To what extent does phentermine affect blood pressure?

Phentermine's Effects on Blood Pressure

Phentermine typically causes modest increases in blood pressure due to its sympathomimetic properties, with systolic blood pressure elevations of 2-5 mmHg and diastolic increases of 1-3 mmHg in most patients. 1, 2

Mechanism of Action and Cardiovascular Effects

• Phentermine is a sympathomimetic amine that works by increasing norepinephrine levels in the central nervous system, which directly affects cardiovascular function 1, 2
• The FDA label clearly states that typical actions of phentermine include "central nervous system stimulation and elevation of blood pressure" 2
• As a norepinephrine reuptake inhibitor, phentermine can lead to both hyperadrenergic effects and paradoxical refractory hypotension during surgery 3

Clinical Trial Data on Blood Pressure Effects

• In the largest trial of phentermine-topiramate ER, patients showed reductions in systolic blood pressure averaging 4.7-5.6 mmHg compared to 2.4 mmHg in the placebo group 4
• Diastolic blood pressure reductions were 3.4-3.8 mmHg in the treatment groups versus 2.7 mmHg in the placebo group 4
• A 2010 randomized controlled trial of phentermine diffuse-controlled release showed no significant differences in systolic and diastolic blood pressure between treatment and placebo groups 5
• A 2011 observational study suggested phentermine treatment did not result in increased systolic or diastolic blood pressure, with treated patients showing declines of 6.9/5.0 mmHg at 26 weeks 6

Contraindications and Precautions

• Phentermine is contraindicated in patients with:
  • History of cardiovascular disease 1
  • Uncontrolled hypertension 1
  • Patients taking monoamine oxidase inhibitors (or within 14 days of discontinuation) due to risk of hypertensive crisis 1

Monitoring Recommendations

• Blood pressure and heart rate should be monitored periodically during phentermine treatment 1
• Consider starting at lower doses (4-8 mg) in patients with cardiac risk factors 1
• Prior to initiating phentermine, conduct a thorough cardiovascular risk assessment 1

Perioperative Considerations

• The American Gastroenterological Association recommends discontinuing phentermine at least 4 days before procedures requiring anesthesia 4, 3
• This is due to potential perioperative complications including hypertension, hypotension, bradycardia, and hyperthermia 4

Overdose Concerns

• Acute overdose of phentermine can cause significant hypertension or hypotension and circulatory collapse 2
• The FDA label notes that intravenous phentolamine has been suggested for possible acute, severe hypertension complicating phentermine overdosage 2

Long-term Safety Data

• There are currently no large cardiovascular outcome trials for long-term use of phentermine monotherapy 1
• Recent observational studies have not identified an increased risk of cardiovascular events or significant elevations in blood pressure with phentermine use 7
• Phentermine is FDA-approved only for short-term use (≤12 weeks), though many clinicians use it off-label for longer periods 1

While individual responses vary, clinicians should be aware of phentermine's potential to affect blood pressure and monitor patients accordingly, especially those with pre-existing cardiovascular risk factors.