Phentermine is a prescription medication used alongside diet and exercise for short-term weight management in people who are obese or overweight with weight-related health issues. The Food and Drug Administration (FDA) has approved phentermine for use up to 12 weeks to promote weight loss. It is classified as a Schedule IV controlled substance due to its mild potential for abuse. A primary concern regarding its use is its known effect on the cardiovascular system, which raises questions about its impact on blood pressure.
How Phentermine Affects the Cardiovascular System
Phentermine is a sympathomimetic amine, meaning it mimics the actions of the natural “fight or flight” hormones in the body. It works by increasing the levels of the neurotransmitter norepinephrine in the central nervous system, which helps to suppress appetite. This increase in norepinephrine also extends its effects to the rest of the body, stimulating the sympathetic nervous system.
The stimulation of the sympathetic nervous system leads to physiological changes that directly affect the heart and blood vessels. These effects include an increase in heart rate, known as tachycardia, and the constriction of blood vessels, which is called vasoconstriction. Vasoconstriction naturally raises the resistance to blood flow, which in turn elevates the systemic blood pressure.
While the potential for increased blood pressure is inherent in the drug’s mechanism, the actual change in a patient can vary. In many cases, the increases are modest, often cited as only 2 to 5 mmHg for systolic pressure and 1 to 3 mmHg for diastolic pressure. However, in some clinical trials, the weight loss achieved with phentermine has resulted in a net decrease in blood pressure for patients with pre-existing hypertension. Despite these varying outcomes, the drug’s fundamental property is to increase blood pressure.
Identifying Clinical Risks and Contraindications
Due to its stimulating effects, phentermine is strictly contraindicated in patients with certain pre-existing medical conditions where the cardiovascular system is already compromised. Patients with a history of cardiovascular disease, including coronary artery disease, arrhythmias, or a prior stroke, should not be prescribed phentermine.
Absolute contraindications also include uncontrolled or severe hypertension, as the drug’s potential to raise blood pressure could lead to a hypertensive crisis or other serious events. Advanced arteriosclerosis (hardening of the arteries) also makes phentermine use dangerous. The drug is also to be avoided in those with congestive heart failure, as its effects could exacerbate the heart’s existing inability to pump blood efficiently.
The contraindication extends to patients with hyperthyroidism because the condition already increases the heart rate and metabolism, and phentermine would add to this stimulation, risking arrhythmias. Furthermore, phentermine must not be used concurrently with or within 14 days of taking a monoamine oxidase inhibitor (MAOI), as this combination can lead to a severe and life-threatening hypertensive crisis. These exclusion criteria exist to prevent serious adverse cardiovascular events like myocardial infarction.
Monitoring Blood Pressure During Treatment
For appropriate candidates beginning phentermine therapy, cardiovascular monitoring is mandatory. Healthcare providers must measure the patient’s heart rate and blood pressure at the initial visit before starting the medication. Regular follow-up checks are then required throughout the duration of the prescription, with particular vigilance during the first 12 weeks of therapy.
Standard practice involves periodic monitoring, such as checking blood pressure at each follow-up visit, which may be weekly or bi-weekly initially. This tracking is particularly important for individuals with controlled hypertension, who may be considered for the drug under strict surveillance. A concerning elevation in blood pressure, or the development of new cardiovascular symptoms, requires immediate attention.
If a patient’s blood pressure rises significantly or becomes uncontrolled, the healthcare provider must intervene promptly. This intervention may involve a reduction in the phentermine dosage or, in cases of sustained or excessive elevation, the complete discontinuation of the drug. Monitoring ensures that the medication’s benefits are not outweighed by cardiovascular risks, guiding the decision to adjust dosage or switch to alternative therapies if necessary.