What is the proper dosing and patient counseling for phentermine (Adipex) initiation?

Phentermine Dosing and Patient Counseling

Phentermine should be dosed at 15-37.5 mg orally once daily, taken in the morning approximately 2 hours after breakfast, with dosage individualized to achieve adequate response with the lowest effective dose. 1

Dosing Recommendations

• Starting dose: 15 mg daily, titrated based on response and tolerability 2, 1
• Maximum dose: 37.5 mg daily 1
• Alternative dosing options:
  • Half tablet (18.75 mg) twice daily
  • Low-dose 8 mg tablet up to 3 times per day (brand name Lomaira) 2
• Special populations: Limit dosage to 15 mg daily for patients with severe renal impairment (eGFR 15-29 mL/min/1.73 m²) 1
• Timing: Take in the morning; avoid late evening administration to prevent insomnia 1
• Administration: Can be taken with or without food 1

Patient Counseling Points

Key Information to Provide

1. Short-term use: Phentermine is FDA-approved only for short-term use (up to 3 months) as an adjunct to diet and lifestyle modification 1

2. Expected weight loss: Approximately 5-6% of body weight at 28 weeks 2

3. Common side effects:

   • Dry mouth (19%)
   • Insomnia (10%)
   • Dizziness/irritability
   • Constipation (15-17%)
   • Headache (16%)
   • Elevated heart rate and blood pressure
   • Paresthesia (23%) 3, 2

4. Serious risks:

   • Primary pulmonary hypertension
   • Valvular heart disease
   • Increased blood pressure
   • Effects on ability to perform hazardous tasks 1

5. Contraindications:

   • Cardiovascular disease
   • During or within 14 days of MAOI use
   • Hyperthyroidism
   • Glaucoma
   • Agitated states
   • History of drug abuse
   • Pregnancy or nursing
   • Known hypersensitivity to sympathomimetic amines 2

6. Drug interactions:

   • Serotonergic drugs
   • Tramadol
   • Alcohol
   • May require reduction in dose of insulin or oral hypoglycemic medications in diabetic patients 2

Monitoring Requirements

• Efficacy assessment: At least monthly for the first 3 months
• Blood pressure and heart rate: Every 2 weeks during the first 12 weeks
• Discontinuation criteria: Less than 5% weight loss at 12 weeks, or if new, unexplained symptoms of dyspnea, angina, syncope, or lower extremity edema develop 2

Discontinuation and Tapering

When discontinuing phentermine, gradual tapering is recommended to minimize withdrawal effects:

Week	Dose
1	18.75mg daily (half tablet)
2	18.75mg every other day
3	Discontinue completely

2

Important Warnings

• Tolerance development: Patients should be informed about the potential for developing tolerance 1
• Risk of dependence: Keep medication in a safe place to prevent theft, accidental overdose, misuse, or abuse 1
• Cardiovascular risks: Particularly important in older adults who may have heightened sensitivity to sympathomimetic effects 2
• CNS effects: Can cause overstimulation, restlessness, dizziness, insomnia, euphoria, dysphoria, tremor, headache, and even psychosis 2

Clinical Pearls

• The combination of phentermine with other weight loss medications is not recommended unless specifically approved (e.g., phentermine-topiramate ER) 1
• Weight loss is greatest in the first weeks of therapy and tends to decrease in succeeding weeks 1
• Recent observational studies have not identified an increased risk of cardiovascular events with phentermine use, though rigorous long-term safety data is limited 4
• For patients who don't respond adequately after 3 months, some evidence suggests that continuing to 6 months may be beneficial, with approximately 40% of 3-month non-responders achieving at least 5% weight loss by 6 months 5