Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
BONTRIL PDM vs FASTIN
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Phentermine is a sympathomimetic amine that acts as an appetite suppressant by stimulating the release of norepinephrine and dopamine in the hypothalamus, reducing food intake. Topiramate is a sulfamate-substituted monosaccharide that enhances GABAergic activity and inhibits glutamatergic neurotransmission via AMPA/kainate receptors, leading to appetite suppression and increased energy expenditure.
Sympathomimetic amine that promotes release of norepinephrine and dopamine from presynaptic nerve terminals in the hypothalamus, suppressing appetite.
FDA-approved: Chronic weight management (BMI ≥30 kg/m² or ≥27 kg/m² with at least one weight-related comorbidity) as an adjunct to a reduced-calorie diet and increased physical activity.,Off-label: None widely recognized.
Short-term adjunct in exogenous obesity,Off-label: Attention deficit hyperactivity disorder (ADHD)
Oral: 5-10 mg once daily in the morning; maximum 20 mg/day. Oral disintegrating tablet: 5-10 mg once daily.
30 mg orally once daily in the morning, administered as a single dose.
Terminal elimination half-life is 12-15 hours in adults, prolonged to 20-30 hours in severe renal impairment (Cr Cl <30 m L/min).
Terminal elimination half-life is approximately 16-20 hours for the immediate-release formulation. With sustained-release forms, effective half-life may extend to 24-34 hours due to prolonged absorption. Clinical context: time to reach steady state is about 3-5 days.
Phentermine: primarily renal excretion (unchanged). Topiramate: metabolized by CYP3A4 (minor), but ~70% excreted unchanged in urine. Also undergoes hydrolysis and glucuronidation.
Hepatic metabolism via CYP3A4 and CYP2D6; active metabolite phendimetrazine (for some formulations).
Renal: ~70% (unchanged), Fecal: ~30% (biliary excretion of metabolites).
Primarily renal (approximately 70-80% unchanged) and biliary/fecal (20-30% as metabolites). Urinary excretion is p H-dependent; acidic urine increases elimination.
98% bound to albumin.
Approximately 40-50% bound to plasma proteins (albumin).
0.25-0.35 L/kg, indicating distribution primarily in extracellular fluid.
Approximately 3-5 L/kg. High Vd indicates extensive tissue distribution, including brain.
Oral: 65-75% (first-pass metabolism); IM: 85-95%.
Oral immediate-release: ~90% (high first-pass metabolism; absolute bioavailability is lower, but systemic exposure is adequate). Oral sustained-release: similar extent but with prolonged absorption.
GFR >30 m L/min: No adjustment. GFR 10-30 m L/min: Use with caution, reduce dose by 50%. GFR <10 m L/min: Contraindicated.
Contraindicated in severe renal impairment (e GFR <30 m L/min/1.73 m²). For moderate impairment (e GFR 30-59 m L/min/1.73 m²), reduce dose to 15 mg once daily.
Child-Pugh A: No adjustment. Child-Pugh B: Reduce dose by 50%. Child-Pugh C: Avoid use.
Contraindicated in Child-Pugh class C cirrhosis. In Child-Pugh class A or B, initiate at 15 mg once daily and titrate cautiously to maximum 30 mg once daily.
Children 6-12 years: 2.5-5 mg once daily; maximum 10 mg/day. Children >12 years: Same as adult dosing.
Not recommended for pediatric patients under 16 years of age due to lack of safety and efficacy data.
Initiate at 2.5 mg once daily; may increase to 5 mg if needed. Use with caution due to increased sensitivity.
Initiating at 15 mg once daily is recommended due to increased sensitivity and potential for central nervous system adverse effects; maximum dose 30 mg once daily.
No black box warning for the combination product. However, topiramate is associated with an increased risk of acute myopia and secondary angle closure glaucoma, and teratogenicity (cleft lip/palate with first-trimester exposure).
None.
Acute myopia and angle-closure glaucoma (topiramate); discontinue if symptoms occur.,Oligohidrosis and hyperthermia (topiramate), especially in pediatric use.,Fetal toxicity (topiramate): increased risk of oral clefts; contraception required for females of reproductive potential.,Suicidal behavior or ideation (topiramate).,Metabolic acidosis (topiramate): monitor serum bicarbonate.,Increase in heart rate (phentermine): use with caution in patients with cardiac disease.,Pulmonary hypertension (phentermine): rare but serious.,Dependence and abuse potential (phentermine, Schedule IV controlled substance).,Glaucoma angle closure risk.,Kidney stones (topiramate): hydrate to prevent.,Cognitive/neuropsychiatric effects (topiramate): difficulty with memory, concentration, or language.
Cardiovascular events (hypertension, tachycardia, stroke), psychiatric adverse effects (psychosis, dependence), primary pulmonary hypertension, valvular heart disease, tolerance, withdrawal symptoms, glaucoma, hyperthyroidism, seizure disorder, diabetes (dose adjustment required), elderly patients (higher sensitivity).
Glaucoma (angle-closure), especially with topiramate component.,Hyperthyroidism (phentermine).,Patients with a history of drug abuse (phentermine).,MAO inhibitor use within 14 days (phentermine).,Pregnancy (topiramate is teratogenic).,Breastfeeding (safety not established).,Known hypersensitivity to phentermine or topiramate.,Cardiovascular disease including arrhythmias, coronary artery disease, or uncontrolled hypertension (phentermine).,Concomitant use of other central nervous system stimulants.
Cardiovascular disease (e.g., coronary artery disease, arrhythmias, hypertension), hyperthyroidism, glaucoma, agitated states, history of drug abuse, MAOIs (concurrent or within 14 days), hypersensitivity to sympathomimetics.
Avoid alcohol and caffeine-containing products. High-fat meals may delay absorption. No other specific food restrictions, but follow a reduced-calorie diet as advised by your healthcare provider.
Avoid excessive caffeine intake (e.g., coffee, tea, cola, energy drinks) as it may potentiate CNS and cardiovascular effects. Grapefruit juice may alter drug metabolism; avoid concurrent consumption. Maintain a balanced, reduced-calorie diet as part of the weight loss plan. Alcohol should be avoided due to potential additive CNS effects.
First trimester: Category X. Contraindicated due to documented teratogenicity (neural tube defects, craniofacial malformations). Second/third trimester: Avoid due to risk of fetal hemorrhage and premature closure of ductus arteriosus.
FDA Pregnancy Category X. First trimester: Increased risk of oral clefts and cardiac malformations with amphetamine use. Second and third trimesters: Risk of premature delivery, low birth weight, and neonatal withdrawal syndrome. Avoid use in pregnancy.
Excreted into breast milk with M/P ratio of 0.8. Contraindicated during breastfeeding due to risk of infant toxicity (renal impairment, bleeding).
Excreted in human milk; M/P ratio not established. Potential for adverse effects in nursing infants (irritability, poor feeding). Contraindicated during breastfeeding.
No established safe dose due to teratogenicity. If inadvertent exposure occurs, immediate discontinuation recommended. No dose adjustment is feasible given contraindication.
Contraindicated in pregnancy; no dose adjustments recommended.
BONTRIL PDM (phendimetrazine tartrate) is a sympathomimetic amine anorectic. Monitor blood pressure and heart rate due to potential increases. Avoid use in patients with history of drug abuse, cardiovascular disease, hyperthyroidism, glaucoma, or MAOI use within 14 days. Taper to avoid abrupt discontinuation. Not recommended for pediatric patients or those with hypertension.
Fastin (phentermine) is a sympathomimetic amine indicated for short-term (up to 12 weeks) monotherapy for obesity. It should be used in conjunction with a reduced-calorie diet and exercise. Avoid co-administration with MAOIs or within 14 days of MAOI use due to hypertensive crisis risk. Use with caution in patients with hypertension, diabetes, or history of drug abuse. Monitor blood pressure and heart rate regularly. Tachyphylaxis may develop; discontinue if tolerance occurs. Do not use in patients with advanced arteriosclerosis, hyperthyroidism, glaucoma, or agitated states.
Take exactly as prescribed; do not exceed recommended dose.,Avoid driving or operating machinery until you know how this medication affects you.,Report chest pain, shortness of breath, or palpitations immediately.,Do not take with other stimulants or diet aids.,Inform your doctor if you become pregnant or plan to breastfeed.,Do not stop suddenly without consulting your doctor.
Take Fastin exactly as prescribed, usually once daily in the morning to avoid insomnia.,Do not crush or chew the extended-release capsule; swallow whole.,Avoid taking late in the day to prevent difficulty sleeping.,Report any chest pain, palpitations, shortness of breath, or dizziness immediately.,Do not increase dose or take more frequently than prescribed; risk of dependence and side effects.,Fastin is for short-term use only (up to 12 weeks) and should be combined with a reduced-calorie diet and exercise.,Do not use if you have taken an MAO inhibitor in the last 14 days.,Avoid alcohol and other CNS stimulants (e.g., caffeine in large amounts) as they may increase side effects.,Do not stop abruptly; follow your doctor's instructions for tapering off.,Keep out of reach of children; misuse can cause severe cardiac toxicity.
No interactions on record
No interactions on record
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about BONTRIL PDM vs FASTIN, answered by our medical review team.
BONTRIL PDM is a Sympathomimetic Anorectic that works by Phentermine is a sympathomimetic amine that acts as an appetite suppressant by stimulating the release of norepinephrine and dopamine in the hypothalamus, reducing food intake. Topiramate is a sulfamate-substituted monosaccharide that enhances GABAergic activity and inhibits glutamatergic neurotransmission via AMPA/kainate receptors, leading to appetite suppression and increased energy expenditure.. FASTIN is a Sympathomimetic Anorectic that works by Sympathomimetic amine that promotes release of norepinephrine and dopamine from presynaptic nerve terminals in the hypothalamus, suppressing appetite.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between BONTRIL PDM and FASTIN depend on the specific clinical indication. These are both Sympathomimetic Anorectic agents and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of BONTRIL PDM is: Oral: 5-10 mg once daily in the morning; maximum 20 mg/day. Oral disintegrating tablet: 5-10 mg once daily.. The standard adult dose of FASTIN is: 30 mg orally once daily in the morning, administered as a single dose.. Dosing should always be individualized based on indication, renal and hepatic function, age, and other patient factors.
No direct drug-drug interaction has been formally documented between BONTRIL PDM and FASTIN in current clinical databases. However, individual patient risk factors including other medications, organ function, and comorbidities should always be evaluated by a qualified healthcare provider.
The maternal-fetal safety profiles differ. BONTRIL PDM is classified as Category C. First trimester: Category X. Contraindicated due to documented teratogenicity (neural tube defects, craniofacial malformations). Second/third trimester: Avoid due to risk of fetal . FASTIN is classified as Category C. FDA Pregnancy Category X. First trimester: Increased risk of oral clefts and cardiac malformations with amphetamine use. Second and third trimesters: Risk of premature delivery, lo. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.