Contraindications to Phentermine Prescribing
Phentermine is absolutely contraindicated in patients with any history of cardiovascular disease (including coronary artery disease, stroke, arrhythmias, congestive heart failure), uncontrolled hypertension, hyperthyroidism, glaucoma, agitated states, history of drug abuse, pregnancy, nursing, and in those taking or within 14 days of monoamine oxidase inhibitors. 1
Absolute Cardiovascular Contraindications
Do not prescribe phentermine to patients with:
- Any history of cardiovascular disease including coronary artery disease, stroke, arrhythmias (including atrial fibrillation), or congestive heart failure 2, 3, 1
- Uncontrolled hypertension (blood pressure readings consistently above goal despite treatment) 2, 1
- Even patients with controlled hypertension require extreme caution due to risk of blood pressure elevation 1
The 2022 AGA guidelines explicitly state phentermine should be avoided in patients with cardiovascular disease history, as there are no large cardiovascular outcome trial data for long-term use 2. Case reports document serious cardiovascular events including new-onset atrial fibrillation in previously healthy individuals 4, and historical data from fenfluramine-phentermine combinations revealed severe valvular heart disease 5.
Endocrine and Metabolic Contraindications
- Untreated hyperthyroidism - phentermine should not be used due to concerns for arrhythmias and seizures 2, 1
- Glaucoma - absolute contraindication per FDA labeling 1
Psychiatric and Substance Use Contraindications
- History of drug abuse - phentermine is a Schedule IV controlled substance with abuse and dependence potential 2, 1
- Agitated states - contraindicated per FDA labeling 1
Medication Interactions - Absolute Contraindications
Never prescribe phentermine in these situations:
- Current use or within 14 days of monoamine oxidase inhibitors (MAOIs) - risk of hypertensive crisis 2, 1
- Concomitant use with other sympathomimetic agents requires extreme caution and intensive monitoring 3
The AGA guidelines emphasize phentermine should be avoided in patients treated with, or within 14 days of, MAOIs 2.
Pregnancy and Lactation - Absolute Contraindications
- Pregnancy - absolute contraindication 1
- Nursing/breastfeeding - absolute contraindication; must discontinue drug or nursing 1
- Women of childbearing potential should be counseled about contraception, particularly when phentermine is combined with topiramate (which is teratogenic) 2
Renal Impairment Contraindications
- Avoid use in patients with eGFR less than 15 mL/min/1.73 m² or end-stage renal disease requiring dialysis 1
- Limit dosage to 15 mg daily for severe renal impairment (eGFR 15-29 mL/min/1.73 m²) 1
Relative Contraindications and High-Risk Situations
Exercise extreme caution or avoid in:
- Geriatric population - limited high-quality data; use with caution due to substantial renal excretion 1
- Patients requiring general anesthesia - special management considerations apply 2
- Seizure disorders - increased risk, particularly when combined with other medications 3
- Patients taking insulin or oral hypoglycemic medications - dose adjustments may be required 1
Known Hypersensitivity
- Known hypersensitivity or idiosyncrasy to sympathomimetic amines - absolute contraindication 1
Common Pitfalls to Avoid
Critical screening errors include:
- Failing to obtain complete cardiovascular history, including remote history of arrhythmias or structural heart disease 2, 4
- Not screening for MAOI use in the preceding 14 days 1
- Overlooking pregnancy status or breastfeeding in women of childbearing age 1
- Missing history of substance abuse or current controlled substance use 1
- Inadequate assessment of thyroid function before initiation 2
- Not checking renal function, particularly in elderly patients 1
The FDA labeling and 2022 AGA guidelines provide the most authoritative guidance, with the FDA label serving as the legal standard for contraindications 1, 2.