Phentermine is a prescription medication used for short-term weight management, typically for individuals with obesity or those who have weight-related health concerns. Classified as a sympathomimetic amine, it acts as a central nervous system stimulant to suppress appetite and increase energy levels. Because of its stimulating properties and chemical similarity to amphetamines, phentermine is a controlled substance. Understanding the conditions, substances, and behaviors to avoid is paramount for safe use, as interactions can lead to life-threatening events like a hypertensive crisis or Serotonin Syndrome. The drug is only prescribed for a few weeks due to its limited long-term usefulness and inherent risks.
Pre-Existing Health Conditions That Prevent Use
Phentermine is an absolute contraindication for patients with a history of cardiovascular disease because its mechanism of action directly stresses the heart and circulatory system. The drug works by increasing the release of neurotransmitters, such as norepinephrine, which leads to an elevated heart rate and increased blood pressure. Therefore, individuals with existing conditions like coronary artery disease, uncontrolled hypertension, congestive heart failure, arrhythmias, or a history of stroke cannot safely take this medication.
Taking phentermine with these conditions significantly increases the risk of a severe cardiac event, including primary pulmonary hypertension and valvular heart disease. Patients with hyperthyroidism must also avoid phentermine because their condition already elevates metabolism and heart rate, which the stimulant action of phentermine would dangerously exacerbate.
Certain neurological and ocular conditions also prohibit its use due to the drug’s effects on the central nervous system and blood pressure. Phentermine is contraindicated for patients with glaucoma, specifically acute angle-closure glaucoma, because its sympathomimetic effects can cause pupil dilation, potentially increasing intraocular pressure to dangerous levels. Due to its stimulant effects, phentermine is also avoided in patients with agitated states or a history of drug abuse, as it can worsen anxiety and carries a risk for dependence.
Medications and Supplements That Interact Dangerously
A number of prescription medications and over-the-counter supplements must be avoided due to dangerous interactions with phentermine. Monoamine Oxidase Inhibitors (MAOIs), a class of antidepressants, are the most notorious interaction. Combining MAOIs with phentermine can lead to a potentially fatal hypertensive crisis because MAOIs block the breakdown of norepinephrine while phentermine increases its release. Patients must observe a mandatory 14-day washout period after stopping an MAOI before beginning phentermine therapy.
Other drugs that increase the risk of overstimulation must also be avoided, including other prescription weight-loss medications and stimulants for Attention-Deficit/Hyperactivity Disorder (ADHD), such as amphetamine or methylphenidate. These combinations magnify side effects like rapid heart rate, high blood pressure, and anxiety, increasing the risk of stroke or heart attack. Additionally, certain over-the-counter decongestants that are sympathomimetic amines, like pseudoephedrine, should be avoided for the same reason.
Phentermine’s effect on brain chemistry necessitates caution with serotonergic drugs, including Selective Serotonin Reuptake Inhibitors (SSRIs) and Serotonin-Norepinephrine Reuptake Inhibitors (SNRIs). Taking these antidepressants alongside phentermine raises the risk of Serotonin Syndrome, a serious condition characterized by symptoms like a fast heart rate, high fever, and agitation. Finally, patients taking insulin or other medications for diabetes must be closely monitored, as phentermine’s appetite-suppressing effect and subsequent reduced food intake can alter blood sugar levels, potentially requiring an adjustment to their diabetes regimen.
Lifestyle Habits to Eliminate or Severely Limit
Alcohol must be avoided or severely limited while taking phentermine, as the combination can intensify the side effects of the stimulant. Mixing this central nervous system stimulant with a depressant like alcohol can worsen symptoms such as dizziness, headache, and difficulty concentrating. Alcohol consumption can also mask the sensation of intoxication, leading to impaired judgment and increased cardiovascular risk.
The consumption of excessive caffeine, found in coffee, energy drinks, and certain supplements, should also be limited. Since phentermine is a stimulant, high doses of caffeine can amplify its effects, leading to an increased risk of nervousness, insomnia, and heart palpitations. Both substances raise blood pressure and heart rate, and combining them can overstimulate the cardiovascular system.
Phentermine can cause side effects like dizziness, blurred vision, and altered reaction time, so patients should avoid operating heavy machinery or driving until they know how the medication affects their alertness. Patients must also avoid strenuous activity in warm environments or during periods of high heat. Phentermine increases the body’s internal heat production and impedes heat dissipation, significantly elevating the risk of developing a serious heat-related illness, such as heat exhaustion or heat stroke.
Actions That Lead to Misuse or Dependency
Phentermine is classified as a Schedule IV controlled substance and must be used exactly as prescribed to prevent misuse and physical dependence. Patients must never take a higher dose than instructed, as this increases the potential for serious side effects, including overdose. Using the medication longer than the prescribed short-term duration, typically a maximum of 12 weeks, also increases the risk of developing tolerance and dependence.
Patients should not abruptly stop taking phentermine, especially after prolonged use, and must taper the dose down under medical guidance. Abrupt cessation can lead to withdrawal symptoms, such as fatigue and depression, necessitating a gradual reduction to ease the body’s transition. Finally, the medication must never be shared with others, as it is prescribed based on an individual patient’s medical history and current health status.