IONAMIN
Clinical safety rating: caution
Comprehensive clinical and safety monograph for IONAMIN (IONAMIN).
Phentermine is a sympathomimetic amine that stimulates the release of norepinephrine from nerve terminals in the hypothalamus, leading to appetite suppression. It also inhibits monoamine oxidase, decreasing the metabolism of norepinephrine.
| Metabolism | Phentermine undergoes extensive hepatic metabolism via N-deamination and aromatic hydroxylation; CYP450 enzymes involved, but specific isoforms not well characterized. |
| Excretion | Renal (approximately 70% as unchanged drug and metabolites), with about 20-30% eliminated through biliary/fecal routes. |
| Half-life | Terminal elimination half-life is approximately 20-24 hours. In patients with renal impairment, half-life may be prolonged, necessitating dose adjustment. |
| Protein binding | Approximately 90-95% bound to plasma proteins, primarily albumin. |
| Volume of Distribution | Approximately 3-5 L/kg, indicating extensive tissue distribution with accumulation in adipose tissue. |
| Bioavailability | Oral bioavailability is approximately 75-85% due to first-pass metabolism. |
| Onset of Action | Oral administration: 1-2 hours to peak plasma concentration; clinical effect (appetite suppression) may be noted within 1-3 hours. |
| Duration of Action | Clinically effective appetite suppression typically lasts 6-12 hours after a single oral dose. Due to long half-life, once-daily dosing provides sustained effect over 24 hours. |
15-30 mg orally once daily in the morning, or as directed by a physician. Usually given as a single daily dose; maximum dose 30 mg/day.
| Dosage form | CAPSULE, EXTENDED RELEASE |
| Renal impairment | Contraindicated in severe renal impairment (GFR <30 mL/min). For moderate impairment (GFR 30-59 mL/min), use with caution and consider dose reduction; specific GFR-based guidelines not established. |
| Liver impairment | Contraindicated in severe hepatic impairment (Child-Pugh class C). Use with caution in moderate impairment (Child-Pugh class B) with dose reduction; specific dosing not defined. No adjustment needed for mild impairment (Child-Pugh class A). |
| Pediatric use | Not recommended for children under 12 years of age. For adolescents 12-17 years: 15 mg orally once daily in the morning; may increase to 30 mg/day if needed. |
| Geriatric use | Start at the lower end of dosing (15 mg once daily) due to potential for increased sensitivity. Monitor closely for adverse effects; titration should be conservative. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for IONAMIN (IONAMIN).
| Breastfeeding | Excreted into breast milk; M/P ratio not established. Potential for adverse effects in nursing infants, including irritability, insomnia, and feeding difficulties. Manufacturer advises discontinuing breastfeeding or avoiding drug. Use only if clearly needed and benefits outweigh risks. |
| Teratogenic Risk | FDA Pregnancy Category X. Contraindicated in pregnancy. First trimester: Increased risk of congenital anomalies, including neural tube defects and cardiac malformations. Second and third trimesters: Potential for fetal growth restriction, preterm delivery, and neonatal withdrawal symptoms. Use is not recommended at any trimester. |
■ FDA Black Box Warning
None.
| Serious Effects |
["History of cardiovascular disease (CAD, arrhythmias, stroke, uncontrolled hypertension).","Hyperthyroidism.","Glaucoma.","History of drug abuse or dependence.","Concomitant use or within 14 days of MAOIs.","Pregnancy (Category X) or nursing mothers.","Known hypersensitivity to sympathomimetic amines.","Anorexia nervosa or bulimia."]
| Precautions | ["Risk of primary pulmonary hypertension (rare, but potentially fatal).","Risk of valvular heart disease (associated with fenfluramine/ dexfenfluramine; not clearly linked to phentermine alone, but caution advised).","Cardiovascular effects: Avoid in patients with known or suspected hypertension, arrhythmias, or coronary artery disease.","Central nervous system effects: Insomnia, nervousness, dizziness, tremor.","Psychiatric reactions: May exacerbate symptoms of psychosis or bipolar disorder.","Abuse potential: Class IV controlled substance due to amphetamine-like properties.","Serotonin syndrome: Risk with concomitant serotonergic drugs (e.g., SSRIs, MAOIs).","Pregnancy: Category X; contraindicated due to risk of fetal harm.","Renal impairment: Use with caution in moderate to severe impairment."] |
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| Fetal Monitoring | Maternal: Monitor blood pressure, heart rate, weight, and signs of abuse/dependence. Fetal: Ultrasound monitoring for growth restriction if inadvertent exposure occurs in pregnancy. Neonatal: Observe for withdrawal symptoms if exposure near term. |
| Fertility Effects | May impair fertility in females due to inhibition of gonadotropin release and potential ovarian cycle disruption. In males, may reduce libido and cause erectile dysfunction. Reversible upon discontinuation. |