pressure and heart rate. It may also cause palpitations, restlessness, and insomnia. Additionally, individuals taking this drug on a long-term basis may develop euphoria and a psychological addiction to it.

More Common Symptoms

• Insomnia
• Increased blood pressure
• Irritability
• Nervousness
• Sense of well-being

Less Common to Rare Symptoms

• Blurred vision
• Change in sexual desire
• Clumsiness
• Confusion
• Diarrhea
• Dizziness
• Dry mouth
• Headache
• Irregular heartbeat
• Nausea or vomiting
• Psychosis
• Skin rash or itching
• Stomach pain
• Tiredness
• Unpleasant taste

Possible Overdose Symptoms

• Confusion
• Convulsions (seizures)
• Dizziness
• Fast Breathing
• Fever
• Hallucinations
• Hostility with urge to attack
• Irregular blood pressure
• Irregular heartbeat
• Lightheadedness or Fainting
• Mental Depression, following a period of excitement
• Tremors, Trembling, or Shaking
• Overactive Reflexes
• Panic
• Restlessness
• Severe nausea, vomiting or diarrhea
• Stomach cramps
• Tiredness or Weakness