Minimum BMI for Prescribing Phentermine
Phentermine is FDA-approved for patients with BMI ≥30 kg/m², or BMI ≥27 kg/m² in the presence of at least one obesity-related comorbidity (such as controlled hypertension, diabetes, or hyperlipidemia). 1
FDA-Approved Indications
The official FDA labeling specifies phentermine as a short-term adjunct (a few weeks) in weight reduction regimens for:
- BMI ≥30 kg/m² (obesity without additional requirements) 1
- BMI ≥27 kg/m² with presence of other risk factors including controlled hypertension, diabetes, or hyperlipidemia 1
This BMI threshold is consistent across all FDA-approved anti-obesity medications for long-term use as well 2.
Clinical Practice Considerations
Duration Beyond FDA Approval
While FDA approval specifies "short-term" use (typically interpreted as 3 months or "a few weeks"), many practitioners prescribe phentermine for longer durations off-label given obesity's chronic nature 2, 3. Recent guidelines from 2024 support use for 6 months or more in most patients 3.
Patient Selection at Minimum BMI Threshold
For patients at the lower BMI threshold (27-29.9 kg/m²), you must document at least one obesity-related comorbidity 2, 1:
- Type 2 diabetes
- Hypertension (controlled)
- Dyslipidemia
- Other obesity-related conditions
Absolute Contraindications Regardless of BMI
Even if BMI criteria are met, phentermine is contraindicated in 1:
- History of cardiovascular disease (coronary artery disease, stroke, arrhythmias, congestive heart failure, uncontrolled hypertension)
- Current or recent (within 14 days) monoamine oxidase inhibitor use
- Hyperthyroidism
- Glaucoma
- Agitated states
- History of drug abuse
- Pregnancy or nursing
Relative Contraindications and Caution
Use caution or avoid in patients with 2, 3:
- Anxiety disorders or insomnia (phentermine may exacerbate these)
- Older adults (due to substantial renal excretion)
- Active or unstable coronary disease
- Uncontrolled hypertension (even mild hypertension requires monitoring)
Monitoring Requirements
Blood pressure and heart rate must be monitored periodically during treatment 3, as phentermine causes mild increases in both parameters 2. Heart rate increases of 5-10 beats per minute have been documented at 9-12 months 4.
Renal Impairment Dosing
For patients with severe renal impairment (eGFR 15-29 mL/min/1.73 m²), limit dosage to 15 mg daily 1.
Common Pitfall to Avoid
Do not prescribe phentermine as monotherapy—it must always be prescribed as an adjunct to behavioral interventions involving medical nutrition therapy and physical activity 3. The medication works through sympathetic nervous system activation to decrease food intake and increase resting energy expenditure 2, but requires lifestyle modification for optimal effectiveness.