From the Guidelines
Phentermine is generally safe for most people when used as prescribed for short-term weight loss, but it's crucial to carefully evaluate individual health conditions and follow guidelines to minimize risks. According to the AGA clinical practice guideline on pharmacological interventions for adults with obesity 1, phentermine is suggested for use with lifestyle modifications in adults with obesity or overweight with weight-related complications, with a conditional recommendation based on low-quality evidence. This guideline emphasizes the importance of monitoring blood pressure and heart rate periodically while taking phentermine and avoiding its use in patients with a history of cardiovascular disease.
Key considerations for the safe use of phentermine include:
- It is approved by the FDA for short-term use (12 weeks), but many practitioners use it off-label for longer periods due to the chronic nature of weight management 1.
- Patients should be carefully selected, avoiding those with cardiovascular disease, and monitoring should include regular checks of blood pressure and heart rate 1.
- Common side effects can include dry mouth, insomnia, constipation, and increased heart rate, and it can be habit-forming, leading to withdrawal symptoms if stopped abruptly.
- Phentermine should always be used as part of a comprehensive weight loss plan that includes diet and exercise, not as a standalone solution.
The decision to use phentermine should be made on a case-by-case basis, considering the potential benefits against the risks, especially in individuals with certain health conditions or those taking other medications. Given the chronic nature of obesity, the use of phentermine beyond the approved 12-week period may be considered in clinical practice, but this should be done with caution and under close medical supervision 1.
From the FDA Drug Label
The limited usefulness of agents of this class, including Phentermine hydrochloride, [ see CLINICAL PHARMACOLOGY ( 12.1,12.2)] should be measured against possible risk factors inherent in their use such as those described below. 4 CONTRAINDICATIONS History of cardiovascular disease (e.g., coronary artery disease, stroke, arrhythmias, congestive heart failure, uncontrolled hypertension) During or within 14 days following the administration of monoamine oxidase inhibitors Hyperthyroidism Glaucoma Agitated states History of drug abuse Pregnancy [see Use in Specific Populations ( 8.1)] Nursing [see Use in Specific Populations ( 8. 3)] Known hypersensitivity, or idiosyncrasy to the sympathomimetic amines 5 WARNINGS AND PRECAUTIONS 5. 1 Coadministration with Other Drug Products for Weight Loss Phentermine is indicated only as short-term (a few weeks) monotherapy for the management of exogenous obesity The safety and efficacy of combination therapy with phentermine and any other drug products for weight loss including prescribed drugs, over-the-counter preparations, and herbal products, or serotonergic agents such as selective serotonin reuptake inhibitors (e.g., fluoxetine, sertraline, fluvoxamine, paroxetine), have not been established. Therefore, coadministration of phentermine and these drug products is not recommended. 5. 2 Primary Pulmonary Hypertension Primary Pulmonary Hypertension (PPH) – a rare, frequently fatal disease of the lungs – has been reported to occur in patients receiving a combination of phentermine with fenfluramine or dexfenfluramine The possibility of an association between PPH and the use of phentermine alone cannot be ruled out; there have been rare cases of PPH in patients who reportedly have taken phentermine alone.
Phentermine is not safe for most people due to the following risk factors:
- History of cardiovascular disease
- Hyperthyroidism
- Glaucoma
- Agitated states
- History of drug abuse
- Pregnancy
- Nursing
- Known hypersensitivity, or idiosyncrasy to the sympathomimetic amines
- Primary Pulmonary Hypertension (PPH)
- Valvular Heart Disease
- Development of Tolerance
- Effect on the Ability to Engage in Potentially Hazardous Tasks
- Risk of Abuse and Dependence
- Usage with Alcohol may result in an adverse drug reaction 2, 2, 2.
From the Research
Safety of Phentermine
- Phentermine is considered safe for most people when used as directed, with studies showing that it is generally well tolerated in adults and pediatric patients 3, 4.
- A study of pediatric patients found that 17% of patients experienced side effects, which resolved after dose reduction or discontinuing phentermine 3.
- Large-scale observational studies have not identified an increased risk of cardiovascular events or elevations in blood pressure associated with phentermine use 4.
- There is no data to support the notion that phentermine is addictive 4.
Efficacy and Safety in Different Populations
- Phentermine has been shown to be effective in achieving weight loss in adults and pediatric patients, with a mean reduction in BMI of 15% in one study 3.
- A study of longer-term phentermine use found that patients using the medication continuously for more than 12 months experienced more weight loss, with no significant difference in hazard ratios for composite cardiovascular disease or death 5.
- Phentermine has been found to be safe and effective in patients with a body mass index (BMI) of 30 kg/m2 or greater, or BMI of 27 kg/m2 or greater with weight-related comorbidities 6.
Comparison to Other Anti-Obesity Medications
- Phentermine is one of the most commonly used anti-obesity medications, and has been found to have a superior weight loss profile compared to some other medications 7.
- Combination phentermine/extended release topiramate has been found to be associated with greater weight loss compared to its constituent monotherapy, with a more favorable adverse effect profile 7.