FASTIN
Clinical safety rating: caution
Comprehensive clinical and safety monograph for FASTIN (FASTIN).
Sympathomimetic amine that promotes release of norepinephrine and dopamine from presynaptic nerve terminals in the hypothalamus, suppressing appetite.
| Metabolism | Hepatic metabolism via CYP3A4 and CYP2D6; active metabolite phendimetrazine (for some formulations). |
| Excretion | Primarily renal (approximately 70-80% unchanged) and biliary/fecal (20-30% as metabolites). Urinary excretion is pH-dependent; acidic urine increases elimination. |
| Half-life | 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. |
| Protein binding | Approximately 40-50% bound to plasma proteins (albumin). |
| Volume of Distribution | Approximately 3-5 L/kg. High Vd indicates extensive tissue distribution, including brain. |
| Bioavailability | 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. |
| Onset of Action | Oral immediate-release: 1-2 hours; sustained-release: 2-4 hours. Time to peak appetite suppression. |
| Duration of Action | Immediate-release: 4-6 hours; sustained-release: 8-12 hours. Note: For sustained-release, clinical effect may persist up to 12-16 hours depending on individual metabolism. |
| Molecular Weight | 186.25 |
30 mg orally once daily in the morning, administered as a single dose.
| Dosage form | CAPSULE |
| Renal impairment | Contraindicated in severe renal impairment (eGFR <30 mL/min/1.73 m²). For moderate impairment (eGFR 30-59 mL/min/1.73 m²), reduce dose to 15 mg once daily. |
| Liver impairment | 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. |
| Pediatric use | Not recommended for pediatric patients under 16 years of age due to lack of safety and efficacy data. |
| Geriatric use | 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. |
| 1st trimester | Associated with increased risk of neural tube defects and cardiovascular malformations. Avoid use due to lack of safety data and potential for fetal harm. |
| 2nd trimester | May cause fetal dependence and withdrawal. Use only if potential benefit justifies risk to fetus. |
| 3rd trimester | May precipitate premature delivery, placental abruption, and neonatal withdrawal. Avoid use near term. |
Clinical note
Comprehensive clinical and safety monograph for FASTIN (FASTIN).
| Placental transfer | Crosses placenta; concentrations in fetal plasma may approach maternal levels. |
| Breastfeeding | Excreted into breast milk; may cause irritability and poor feeding in infants. Use with caution, weighing maternal need against potential risk to infant. |
■ FDA Black Box Warning
None.
| Serious Effects |
Advanced arteriosclerosisSymptomatic cardiovascular diseaseModerate to severe hypertensionHyperthyroidismGlaucomaAgitated statesHistory of drug abuseWithin 14 days of MAO inhibitor use
| Precautions | 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). |
| Food/Dietary | 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. |
Loading safety data…
| Lactation Rating |
| L3 (Moderately Safe) |
| Teratogenic Risk | 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. |
| Fetal Monitoring | Monitor maternal blood pressure, heart rate, and weight. Assess for signs of abuse or dependence. Fetal ultrasound for growth and anomalies if exposed. |
| Fertility Effects | May impair fertility in females due to altered hormone levels or weight loss; no direct evidence of male fertility impairment. |
| Clinical Pearls | 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. |
| Patient Advice | 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. |