What are the important side effects to screen for when prescribing phentermine (phentermine hydrochloride) for weight loss?

Important Side Effects to Screen for with Phentermine

When prescribing phentermine for weight loss, patients should be closely monitored for cardiovascular effects (elevated blood pressure, palpitations, tachycardia), central nervous system effects (insomnia, anxiety, restlessness), and potential drug interactions, as these represent the most significant safety concerns. 1, 2

Cardiovascular Side Effects

• Blood pressure and heart rate: Regular monitoring is essential as phentermine can cause:

  • Elevated blood pressure
  • Tachycardia
  • Palpitations
  • Potential ischemic events 1, 2

• Pulmonary hypertension: Though rare, patients should be evaluated for:

  • New or unexplained dyspnea
  • Decreased exercise tolerance
  • Syncope
  • Lower extremity edema 2

• Valvular heart disease: Monitor for signs of regurgitant cardiac valvular disease, particularly affecting mitral, aortic, and tricuspid valves 2

Central Nervous System Effects

• Stimulant effects:

  • Insomnia (avoid late evening administration)
  • Restlessness and anxiety
  • Dizziness
  • Headache
  • Overstimulation 1, 2

• Mood alterations:

  • Euphoria or dysphoria
  • Potential psychosis in susceptible individuals
  • Tremor 2

Gastrointestinal Side Effects

• Dry mouth (most common)
• Unpleasant taste
• Constipation
• Diarrhea
• Other gastrointestinal disturbances 1, 2

Other Important Side Effects

• Allergic reactions: Monitor for urticaria 2
• Endocrine effects: Impotence and changes in libido 1, 2
• Tolerance development: Effectiveness typically diminishes within a few weeks; do not exceed recommended dosage if this occurs 2

Drug Interactions to Screen For

• Severe interactions (contraindicated):

  • MAOIs (risk of hypertensive crisis) - allow ≥14 days between discontinuation and phentermine initiation 1, 2
  • Tramadol (most common severe interaction reported) 3

• Moderate interactions:

  • Dextromethorphan (commonly co-prescribed) 3
  • Insulin and oral hypoglycemics (may require dose adjustment) 2
  • Adrenergic neuron blocking drugs (decreased hypotensive effect) 2
  • Alcohol (potential adverse reactions) 2

Monitoring Recommendations

1. Baseline assessment:

   • Complete cardiovascular evaluation
   • Blood pressure and heart rate
   • Screen for contraindications

2. Follow-up monitoring:

   • Regular blood pressure and heart rate checks
   • Evaluate weight loss at 12 weeks (discontinue if <5% weight loss achieved) 1, 4
   • Monitor for development of tolerance 2
   • Assess for any new cardiovascular symptoms

Contraindications

Phentermine is contraindicated in patients with:

• History of cardiovascular disease (coronary artery disease, stroke, arrhythmias, heart failure, uncontrolled hypertension)
• Hyperthyroidism
• Glaucoma
• Agitated states
• History of drug abuse
• Pregnancy and breastfeeding
• Concomitant use of MAOIs or within 14 days of MAOI use
• Known hypersensitivity to sympathomimetic amines 1, 2

Special Considerations

• Duration of therapy: FDA-approved for short-term use only (≤12 weeks) 1
• Cognitive effects: May impair ability to operate machinery or drive 2
• Potential for abuse: Though recent data suggests addiction risk may be lower than previously thought, prescribe the minimum effective amount 2, 5
• Renal impairment: Limit dosage to 15 mg daily for patients with severe renal impairment 2

By systematically screening for these side effects and following appropriate monitoring protocols, clinicians can optimize the safety profile of phentermine when used for weight management in appropriate patients.