How should phentermine be dosed, timed, and monitored for short‑term weight loss in an adult with BMI ≥ 30 kg/m² (or ≥ 27 kg/m² with comorbidities)?

How to Take Phentermine for Short‑Term Weight Loss

Phentermine should be taken once daily in the morning (15–37.5 mg) or as low‑dose 8 mg up to three times daily, always as an adjunct to a reduced‑calorie diet and increased physical activity, with mandatory cardiovascular screening before initiation and periodic blood‑pressure monitoring throughout treatment.

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Patient Eligibility and Pre‑Treatment Screening

BMI Thresholds

• Adults with BMI ≥ 30 kg/m² qualify for phentermine without additional requirements. 1
• Adults with BMI ≥ 27 kg/m² qualify only if they have at least one weight‑related comorbidity (hypertension, dyslipidemia, or type 2 diabetes). 1

Mandatory Cardiovascular Assessment

• Obtain baseline blood pressure and heart rate measurements before starting phentermine because the drug activates the sympathetic nervous system and can raise both parameters. 2
• Screen for cardiovascular contraindications: active coronary artery disease, stroke, arrhythmias, congestive heart failure, or uncontrolled hypertension (systolic ≥ 140 mm Hg or diastolic ≥ 90 mm Hg). 3
• Younger patients without active coronary disease are more appropriate candidates for phentermine therapy. 2

Additional Contraindication Screening

• Exclude hyperthyroidism before initiating phentermine because sympathomimetic effects can precipitate arrhythmias or seizures. 2, 3
• Assess for glaucoma because sympathomimetic agents may trigger angle‑closure glaucoma. 2, 3
• Evaluate for agitated states or anxiety disorders because phentermine's stimulant properties can worsen these conditions. 1, 2, 3
• Confirm the patient is not taking monoamine oxidase inhibitors (MAOIs) or has not used them within the past 14 days, as concurrent use creates a risk of hypertensive crisis. 2, 3
• Verify pregnancy status in all women of reproductive potential because phentermine is contraindicated in pregnancy. 2, 3

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Dosing and Administration

Standard Dosing Regimen

• Start phentermine 15 mg or 37.5 mg orally once daily in the morning to minimize insomnia risk. 2, 3
• Individualize the dose to achieve adequate response with the lowest effective dose. 3
• Phentermine can be taken with or without food because absorption is not significantly affected by meals. 2

Low‑Dose Alternative

• Low‑dose phentermine 8 mg tablets can be prescribed up to three times daily (e.g., at 1:00 p.m. and 4:00 p.m.) to target high‑risk afternoon and evening eating periods. 2, 4
• Scored 8 mg tablets allow doses as low as 4 mg for patients who require minimal dosing. 2

Renal Impairment

• Limit the dosage to 15 mg daily for patients with severe renal impairment (eGFR 15–29 mL/min/1.73 m²) because phentermine undergoes substantial renal excretion. 3

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Treatment Duration and Monitoring

FDA‑Approved Duration

• Phentermine is FDA‑approved only for short‑term use (up to 12 weeks) as an adjunct to caloric restriction and exercise. 2, 3
• Many experienced clinicians prescribe phentermine off‑label for 3–6 months or longer because obesity is now understood as a chronic metabolic disease requiring long‑term management. 1, 2

Cardiovascular Monitoring

• Measure blood pressure and heart rate at every clinical visit throughout treatment because phentermine causes mild sympathomimetic‑mediated increases in these parameters. 2
• Observational data show that systolic and diastolic blood pressure often decrease during phentermine therapy (average reductions of approximately ‑7.3 mm Hg systolic and ‑5.4 mm Hg diastolic at 52 weeks), likely secondary to weight loss. 2

Efficacy Assessment

• Discontinue phentermine if weight loss is < 5% after 12 weeks on the maximum dose because early non‑responders are unlikely to benefit from continued therapy. 2, 5
• Expected outcomes with phentermine 15 mg daily include mean weight loss of approximately 5.1% at 28 weeks, with 46% of patients achieving ≥ 5% weight loss and 20.8% achieving ≥ 10% weight loss. 2

Laboratory Testing

• Routine metabolic panels, lipid profiles, liver function tests, or other laboratory studies are not mandated by current phentermine monitoring guidelines; focus should remain on cardiovascular parameters and weight‑loss efficacy. 2

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Lifestyle Interventions (Mandatory Adjuncts)

• Phentermine must never be prescribed as stand‑alone treatment and should always be combined with behavioral interventions involving medical nutrition therapy, physical activity, and stress management. 1
• Patients should follow a reduced‑calorie diet (approximately 500‑kcal daily deficit below maintenance requirements). 1
• Patients should engage in at least 150 minutes per week of moderate‑intensity physical activity. 1

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Safety Considerations and Adverse Effects

Common Side Effects

• Dry mouth, insomnia, headache, constipation, anxiety, and palpitations occur commonly and are related to sympathomimetic stimulation. 5
• Dry mouth can be managed with adequate hydration. 5
• Insomnia can be minimized by avoiding late evening dosing. 5

Serious Adverse Effects Requiring Immediate Discontinuation

• Dyspnea (shortness of breath), angina pectoris, syncope, or severe hypertension may indicate cardiovascular compromise and warrant immediate cessation of phentermine. 5, 3
• Persistent severe abdominal pain may signal pancreatitis or other serious gastrointestinal complications. 5

Drug Interactions

• Phentermine should not be used with MAOIs or other sympathomimetic amines due to the risk of serious interactions. 2, 3
• Concomitant alcohol use may result in an adverse drug reaction. 3

Special Populations Requiring Caution

• Individuals with anxiety disorders or depression, those taking antidepressants, older adults, those at risk for cardiovascular disease, and those at risk for glaucoma require more cautious use and closer monitoring. 1

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Treatment Continuation and Cycling

Decision‑Making for Extended Use

• Continue phentermine if ≥ 5% weight loss is achieved and no cardiovascular contraindications develop because there is no requirement to stop at 12 weeks. 2
• Treatment decisions should be based on efficacy and ongoing cardiovascular safety monitoring rather than arbitrary time limits. 2
• No mandatory waiting period is required before restarting phentermine if it was previously discontinued for reasons other than safety concerns; repeat baseline cardiovascular assessment and screen for new contraindications before restarting. 2

Discontinuation Criteria

• Development of cardiovascular disease, uncontrolled hypertension, or other absolute contraindications mandates immediate discontinuation. 2
• Inadequate weight loss (< 5% after 12 weeks on maximum dose) or intolerable side effects that do not resolve with dose reduction are reasons for discontinuation. 2
• Pregnancy or planning pregnancy is an absolute contraindication for phentermine use. 2, 3

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Cost and Access Considerations

• Phentermine is low‑cost and widely available, making it especially useful for patients with lower income and those who lack access to newer, costlier medications. 1
• The low cost and wide availability favor the use of phentermine for 6 months or more in most patients when balanced against equity and acceptability considerations. 1

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Common Pitfalls to Avoid

• Do not confuse phentermine monotherapy with the discontinued "fen‑phen" combination (phentermine‑fenfluramine); valvular heart disease and pulmonary hypertension were attributed to fenfluramine, not phentermine. 2
• Do not prescribe phentermine to patients with any history of cardiovascular disease because safer alternatives (e.g., orlistat, GLP‑1 receptor agonists) should be considered. 2
• Do not delay follow‑up beyond 12 weeks to assess initial treatment response, as early non‑responders are unlikely to benefit from continued therapy. 2
• Do not overlook the need to counsel patients that phentermine is FDA‑approved only for short‑term use (3 months), though off‑label longer duration is common in clinical practice. 2