What is the appropriate dosing to initiate phentermine (Adipex-P) therapy?

From the Guidelines

Phentermine should be started at a dose of 15 mg orally once daily in the morning, as this dosage has been associated with a 6.0-kg weight loss at 28 weeks, compared with a placebo 1. The recommended dosage of phentermine is 15 to 37.5 mg orally once daily in the morning, with the option to adjust based on response and tolerability.

• The medication should be taken early in the day to avoid insomnia.
• Phentermine is approved for short-term use (usually up to 12 weeks) as an adjunct to a comprehensive weight management program that includes dietary changes and increased physical activity.
• Before starting phentermine, patients should undergo a thorough medical evaluation to ensure they don't have contraindications such as cardiovascular disease, hyperthyroidism, glaucoma, history of drug abuse, or concurrent use of MAOIs.
• Blood pressure and heart rate should be monitored regularly during treatment, as phentermine can cause a mild increase in heart rate and blood pressure 1.
• Common side effects of phentermine include dry mouth, difficulty sleeping, dizziness, and irritability, and should be discussed with patients before initiating therapy 1.
• Appropriate candidates for phentermine include patients with obesity who need assistance with appetite suppression, particularly those without evidence of active or unstable coronary disease, or uncontrolled hypertension 1.

From the FDA Drug Label

The usual adult dose is 15 mg to 30 mg at approximately 2 hours after breakfast for appetite control. Administration of one capsule (30 mg) daily has been found to be adequate in depression of the appetite for 12 to 14 hours. The appropriate dosing to start phentermine is 15 mg to 30 mg at approximately 2 hours after breakfast for appetite control 2 2.

• The dose should be individualized to obtain an adequate response with the lowest effective dose.
• Late evening medication should be avoided because of the possibility of resulting insomnia.

From the Research

Phentermine Dosing

The appropriate dosing for phentermine can vary depending on the specific formulation and combination with other medications.

• In the study 3, patients were assigned to receive either phentermine 7.5 mg plus topiramate 46.0 mg or phentermine 15.0 mg plus topiramate 92.0 mg once daily.
• Another study 4 mentions that phentermine hydrochloride is approved for short-term treatment of obesity at doses as high as 37.5 mg/day.
• A retrospective study 5 analyzed data from patients taking phentermine alone, with no specific dose mentioned.

Combination Therapy

Phentermine is often used in combination with other medications, such as topiramate, to enhance its weight-loss effects.

• The study 3 found that the combination of phentermine and topiramate resulted in significant weight loss and improvement in metabolic risk factors.
• Another study 6 describes phentermine and topiramate extended-release as a new treatment for obesity, which can produce and sustain approximately 10% loss of body weight.
• The combination of phentermine and fenfluramine was also studied 5, but this combination is no longer commonly used due to safety concerns.

Safety and Efficacy

The safety and efficacy of phentermine have been evaluated in various studies.

• The study 3 reported common adverse events such as dry mouth, paraesthesia, and constipation, but also found that the combination of phentermine and topiramate was generally well-tolerated.
• Another study 7 found that phentermine monotherapy was effective and safe for weight loss in pediatric patients when combined with lifestyle interventions.
• The study 5 found that phentermine alone was effective for weight loss when combined with a very-low-calorie diet, with no significant differences in adverse events or dropout rate compared to the combination of phentermine and fenfluramine.