Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
FASTIN vs TEPANIL
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Sympathomimetic amine that promotes release of norepinephrine and dopamine from presynaptic nerve terminals in the hypothalamus, suppressing appetite.
TEPANIL (diethylpropion) is a sympathomimetic amine that acts as a norepinephrine reuptake inhibitor in the hypothalamus, increasing norepinephrine levels in the synaptic cleft, which stimulates beta-2 adrenergic receptors, leading to appetite suppression.
Short-term adjunct in exogenous obesity,Off-label: Attention deficit hyperactivity disorder (ADHD)
FDA-approved: Short-term (8-12 weeks) adjunctive therapy for weight management in patients with a body mass index (BMI) ≥30 kg/m² or BMI ≥27 kg/m² in the presence of obesity-related risk factors (e.g., hypertension, diabetes, dyslipidemia). Off-label: None commonly recognized.
30 mg orally once daily in the morning, administered as a single dose.
25 mg orally three times daily, 1 hour before meals, or 75 mg extended-release orally once daily in the morning.
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.
4-6 hours; clinical context: requires multiple daily dosing for sustained anorectic effect
Hepatic metabolism via CYP3A4 and CYP2D6; active metabolite phendimetrazine (for some formulations).
Hepatic metabolism via CYP450 isoenzymes, primarily N-dealkylation and deamination. Active metabolites include N-ethyl- and N,N-diethyl- derivatives.
Primarily renal (approximately 70-80% unchanged) and biliary/fecal (20-30% as metabolites). Urinary excretion is p H-dependent; acidic urine increases elimination.
Renal: 90% (as metabolites and unchanged drug), Fecal: <10%
Approximately 40-50% bound to plasma proteins (albumin).
30-40% bound to albumin
Approximately 3-5 L/kg. High Vd indicates extensive tissue distribution, including brain.
3-4 L/kg; indicates extensive tissue distribution
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.
Oral: 60-70% due to first-pass metabolism
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.
Contraindicated in end-stage renal disease. For GFR <30 m L/min: not recommended. For GFR 30-59 m L/min: use with caution and monitor for adverse effects.
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.
Contraindicated in severe hepatic impairment. For Child-Pugh Class B: reduce dose by 50% or consider alternative. For Child-Pugh Class A: no adjustment required.
Not recommended for pediatric patients under 16 years of age due to lack of safety and efficacy data.
Not recommended for use in children below 16 years of age due to lack of safety and efficacy data.
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.
Starting dose of 25 mg once daily in the morning, with slow titration upwards. Monitor for cardiovascular and psychiatric effects due to increased sensitivity.
None.
None.
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).
Pulmonary hypertension: Cases of primary pulmonary hypertension (PPH) have been reported; avoid in patients with known pulmonary hypertension.,Valvular heart disease: Association with regurgitant cardiac valvular disease; avoid in patients with structural cardiac abnormalities.,Serotonin syndrome: Risk when co-administered with serotonergic drugs or MAOIs; allow 14 days after MAOI discontinuation.,CNS stimulation: May cause dizziness, insomnia, and euphoria; avoid with alcohol or other CNS stimulants.,Tolerance/dependence: Tolerance develops with prolonged use; potential for psychological dependence; limit use to 8-12 weeks.,Hypertension: Monitor blood pressure; exacerbate pre-existing hypertension.
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.
History of pulmonary hypertension or valvular heart disease.,Hyperthyroidism.,Glaucoma.,Agitated states.,History of drug abuse.,Concurrent use or within 14 days of MAOIs.,Hypersensitivity to diethylpropion or sympathomimetic amines.,Pregnancy and lactation.
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.
Avoid caffeine, as it may increase stimulant effects and risk of palpitations. Avoid alcohol, which can potentiate CNS effects and increase seizure risk. Take with food if gastrointestinal upset occurs.
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.
Pregnancy Category X. TEPANIL (diethylpropion) is contraindicated in pregnant women due to anorectic effects potentially causing fetal malnutrition and growth restriction. First trimester exposure may increase risk of neural tube defects, though human data limited. Second and third trimester exposure may lead to reduced birth weight and neonatal withdrawal symptoms including irritability and tremors.
Excreted in human milk; M/P ratio not established. Potential for adverse effects in nursing infants (irritability, poor feeding). Contraindicated during breastfeeding.
Excreted into breast milk; milk-to-plasma ratio not established. Potential for adverse effects in nursing infants including irritability and feeding difficulties. Contraindicated in breastfeeding due to risk of infant exposure and lack of safety data.
Contraindicated in pregnancy; no dose adjustments recommended.
No dose adjustment is recommended or studied in pregnancy as drug is contraindicated. Pharmacokinetic changes in pregnancy (increased volume of distribution, enhanced hepatic metabolism) would likely reduce drug exposure; however, given fetal risks, use is not justified. Avoid use entirely.
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.
TEPANIL is a schedule IV controlled substance; assess for history of substance abuse before prescribing. Avoid use in patients with cardiovascular disease, hyperthyroidism, glaucoma, or agitated states. Monitor blood pressure and heart rate regularly. Use only for short-term (8-12 weeks) as tolerance develops. Do not combine with MAOIs or within 14 days of MAOI use. May cause insomnia; advise last dose before 6 PM.
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.
Take exactly as prescribed; do not increase dose or frequency due to abuse potential.,May cause dizziness or blurred vision; avoid driving until you know how this medicine affects you.,Report chest pain, shortness of breath, or leg swelling immediately.,Avoid alcohol and caffeine-containing products while taking this medication.,Do not stop abruptly; taper under medical supervision to avoid withdrawal symptoms.,Tell your doctor if you have a history of drug abuse or mental health issues.
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 FASTIN vs TEPANIL, answered by our medical review team.
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.. TEPANIL is a Sympathomimetic anorectic that works by TEPANIL (diethylpropion) is a sympathomimetic amine that acts as a norepinephrine reuptake inhibitor in the hypothalamus, increasing norepinephrine levels in the synaptic cleft, which stimulates beta-2 adrenergic receptors, leading to appetite suppression.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between FASTIN and TEPANIL depend on the specific clinical indication. These are agents from distinct pharmacological classes and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of FASTIN is: 30 mg orally once daily in the morning, administered as a single dose.. The standard adult dose of TEPANIL is: 25 mg orally three times daily, 1 hour before meals, or 75 mg extended-release orally once daily in the morning.. 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 FASTIN and TEPANIL 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. 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. TEPANIL is classified as Category C. Pregnancy Category X. TEPANIL (diethylpropion) is contraindicated in pregnant women due to anorectic effects potentially causing fetal malnutrition and growth restriction. First tr. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.