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. |
| Molecular Weight | 203.28 |
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 | Teratogenic risk cannot be ruled out; avoidance recommended due to potential for neural tube defects and cardiovascular anomalies. |
| 2nd trimester | Avoid use; risk of maternal hypertension and placental insufficiency. |
| 3rd trimester | Contraindicated due to risk of pulmonary hypertension, neonatal withdrawal, and growth restriction. |
Clinical note
MAOIs can cause hypertensive crisis High potential for abuse and dependence.
| FDA category | Contraindicated |
| Placental transfer | Crosses placenta; documented in animal studies and human case reports (e.g., umbilical cord levels 10-100% of maternal levels). |
| Breastfeeding |
■ FDA Black Box Warning
No FDA black box warning.
| Common Effects | Insomnia |
| Serious Effects |
PregnancyBreastfeedingHistory of cardiovascular disease (e.g., coronary artery disease, stroke, arrhythmias)Severe hypertensionHyperthyroidismGlaucomaAgitated statesHistory of drug abuseMonoamine oxidase inhibitor (MAOI) use within 14 daysHypersensitivity to sympathomimetics
| 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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| Excreted into breast milk; may cause irritability, poor feeding, and reduced milk production. Contraindicated during breastfeeding. |
| Lactation Rating | L5 - Contraindicated |
| 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. |
| 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. |
| Food/Dietary | Avoid high-fat meals as they may delay absorption. No specific food interactions, but concurrent use of alcohol may enhance CNS depressant effects. Maintain a calorie-restricted diet as prescribed. No tyramine restrictions like with some other weight-loss drugs. |
| Clinical Pearls | Phentermine resin complex is a controlled substance (CIV) for short-term obesity management; avoid in patients with history of cardiovascular disease, hyperthyroidism, glaucoma, or agitated states. Monitor blood pressure and heart rate; tolerance may develop, requiring drug holiday. Onset of action is 1-2 hours; duration 12-14 hours. Do not co-administer with MAOIs or within 14 days of their use. Risk of serotonin syndrome when combined with other serotonergic agents. |
| Patient Advice | Take capsule whole on an empty stomach or after breakfast; avoid late evening doses to prevent insomnia. · Do not crush or chew capsule; it is a long-acting formulation. · Report symptoms of chest pain, palpitations, shortness of breath, or leg edema immediately. · Avoid alcohol and other CNS stimulants; may increase blood pressure and heart rate. · This medication is only for short-term use (up to 12 weeks) as an adjunct to diet and exercise. · Do not discontinue abruptly; taper under medical supervision to avoid withdrawal. · Keep in a secure place; is a federally controlled substance and can be abused. · Inform all healthcare providers you are taking this medication before any surgery or procedure. |