Phentermine Dosing Guidelines

Introduction to Phentermine Dosing

The recommended dosing for phentermine is 15 to 37.5 mg orally once daily in the morning, approximately 2 hours after breakfast, with the option to start at lower doses (as low as 8-9.375 mg) and titrate based on efficacy and tolerability 1, 2

The standard dose of phentermine is 15 to 37.5 mg once daily in the morning 1, 2
Avoid late evening dosing due to risk of insomnia 1, 2
Start low and titrate up to achieve adequate response with the lowest effective dose 3, 4, 5
A quarter tablet of 9.375 mg daily may be adequate for some patients 3, 4
A half tablet of 18.75 mg daily is an intermediate option 3, 4
Split dosing of half tablet (18.75 mg) twice daily is also an option 3, 4
The low-dose formulation (Lomaira) can be prescribed up to 3 times per day (approximately 30 minutes before meals) 1, 2, 6
Dosing can be as low as 4 mg using the scored tablets 6

Phentermine is approved for short-term use only (3 months) due to lack of long-term safety trials 1, 2, 7, 5
Phentermine is a Schedule IV controlled substance 8, 1, 2
Many practitioners prescribe phentermine for longer durations as off-label therapy for continued weight management 3, 7, 5, 6

Discontinue phentermine if inadequate weight loss is achieved 8, 3
If approximately 5% weight loss or less at 12 weeks, discontinue and consider alternative medication or other treatments 8

Absolute contraindications include history of cardiovascular disease, during or within 14 days of MAOI use, hyperthyroidism, glaucoma, pregnancy and nursing, history of drug abuse, and agitated states 8, 3, 7, 5

Ideal candidates for phentermine include younger patients without active or unstable coronary disease, no uncontrolled hypertension, patients needing appetite suppression assistance, and avoiding patients with anxiety or insomnia (may be exacerbated) 1, 2, 6, 7
Avoid phentermine in patients with cardiovascular disease, safer alternatives include lorcaserin and orlistat 8

Most frequent side effects include dry mouth, difficulty sleeping/insomnia, dizziness, irritability, constipation, and palpitations 8, 1, 3, 2, 6

Mean weight loss is 5.1% at 28 weeks compared with placebo 8
In a 28-week trial with 15 mg daily, 46% achieved ≥5% weight loss and 20.8% achieved ≥10% weight loss 1, 2, 4

The American Gastroenterological Association recommends measuring blood pressure and heart rate at baseline and periodically throughout phentermine treatment, as it causes sympathomimetic effects that can increase both parameters 9
The American Gastroenterological Association advises against increasing the phentermine dose in patients with uncontrolled hypertension or emerging cardiovascular symptoms 9

Consider switching to phentermine-topiramate ER (Qsymia) rather than uptitrating phentermine monotherapy, as this combination provides superior weight loss with a structured titration schedule, according to the American Gastroenterological Association 10

The American Gastroenterological Association (AGA) explicitly advises against increasing phentermine dose in patients with uncontrolled hypertension 11
The AGA also advises against increasing phentermine dose in patients with emerging cardiovascular symptoms during treatment 12
The AGA recommends avoiding phentermine use entirely in patients with a history of cardiovascular disease, as it is an absolute contraindication 11, 12

The AGA recommends considering a switch to phentermine-topiramate ER (Qsymia) rather than uptitrating phentermine monotherapy, as this combination provides superior weight loss with a structured titration schedule 11, 12
Phentermine-topiramate ER has demonstrated better efficacy in clinical trials, with blood pressure generally declining with treatment, and only modest heart rate increases at higher doses 11, 12

Phentermine causes sympathomimetic effects that can increase blood pressure and heart rate, and monitoring is necessary throughout treatment 11, 12