Can a patient who previously took phentermine (phentermine) 37.5mg and stopped, restart at the same dose or should they decrease it?

Restarting Phentermine After Discontinuation

Patients who previously took phentermine 37.5mg can restart at a lower dose of 15-18.75mg rather than immediately resuming the full 37.5mg dose, with individualized titration based on tolerance and response.

Dosing Strategy for Restarting Phentermine

Initial Restart Dose

• Start with 15mg or lower (9.375-18.75mg) when reinitiating phentermine, even if the patient previously tolerated 37.5mg 1
• The recommended dosage range is 15-37.5mg orally once daily, but dosage should be individualized to achieve adequate response with the lowest effective dose 1
• For some patients, a quarter tablet (9.375mg) or half tablet (18.75mg) may be adequate 1

Rationale for Lower Restart Dose

• Starting lower minimizes cardiovascular risks including heart rate elevation and blood pressure changes that can occur with sympathomimetic agents 1
• Phentermine's effect can decline with time, and there is addiction potential, making cautious reinitiation prudent 2
• A lower starting dose allows assessment of current tolerance, as the patient's cardiovascular status or medication regimen may have changed during the discontinuation period 3

Titration Approach

Dose Escalation Strategy

• Begin with 15mg daily and assess response over 2-4 weeks 1
• If well-tolerated and additional weight loss is needed, can increase to 18.75mg, then to 37.5mg 1
• Alternative dosing includes split dosing of half tablet twice daily (e.g., 9.375mg BID) 1

Monitoring During Titration

• Monitor blood pressure and heart rate at each dose adjustment, as phentermine can cause modest increases in heart rate and potential blood pressure changes 1
• Assess for adverse effects including insomnia, dry mouth, constipation, and cardiovascular symptoms 1

Important Safety Considerations

Cardiovascular Precautions

• Avoid phentermine in patients with cardiovascular disease or uncontrolled hypertension 1
• Heart rate increases of 5-10 beats per minute have been observed, particularly at higher doses and at 9-12 months of treatment 4
• Blood pressure should be monitored regularly, though observational data suggest phentermine monotherapy does not demonstrate significant increases in most treated individuals 1

Duration of Therapy

• Phentermine is FDA-approved for short-term use (3 months), though many providers prescribe it longer as off-label therapy 1
• Studies show that approximately 40% of 3-month non-responders can achieve ≥5% body weight reduction if continued to 6 months 5
• However, approximately 10% of patients experience weight regain (potential tolerance) between 3-6 months 5

Common Pitfalls to Avoid

Drug-Drug Interactions

• Do not use phentermine during or within 14 days of monoamine oxidase inhibitors (MAOIs) due to risk of hypertensive crisis 1
• Avoid concurrent use with other sympathomimetic amines 1
• Be aware of potential moderate to severe drug interactions, particularly with tramadol (severe interaction), dextromethorphan (moderate), and other serotonergic agents 3

Timing and Administration

• Administer phentermine early in the day to minimize insomnia risk 1
• Some patients benefit from afternoon dosing (1:00 PM and 4:00 PM) to target high-risk eating periods 6

Contraindications

• Avoid in pregnancy, hyperthyroidism, glaucoma, agitated states, and history of drug abuse 1
• Use caution in geriatric populations due to limited high-quality data 1