OBY-TRIM
Clinical safety rating: caution
Comprehensive clinical and safety monograph for OBY-TRIM (OBY-TRIM).
OBY-TRIM is a combination of phentermine, a sympathomimetic amine anorectic that stimulates the hypothalamus to release norepinephrine, suppressing appetite, and a diuretic (often a thiazide) to reduce fluid retention.
| Metabolism | Phentermine: primarily hepatic metabolism via CYP450 isoenzymes (CYP3A4, CYP2D6); diuretic: varies (e.g., thiazide is not extensively metabolized). |
| Excretion | Primarily renal (90% as unchanged drug and metabolites), with about 10% biliary/fecal. In an acidic urine, elimination half-life increases. |
| Half-life | Terminal half-life is approximately 20–30 hours in adults with normal renal function. This allows for once-daily dosing in most patients. |
| Protein binding | Approximately 90–95% bound to plasma proteins, mainly albumin. |
| Volume of Distribution | Vd ~3–5 L/kg, indicating extensive tissue distribution, particularly to lungs and placenta. |
| Bioavailability | Oral: ~60–70% due to first-pass metabolism. |
| Onset of Action | Oral: 1–2 hours. Peak effect achieved within 4–6 hours. |
| Duration of Action | Clinical effects last approximately 12–24 hours after a single oral dose. With repeated dosing, effects persist throughout the dosing interval. |
Oral: 1 capsule (phentermine 18.75 mg / chlorpheniramine 6.25 mg) twice daily, 30 minutes before meals.
| Dosage form | CAPSULE |
| Renal impairment | Contraindicated if GFR < 30 mL/min. For GFR 30-89 mL/min, no dose adjustment required; monitor for electrolyte imbalances. |
| Liver impairment | Child-Pugh A: No adjustment. Child-Pugh B: Use with caution; reduce dose by 50%. Child-Pugh C: Contraindicated. |
| Pediatric use | Not recommended for patients under 18 years of age due to lack of safety and efficacy data. |
| Geriatric use | Use with caution due to increased sensitivity and higher risk of cardiovascular effects. Start at lowest effective dose; monitor blood pressure and heart rate. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for OBY-TRIM (OBY-TRIM).
| Breastfeeding | Excretion into breast milk unknown for the combination. Topiramate is present in human milk at concentrations similar to maternal plasma; M/P ratio approximately 0.86. Risk of infant sedation, irritability, and poor feeding. Use during breastfeeding is not recommended. |
| Teratogenic Risk | OBY-TRIM (phentermine and topiramate extended-release) is contraindicated in pregnancy. First trimester exposure increases risk of oral clefts (topiramate). Risk of hypospadias from phentermine is unconfirmed. Second and third trimester exposure may cause fetal growth restriction, metabolic acidosis, and electrolyte disturbances. |
■ FDA Black Box Warning
None.
| Serious Effects |
["Hypersensitivity to phentermine or other sympathomimetic amines","Concurrent use or within 14 days of MAOIs","History of cardiovascular disease (e.g., coronary artery disease, stroke, arrhythmias)","Hyperthyroidism","Glaucoma","Agitated states","History of drug abuse","Pregnancy or lactation","Use in children under 16 years"]
| Precautions | ["Risk of primary pulmonary hypertension (PPH) and valvular heart disease with serotonergic anorectics","Tolerance to anorectic effect may develop; do not exceed recommended dose","May impair ability to drive or operate machinery due to CNS stimulation","Monitor blood pressure and heart rate; use with caution in hypertension","Risk of serotonin syndrome when combined with MAOIs, SSRIs, or other serotonergic drugs","Rebound weight gain after discontinuation","Abuse potential due to phentermine's amphetamine-like effects"] |
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| Fetal Monitoring | Monitor maternal weight, blood pressure, heart rate, electrolytes, and renal function. Fetal surveillance includes ultrasound for growth restriction and screening for structural anomalies. Assess for signs of metabolic acidosis in mother and newborn. |
| Fertility Effects | Topiramate may reduce efficacy of oral contraceptives due to enzyme induction; advise non-hormonal contraception. Phentermine effects on fertility not established. Weight loss from therapy may improve fertility in some women. |