What are the guidelines for using Phentermine (phentermine) for weight loss?

Guidelines for Using Phentermine for Weight Loss

Phentermine is indicated only as a short-term (a few weeks) adjunct for weight reduction in patients with BMI ≥30 kg/m² or ≥27 kg/m² with risk factors, and should not be used long-term due to limited safety data and potential cardiovascular risks. 1

Indications and Patient Selection

• Appropriate for patients with:
  • BMI ≥30 kg/m² OR
  • BMI ≥27 kg/m² with additional risk factors (controlled hypertension, diabetes, hyperlipidemia) 1
• Must be used as part of a comprehensive program including:
  • Exercise
  • Behavioral modification
  • Caloric restriction 1

Dosing Guidelines

• Starting dose: 15 mg daily, taken approximately 2 hours after breakfast 2
• Maximum dose: 37.5 mg daily 3, 2
• Alternative dosing options:
  • Half tablet (18.75 mg) twice daily
  • Low-dose 8 mg tablet up to 3 times daily (brand name Lomaira) 2

Expected Outcomes

• Mean weight loss: 5-6% of body weight at 28 weeks 2
• In clinical trials, phentermine demonstrated:
  • 7.5 mg daily: 6.6% weight loss (vs 1.8% with placebo)
  • 15 mg daily: 7.4% weight loss (vs 3.0% with placebo) 3
• Patients reporting greater baseline hunger and less dietary restraint tend to achieve better results with phentermine 4

Monitoring and Follow-up

• Assess efficacy and safety at least monthly for the first 3 months 2
• Monitor blood pressure and heart rate every 2 weeks during the first 12 weeks 2
• Discontinuation criteria:
  • Less than 5% weight loss at 12 weeks
  • Development of new, unexplained symptoms (dyspnea, angina, syncope, lower extremity edema) 2

Contraindications

• Unmanaged hypertension
• Hypoglycemia (when used with insulin or sulfonylureas)
• Use with monoamine oxidase inhibitors
• High risk for glaucoma
• History of cardiovascular disease
• Pregnancy and nursing 3, 2, 1

Common Side Effects

• Dry mouth (most common)
• Insomnia
• Dizziness
• Irritability
• Elevated heart rate and blood pressure
• Constipation
• Headache
• Gastrointestinal issues (nausea, diarrhea, abdominal pain) 3, 2

Drug Interactions

• Severe interactions with:
  • Tramadol (most commonly co-prescribed with severe interaction potential, 46.79% of patients) 5
  • Serotonergic drugs
  • MAO inhibitors
  • Other stimulants (including ADHD medications like Adderall) 2, 5
• May require dose adjustment of insulin or oral hypoglycemic medications in diabetic patients 2

Discontinuation Protocol

When stopping phentermine, gradual tapering is recommended:

• Week 1: Reduce to half dose (18.75 mg daily)
• Week 2: Take half dose every other day
• Week 3: Discontinue completely 2

Important Considerations and Cautions

• Despite common practice, phentermine is FDA-approved only for short-term use (a few weeks) 1
• No evidence supports phentermine being addictive or causing withdrawal symptoms (other than increased hunger) when discontinued properly 6
• For long-term weight management, newer options like GLP-1 receptor agonists (semaglutide, tirzepatide) show greater efficacy and may have better safety profiles 3, 2
• Korean studies showed significant weight reduction (-6.7 kg) and waist circumference reduction (-6.2 cm) with short-term phentermine use 7

Special Populations

• Not recommended for long-term use in older adults due to cardiovascular risks and limited safety data 2
• Contraindicated in patients younger than 24 years due to risk of suicidal behavior/ideation 3
• Use with caution in patients with kidney disease due to increased risk of gastrointestinal side effects and potential risk of acute kidney injury from dehydration 3