Is it safe to initiate phentermine (anorexiant) in a patient taking antihypertensive (blood pressure) medication?

From the Guidelines

Starting phentermine in a patient taking antihypertensive medication requires caution and medical supervision, and it is generally not recommended for patients with uncontrolled hypertension.

Key Considerations

• Phentermine can increase blood pressure and heart rate, potentially counteracting the effects of antihypertensive medications 1.
• If blood pressure is well-controlled, some physicians may consider prescribing phentermine, but this decision should be made on an individual basis after a thorough evaluation.
• Certain blood pressure medications like beta-blockers (such as metoprolol or propranolol) may have particularly concerning interactions with phentermine.
• Before starting phentermine, patients should have their blood pressure well-controlled, typically below 140/90 mmHg, and should be monitored more frequently after starting the medication, often weekly for the first month.
• Phentermine is typically prescribed at doses of 15-37.5 mg daily, taken in the morning to avoid insomnia.

Physiological Concerns

• Phentermine works as a sympathomimetic amine, stimulating the release of norepinephrine, which can increase blood pressure through vasoconstriction and increased cardiac output, directly opposing the mechanism of many antihypertensive medications.
• The use of phentermine should be avoided in patients with a history of cardiovascular disease or uncontrolled hypertension 1.

Monitoring and Follow-up

• Blood pressure and heart rate should be monitored periodically when taking phentermine.
• Patients should be advised regarding off-label use and limited data supporting this approach if phentermine is prescribed for longer than 12 weeks.

From the FDA Drug Label

5.8 Use in Patients with Hypertension Use caution in prescribing Phentermine for patients with even mild hypertension (risk of increase in blood pressure). 7.4 Adrenergic Neuron Blocking Drugs Phentermine may decrease the hypotensive effect of adrenergic neuron blocking drugs.

Initiating phentermine in a patient taking antihypertensive medication is not recommended without caution. The FDA drug label warns that phentermine may increase blood pressure and decrease the hypotensive effect of adrenergic neuron blocking drugs, which are used to treat hypertension. Therefore, use caution and monitor blood pressure closely when prescribing phentermine to patients taking antihypertensive medication 2.

From the Research

Safety of Initiating Phentermine in Patients Taking Antihypertensive Medication

• The safety of initiating phentermine in patients taking antihypertensive medication is a concern due to potential interactions and effects on blood pressure 3, 4, 5, 6.
• Studies have shown that phentermine can reduce blood pressure in patients with hypertension, with one study finding a decrease in systolic blood pressure of -6.9 mmHg and diastolic blood pressure of -5.0 mmHg at 26 weeks 5.
• Another study found that phentermine/topiramate reduced systolic blood pressure by -2.0 to -4.2 mmHg and diastolic blood pressure by -1.3 to -1.9 mmHg compared to placebo 3.
• A randomized, multicenter, double-blind study found that phentermine/topiramate extended-release reduced ambulatory blood pressure monitoring (ABPM) compared to placebo and phentermine alone 6.
• However, the European Medicines Agency refused marketing authorization for phentermine/topiramate due to safety concerns, and the application for European marketing authorization for lorcaserin was withdrawn due to a negative overall benefit/risk balance 3, 4.
• It is essential to monitor blood pressure and adjust antihypertensive medication as needed when initiating phentermine in patients taking antihypertensive medication 7.

Effects on Blood Pressure

• Phentermine has been shown to reduce blood pressure in patients with hypertension, with significant decreases in systolic and diastolic blood pressure 3, 5, 6.
• The combination of phentermine and topiramate has been found to reduce blood pressure more effectively than phentermine alone 3, 6.
• However, the effects of phentermine on blood pressure can vary depending on the individual and the presence of other health conditions 4, 7.

Adverse Events

• Common adverse events associated with phentermine include dry mouth, constipation, nausea, dizziness, paresthesia, and headache 3, 4, 6.
• The incidence of adverse events can be higher in patients taking phentermine compared to placebo, particularly for gastrointestinal side effects 3, 4.
• Monitoring for adverse events and adjusting medication as needed is crucial when initiating phentermine in patients taking antihypertensive medication 7.