PHENTERMINE RESIN COMPLEX
Clinical safety rating: avoid
MAOIs can cause hypertensive crisis High potential for abuse and dependence.
Phentermine resin complex is a sympathomimetic amine that stimulates the hypothalamus to release norepinephrine, suppressing appetite. It may also inhibit neuronal reuptake of norepinephrine and dopamine in the arcuate nucleus, reducing food intake.
| Metabolism | Phentermine is extensively metabolized in the liver via oxidative deamination and hydroxylation to inactive metabolites, including p-hydroxy-phentermine and ketone derivatives. CYP450 isoenzymes may have a minor role; exact pathways not fully characterized. |
| Excretion | Primarily renal elimination as unchanged drug and metabolites; approximately 80% of a dose is excreted in urine (10-40% unchanged, remainder as metabolites), with the rest in feces via biliary elimination. |
| Half-life | Terminal elimination half-life is approximately 19-24 hours for the resin complex formulation due to sustained release, allowing once-daily dosing. |
| Protein binding | Approximately 90-95% bound to plasma proteins, primarily albumin. |
| Volume of Distribution | Volume of distribution is 3-4 L/kg, indicating extensive tissue distribution. |
| Bioavailability | Oral bioavailability of phentermine resin complex is approximately 70-80%, with lower peak concentrations compared to immediate-release formulations due to controlled release. |
| Onset of Action | Oral: Onset of appetite suppression occurs within 1-2 hours after dosing. |
| Duration of Action | Duration of appetite suppression is approximately 12-14 hours after oral administration of the resin complex. |
Oral: 15-30 mg once daily before breakfast or 1-2 hours after breakfast.
| Dosage form | CAPSULE, EXTENDED RELEASE |
| Renal impairment | Contraindicated in severe renal impairment (CrCl <30 mL/min). For mild to moderate impairment, use with caution and consider lower doses. |
| Liver impairment | Contraindicated in severe hepatic impairment (Child-Pugh class C). For mild to moderate (Child-Pugh A or B), use with caution and consider dose reduction. |
| Pediatric use | Not recommended for use in pediatric patients under 16 years of age. |
| Geriatric use | Use with caution due to increased sensitivity; consider starting at lower doses (e.g., 15 mg daily) and monitor for adverse effects. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
MAOIs can cause hypertensive crisis High potential for abuse and dependence.
| FDA category | Contraindicated |
| Breastfeeding | It is unknown whether phentermine is excreted in human breast milk. Due to potential for serious adverse reactions in nursing infants, including CNS stimulation and cardiovascular effects, breast-feeding is not recommended during therapy. No M/P ratio is available. |
| Teratogenic Risk | Phentermine resin complex is FDA Pregnancy Category X. Studies in animals have demonstrated teratogenic effects, and there are no adequate studies in pregnant women. The drug is contraindicated during pregnancy due to risk of fetal harm, including potential for congenital malformations and embryotoxicity. Use should be avoided in all trimesters. |
■ FDA Black Box Warning
No FDA black box warning.
| Common Effects | Insomnia |
| Serious Effects |
["Pregnancy (Category X; may cause fetal harm) and breastfeeding.","During or within 14 days of MAO inhibitor therapy (hypertensive crisis risk).","History of cardiovascular disease (e.g., coronary artery disease, stroke, arrhythmias, congestive heart failure, uncontrolled hypertension).","Hyperthyroidism.","Glaucoma.","Agitated states or history of drug abuse.","Diabetes mellitus type 2 (relative contraindication due to potential for increased insulin resistance).","Concomitant use with other CNS stimulants or serotonergic drugs (increased risk of pulmonary hypertension/valvulopathy).","History of pulmonary hypertension or valvular heart disease.","Hypersensitivity to sympathomimetic amines."]
| Precautions | ["Primary pulmonary hypertension: Rare, potentially fatal; discontinue if new-onset dyspnea, angina, or lower extremity edema.","Valvular heart disease: Associated with serotonergic drugs; avoid combination with other serotonergic agents (e.g., SSRIs, MAOIs).","Cardiovascular effects: May increase heart rate and blood pressure; use with caution in hypertension, arrhythmias, or coronary artery disease.","CNS stimulation: May cause insomnia, dizziness, tremor; avoid driving until effects known.","Tolerance and dependence: Tolerance may develop; do not exceed recommended dose due to abuse potential.","Abrupt discontinuation: May cause extreme fatigue or depression; taper if possible.","Psychiatric effects: Use caution in patients with history of depression, bipolar disorder, or psychosis.","Seizure threshold: Theoretical risk of lowering seizure threshold; use caution with anticonvulsants.","Renal impairment: Use with caution in moderate to severe impairment; avoid in end-stage renal disease."] |
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| Fetal Monitoring | If inadvertently used during pregnancy, fetal ultrasound to assess for anomalies. Monitor maternal blood pressure and heart rate due to sympathomimetic effects. Assess for fetal distress with non-stress test or biophysical profile if exposure occurred later in pregnancy. |
| Fertility Effects | No specific studies on human fertility. In animal studies, no adverse effects on fertility were noted. However, as a CNS stimulant and anorectic, it may impact ovulation through metabolic effects; clinical significance is uncertain. |