From the Guidelines
Phentermine is a recommended medication for weight loss in adults with obesity, with a suggested dosage of 15-37.5 mg once daily, and should be used in conjunction with lifestyle modifications. Before starting phentermine, baseline labs should include a comprehensive metabolic panel, lipid profile, thyroid function tests, and an EKG to assess cardiovascular health, as recommended by the AGA clinical practice guideline on pharmacological interventions for adults with obesity 1. Blood pressure and heart rate should be measured, as phentermine can increase both. Regarding efficacy, phentermine typically produces weight loss of 3-5% of body weight over 12 weeks when combined with lifestyle modifications, with 46% of subjects achieving 5% weight loss and 20.8% achieving 10% weight loss in a 28-week randomized controlled trial 1.
The standard dosage is 15-37.5 mg once daily, taken in the morning to avoid insomnia. Treatment duration should be limited to 12 weeks due to potential for tolerance and side effects, although many practitioners use phentermine longer than 12 weeks in an off-label fashion 1. Phentermine works by suppressing appetite through increased norepinephrine in the central nervous system. It should be avoided in patients with uncontrolled hypertension, cardiovascular disease, hyperthyroidism, glaucoma, history of drug abuse, or during pregnancy. Regular monitoring of blood pressure, heart rate, and weight is essential during treatment, with follow-up visits recommended every 4 weeks to assess efficacy and side effects.
Some key points to consider when prescribing phentermine include:
- Monitoring blood pressure and heart rate periodically while taking phentermine
- Avoiding use in patients with a history of cardiovascular disease
- Using phentermine in conjunction with lifestyle modifications
- Limiting treatment duration to 12 weeks, although off-label use for longer durations may be considered
- Regular follow-up visits to assess efficacy and side effects.
From the FDA Drug Label
Phentermine Hydrochloride, USP 15 mg and 30 mg is indicated as a short-term (a few weeks) adjunct in a regimen of weight reduction based on exercise, behavioral modification and caloric restriction in the management of exogenous obesity for patients with an initial body mass index greater than or equal to 30 kg/m 2, or greater than or equal to 27 kg/m 2 in the presence of other risk factors (e.g., controlled hypertension, diabetes, hyperlipidemia). The usual adult dose is 15 mg to 30 mg at approximately 2 hours after breakfast for appetite control.
Starting Labs and Efficacy for Phentermine:
- The FDA label does not specify particular starting labs for phentermine.
- Efficacy is based on short-term weight reduction, with the understanding that tolerance to the anorectic effect usually develops within a few weeks 2.
- Key Considerations:
- Phentermine is indicated for patients with a BMI ≥ 30 kg/m² or ≥ 27 kg/m² with other risk factors.
- The drug should be used as part of a regimen including exercise, behavioral modification, and caloric restriction.
- Dosage should be individualized, with a usual adult dose of 15 mg to 30 mg daily.
- Patients should be monitored for potential side effects, including primary pulmonary hypertension, valvular heart disease, and abuse/dependence 2.
From the Research
Phentermine for Weight Loss
- Phentermine is a medication used for weight loss, often in combination with topiramate 3, 4, 5.
- Studies have shown that phentermine/topiramate combination therapy results in significant weight loss compared to placebo, with an average weight loss of 7.73 kg 5.
- The weight loss achieved with phentermine/topiramate is dose-dependent, with higher doses resulting in greater weight loss 3, 4, 5.
Efficacy of Phentermine
- Phentermine/topiramate therapy has been shown to result in significant reductions in waist circumference, fasting triglycerides, and fasting glucose levels 3, 4, 5.
- A systematic review and meta-analysis found that phentermine/topiramate therapy resulted in an average weight loss of 7.73 kg, with significant improvements in metabolic parameters 5.
- Another study found that phentermine use is associated with clinically significant weight loss in adults, with the medication being generally well tolerated 6.
Safety and Adverse Events
- Common adverse events associated with phentermine/topiramate therapy include paresthesia, dry mouth, constipation, dysgeusia, and insomnia 3, 4, 5.
- A systematic review and meta-analysis found that phentermine/topiramate therapy increased the risk of nervous system-related adverse events, including dysgeusia, paresthesia, and dizziness 5.
- Large-scale observational studies have not identified an increased risk of cardiovascular events or elevations in blood pressure associated with phentermine use 6.
Comparison with Other Weight Loss Medications
- A systematic review compared the effectiveness of semaglutide, liraglutide, orlistat, and phentermine for weight loss in obese individuals, finding that all agents presented distinct benefits and side effects 7.
- The review highlighted the importance of integrating pharmacotherapy with lifestyle interventions to achieve sustainable weight management and identified areas for future research to optimize therapeutic outcomes for individuals with obesity 7.