Guidelines for Using Phentermine for Weight Loss
Phentermine should be used as a short-term (12 weeks) adjunct to lifestyle modifications for weight management in adults with obesity (BMI ≥30 kg/m²) or overweight (BMI ≥27 kg/m²) with weight-related complications, and should be avoided in patients with cardiovascular disease. 1, 2
Indications and Patient Selection
- Phentermine is indicated for patients with BMI ≥30 kg/m² or BMI ≥27 kg/m² with weight-related complications such as hypertension, diabetes, dyslipidemia, or obstructive sleep apnea 1, 2
- Phentermine should always be used in combination with an intensive lifestyle program including exercise, behavioral modification, and caloric restriction 1, 2
- Phentermine is FDA-approved only for short-term use (12 weeks), though many practitioners use it longer in an off-label fashion given the chronic nature of weight management 1
Dosing and Administration
- The recommended dosage is 15-37.5 mg orally once daily, typically taken in the morning 1, 2
- Start with the lowest effective dose and individualize based on response - some patients may respond to as little as 9.375 mg (quarter tablet) or 18.75 mg (half tablet) 1
- A split dose of half tablet twice daily is also an option 1
- Low-dose phentermine (8 mg) can be prescribed up to 3 times per day 3, 4
Monitoring and Efficacy Assessment
- Blood pressure and heart rate should be monitored periodically while taking phentermine 1, 5
- If there is less than 5% weight loss at 12 weeks on the maximum dose, consider discontinuing phentermine and trying alternative treatments 1, 6
- Research shows that longer-term users (>12 months) experienced 7.4% more weight loss than those using it for ≤3 months, suggesting potential benefit to extended use in appropriate patients 7
Contraindications and Precautions
Phentermine should be avoided in patients with: 1, 8
- History of cardiovascular disease
- Uncontrolled hypertension
- Hyperthyroidism
- Glaucoma
- Agitated states
- History of drug abuse
- During or within 14 days of taking MAOIs
- Pregnancy or nursing
Phentermine should not be used concomitantly with other sympathomimetic amines or alcohol 1, 9
Side Effects and Safety
- Common side effects include dizziness, dry mouth, difficulty sleeping, irritability, nausea/vomiting, diarrhea, and constipation 1, 5
- Taking phentermine early in the day can help minimize insomnia 5, 3
- Potential drug-drug interactions include moderate to severe interactions with drugs like dextromethorphan, tramadol, and phenylpropanolamine 9
- Rare but serious adverse events have been reported with inappropriate long-term use, including ischemic colitis 10
Off-label Extended Use
- While FDA-approved for only 12 weeks, the American Gastroenterological Association acknowledges that many practitioners use phentermine longer than 12 weeks in an off-label fashion given the chronic nature of weight management 1, 5
- A 2019 study found no increased risk of cardiovascular disease or death in patients using phentermine for longer than 3 months, supporting potential safety of longer-term use in low-risk individuals 7
- For extended use, regular monitoring of cardiovascular parameters is essential 1, 5
Patient Education
- Patients must be informed that phentermine is intended as a short-term adjunct to lifestyle modifications 2
- Patients should be counseled about potential side effects and when to contact their physician 2
- Phentermine should be kept in a safe place to prevent theft, accidental overdose, misuse, or abuse 2