Baseline EKG Before Starting Phentermine
A baseline EKG is recommended for patients ≥40 years of age or those with cardiovascular risk factors before initiating phentermine, based on ACC/AHA Class I guidelines for evaluating patients starting medications with known cardiovascular effects.
Age-Based Recommendations
Patients ≥40 years old should receive a baseline EKG before starting phentermine, as this represents a Class I indication per ACC/AHA guidelines for asymptomatic persons in this age group and for patients starting therapy known to produce cardiovascular effects 1, 2.
Patients <40 years without cardiovascular risk factors do not require routine baseline EKG, as ACC/AHA guidelines classify this as Class III (not indicated) for young healthy persons without cardiac disease or risk factors 1.
Risk Factor-Based Screening
Obtain a baseline EKG regardless of age if any of the following cardiovascular risk factors are present:
- History of cardiovascular disease (coronary artery disease, heart failure, cerebrovascular disease) 1, 2
- Uncontrolled hypertension 1
- Diabetes mellitus 2
- Hyperlipidemia 2
- Current tobacco use 2
- Family history of premature coronary artery disease 2
- Untreated hyperthyroidism (absolute contraindication to phentermine due to arrhythmia risk) 1
Rationale for EKG Screening
Phentermine's sympathomimetic properties create specific cardiac concerns:
Phentermine produces adrenergic hemodynamic effects that can increase heart rate and potentially affect blood pressure, though clinical trial data show blood pressure generally declines with small heart rate increases 1.
The medication can cause palpitations and, rarely, atrial fibrillation as documented in case reports 3.
ACC/AHA guidelines classify medications with known cardiac effects (including sympathomimetics) as Class I indications for baseline EKG to assess response to therapy 1.
Clinical Practice Considerations
The 2022 AGA guidelines emphasize cardiovascular safety monitoring:
Phentermine should be avoided entirely in patients with a history of cardiovascular disease or uncontrolled hypertension 1.
Blood pressure and heart rate require periodic monitoring during treatment, making baseline values essential for comparison 1.
No large cardiovascular outcome trials exist for long-term phentermine use, necessitating caution and baseline cardiac assessment 1.
Common Pitfalls to Avoid
Do not skip EKG screening in patients ≥40 years even if they appear healthy, as this represents standard of care for initiating sympathomimetic medications 1, 2.
Do not prescribe phentermine to patients with untreated hyperthyroidism without first obtaining thyroid function tests and EKG, as this combination significantly increases arrhythmia and seizure risk 1.
Do not rely solely on patient-reported cardiac history—actively screen for cardiovascular risk factors including diabetes, hypertension, smoking, and family history, as these mandate baseline EKG 2.
Do not initiate phentermine in patients taking or recently discontinued (within 14 days) monoamine oxidase inhibitors due to dangerous drug interactions 1.
Monitoring After Initiation
Periodic blood pressure and heart rate monitoring should continue throughout treatment, with the baseline EKG serving as reference for any concerning changes 1.
Consider repeat EKG if patients develop cardiac symptoms (palpitations, chest pain, dyspnea) during treatment, as these may indicate arrhythmias or other cardiac complications 3.