What are the age limits for using phentermine (phentermine) for weight loss?

Age Limits for Phentermine Use in Weight Loss

Phentermine is indicated for adults 18 years of age or older with overweight or obesity, and should not be used in patients 16 years of age and under. 1

FDA-Approved Indications and Age Restrictions

• Phentermine is approved as a short-term (a few weeks) adjunct for weight reduction in adults with an initial BMI ≥30 kg/m², or ≥27 kg/m² with weight-related risk factors (e.g., controlled hypertension, diabetes, hyperlipidemia) 1
• The FDA label explicitly states that "phentermine is not recommended for use in patients 16 years of age and under" 1
• Clinical guidelines consistently refer to phentermine use in "adults 18 years of age or older with overweight or obesity" 2

Upper Age Considerations

• While there is no specific upper age limit stated in the FDA labeling, special considerations apply to geriatric patients 1
• Use phentermine with caution in elderly patients due to substantial renal excretion of the medication 1
• Clinical trials for phentermine-topiramate extended-release enrolled subjects up to age 70, but high-quality data specifically guiding phentermine use in the geriatric population is limited 2

Dosing Considerations Based on Age and Other Factors

• For adults, dosage should be individualized to obtain adequate response with the lowest effective dose 1
• The usual adult dose is 15-30 mg taken approximately 2 hours after breakfast 1
• For elderly patients or those with renal impairment, lower doses may be appropriate 1
• For patients with severe renal impairment (eGFR 15-29 mL/min/1.73 m²), the maximum recommended dose is 15 mg daily 1

Safety Considerations Relevant to Age

• Phentermine should be avoided in patients with:

  • History of cardiovascular disease 1
  • Uncontrolled hypertension 1
  • Hyperthyroidism 1
  • Glaucoma 1
  • History of drug abuse 1
  • During pregnancy or nursing 1

• Blood pressure and heart rate should be monitored periodically in all patients, but this is particularly important in older adults who may have underlying cardiovascular issues 3

Duration of Treatment

• Phentermine is FDA-approved for short-term use (typically 12 weeks) 3, 1
• Many clinicians use phentermine for longer periods (beyond 12 weeks) as off-label therapy for continued weight management 2, 3
• Mexican clinical practice guidelines discuss phentermine use for six months or longer compared to lifestyle interventions 2
• Studies have shown efficacy and acceptable safety profiles for up to 6 months in adult populations 4

Common Pitfalls and Caveats

• Phentermine should not be confused with the previously available "fen-phen" combination (phentermine with fenfluramine), which was associated with valvular heart disease 3
• Late evening administration should be avoided due to the risk of insomnia 1, 5
• Documentation of specific benefits, tolerance, and adverse effects is recommended if using phentermine long-term (beyond the FDA-approved duration) 3
• Phentermine should not be used concomitantly with MAOIs or within 14 days of their administration 1

In summary, phentermine is indicated only for adults 18 years and older, is not recommended for patients 16 years and under, and should be used with caution in elderly patients due to renal excretion considerations.