Contraindications for Phentermine
Phentermine is absolutely contraindicated in patients with cardiovascular disease, uncontrolled hypertension, during pregnancy, within 14 days of MAOI use, hyperthyroidism, glaucoma, and agitated states. 1
Absolute Contraindications
Cardiovascular Conditions
- History of cardiovascular disease (including coronary artery disease, arrhythmias, heart failure, stroke) 2, 1
- Uncontrolled hypertension - even mild hypertension requires extreme caution 2, 1
- The 2015 Endocrine Society guideline makes a strong recommendation against using phentermine in patients with uncontrolled hypertension or history of heart disease, emphasizing harm avoidance over potential short-term weight loss benefits 2
Drug Interactions
- Concurrent or recent MAOI use (within 14 days) - risk of hypertensive crisis 1
- Concomitant use with other sympathomimetics (including phenylpropanolamine) - risk of severe adverse reactions 3
Pregnancy and Lactation
- Pregnancy (Category X) - weight loss offers no benefit and may cause fetal harm; phentermine has amphetamine-like pharmacologic activity 1
- Breastfeeding - amphetamines are present in human milk; risk of serious adverse reactions in nursing infants 1
Other Medical Conditions
- Hyperthyroidism - sympathomimetic effects are amplified
- Glaucoma - risk of increased intraocular pressure
- Agitated states or history of drug abuse - phentermine is chemically related to amphetamine with abuse potential 1
Relative Contraindications Requiring Extreme Caution
Renal Impairment
- Severe renal impairment (eGFR 15-29 mL/min/1.73 m²) - limit dose to 15 mg daily 1
- End-stage renal disease or eGFR <15 mL/min/1.73 m² - avoid use entirely 1
Psychiatric Conditions
- History of psychiatric disorders - risk of overstimulation, restlessness, psychosis 1
- Patients on SSRIs/SNRIs - while not an absolute contraindication, careful monitoring is required 2
Diabetes
- Patients on insulin or oral hypoglycemics - may require medication adjustment due to weight loss effects 1
Critical Monitoring Requirements
When phentermine is prescribed (in appropriate candidates):
- Blood pressure and heart rate must be monitored periodically 4, 2
- Baseline cardiovascular assessment is essential before initiation
- Discontinue if tolerance develops - do not increase dose 1
- Discontinue 4-7 days before elective surgery due to risk of intraoperative hypotension unresponsive to standard vasopressors 5
Common Pitfalls to Avoid
Do not prescribe in patients with "controlled" cardiovascular disease - the AGA and Endocrine Society guidelines specifically state to avoid in patients with any history of cardiovascular disease, not just uncontrolled disease 4, 2
Do not combine with alcohol - concomitant use may result in adverse drug reactions 1
Do not use in pediatric populations - safety and effectiveness not established; not recommended for chronic pediatric obesity 1
Do not ignore the perioperative risk - phentermine can cause refractory hypotension during anesthesia that may not respond to ephedrine 5
The FDA label and clinical practice guidelines prioritize safety over the modest weight loss benefits, particularly given phentermine's sympathomimetic properties and lack of long-term cardiovascular outcomes data 2, 1.