Phentermine Prescribing Laws
Phentermine is a Schedule IV controlled substance that requires standard DEA registration and controlled substance prescribing authority, with no special state-specific restrictions beyond federal requirements for controlled substances. 1
Federal Regulatory Status
- Phentermine is classified as a Schedule IV controlled substance under the Controlled Substances Act, indicating recognized medical use with low potential for abuse and dependence 2
- Prescribers must maintain active DEA registration to prescribe phentermine 1
- Standard controlled substance documentation and record-keeping requirements apply 1
FDA-Approved Indications and Patient Selection
- Phentermine is FDA-approved only for short-term use (a few weeks) as an adjunct to exercise, behavioral modification, and caloric restriction 1
- Approved for patients with BMI ≥30 kg/m² OR BMI ≥27 kg/m² with weight-related comorbidities (controlled hypertension, diabetes, hyperlipidemia) 1
- Not recommended for patients under 16 years of age 1
Mandatory Pre-Prescribing Requirements
Cardiovascular Screening
- Measure baseline blood pressure and heart rate before initiating therapy 3
- Screen for history of cardiovascular disease including coronary artery disease, stroke, arrhythmias, congestive heart failure, or uncontrolled hypertension—all are absolute contraindications 1
- Younger patients without active coronary disease are more appropriate candidates 3
Additional Contraindication Screening
- Assess pregnancy status in all females of reproductive potential—phentermine is absolutely contraindicated in pregnancy 3, 1
- Screen for hyperthyroidism (contraindication) 3, 1
- Assess for glaucoma (contraindication due to sympathomimetic effects) 3, 1
- Evaluate for agitated states, anxiety disorders, or history of drug abuse (all contraindications) 3, 1
- Confirm no current MAOI use or use within 14 days—risk of hypertensive crisis 3, 1
- Assess renal function, as dosing adjustments are required for renal impairment 1
Dosing and Administration
- Standard adult dose: 15-30 mg orally once daily, approximately 2 hours after breakfast 1
- Avoid late evening dosing due to insomnia risk 1
- Maximum dose of 15 mg daily for severe renal impairment (eGFR 15-29 mL/min/1.73 m²) 1
- Avoid use in end-stage renal disease or eGFR <15 mL/min/1.73 m² 1
Monitoring Requirements During Treatment
- Monitor blood pressure and heart rate periodically at every visit throughout treatment 3, 4
- Discontinue if patient fails to achieve 5% weight loss after 12 weeks on maximum dose 3
- For females of reproductive potential on extended therapy, provide ongoing pregnancy counseling 3
Duration of Therapy Considerations
- FDA approval is limited to short-term use (≤12 weeks) 1, 5
- Many clinicians prescribe off-label for 3-6 months or longer based on understanding obesity as a chronic disease 3
- If prescribing beyond 12 weeks off-label, document specific benefits, tolerance profile, and counsel patients regarding limited long-term safety data 3
- This off-label extended use requires careful patient selection—only those without cardiovascular disease 3
Perioperative Management
- Discontinue phentermine at least 4 days before procedures requiring anesthesia 4
- Risk of perioperative complications includes hypertension, hypotension, bradycardia, and hyperthermia due to norepinephrine reuptake inhibition 4
Common Prescribing Pitfalls
- Do not confuse phentermine monotherapy with the discontinued "fen-phen" combination—valvular heart disease and pulmonary hypertension risks were attributed to fenfluramine, not phentermine 3
- Failing to obtain complete cardiovascular history including remote arrhythmias or structural heart disease is a critical error 2
- Inadequate thyroid function assessment before initiation 2
- Phentermine should not be combined with other weight loss products, including over-the-counter preparations, herbal products, or serotonergic agents like SSRIs—safety and efficacy of combinations not established 1