TEGRETOL-XR
Clinical safety rating: caution
Comprehensive clinical and safety monograph for TEGRETOL-XR (TEGRETOL-XR).
Carbamazepine stabilizes inactivated state of voltage-gated sodium channels, thereby inhibiting repetitive neuronal firing and reducing synaptic transmission.
| Metabolism | Hepatic via CYP3A4; also metabolized by CYP2C8. Carbamazepine induces its own metabolism and that of other drugs via CYP3A4. |
| Excretion | Renal: ~72% as unchanged drug and metabolites (primarily glucuronides). Fecal: ~28% via bile (enterohepatic recirculation possible). |
| Half-life | Initial: 25-65 hours; chronic dosing: 12-17 hours due to autoinduction. Steady-state reached in 2-4 weeks. |
| Protein binding | 75-90% primarily to albumin; binding is concentration-dependent and saturable at high doses. |
| Volume of Distribution | 0.8-1.4 L/kg; approximately 0.8 L/kg in adults. Signifies distribution into total body water and tissues. |
| Bioavailability | Extended-release tablets: ~89% relative to immediate-release; food does not affect extent but may delay absorption (Cmax reduced by ~30%). |
| Onset of Action | Oral extended-release: 1-2 weeks for therapeutic anticonvulsant effect; acute analgesic effect for trigeminal neuralgia: 24-48 hours. |
| Duration of Action | Extended-release formulation maintains therapeutic levels for 12-24 hours with twice-daily dosing. Autoinduction may reduce duration over time. |
| Molecular Weight | 236.27 |
200-400 mg orally twice daily; maximum 1200 mg/day for monotherapy, 1600 mg/day for combination therapy.
| Dosage form | TABLET, EXTENDED RELEASE |
| Renal impairment | CrCl <10 mL/min: avoid use. CrCl 10-50 mL/min: reduce dose by 25-50% and monitor levels. |
| Liver impairment | Child-Pugh A: no adjustment. Child-Pugh B: reduce dose by 50%. Child-Pugh C: contraindicated. |
| Pediatric use | 6-12 years: initial 100 mg twice daily, increase by 100 mg/day weekly; maintenance 15-30 mg/kg/day in 2 divided doses. <6 years: 10-20 mg/kg/day in 2-3 divided doses; maximum 35 mg/kg/day. |
| Geriatric use | Start at lower end of dosing range (200 mg/day) due to increased sensitivity; monitor for hyponatremia and ataxia. |
| 1st trimester | Carbamazepine is teratogenic; risk of neural tube defects, craniofacial anomalies, and developmental delay. Use lowest effective dose, supplement with folic acid 4-5 mg/day. |
| 2nd trimester | Monitor for fetal growth restriction and bleeding due to vitamin K deficiency. Consider prenatal vitamin K supplementation near term. |
| 3rd trimester | Neonatal hemorrhage and withdrawal syndrome reported. Administer vitamin K to neonate at birth. |
Clinical note
Comprehensive clinical and safety monograph for TEGRETOL-XR (TEGRETOL-XR).
| Placental transfer | Crosses placenta readily, reaching fetal concentrations 50-80% of maternal serum levels. |
| Breastfeeding | Carbamazepine is excreted into breast milk in low concentrations (2-5% of maternal dose). Cases of neonatal hepatotoxicity and transient cholestatic hepatitis reported; monitor infant for jaundice, drowsiness, and poor feeding. Benefit generally outweighs risk. |
■ FDA Black Box Warning
Aplastic anemia and agranulocytosis have been reported in association with carbamazepine therapy. Patients should be informed of the early signs and symptoms of hematologic disorders.
| Serious Effects |
Hypersensitivity to carbamazepine or tricyclic antidepressantsHistory of bone marrow suppressionConcomitant use of MAOIs (within 14 days)AV heart blockPorphyria
| Precautions | Hematologic toxicity: monitor CBC at baseline and periodically, Severe dermatologic reactions (e.g., SJS, TEN, DRESS) especially in HLA-B*1502-positive patients, Hyponatremia, SIADH, Hepatic effects: monitor LFTs, Cardiovascular effects: AV block, bradycardia, Suicidal ideation/behavior, Central nervous system effects: dizziness, drowsiness, ataxia, Increased intraocular pressure, Potential for fetal harm (pregnancy category D), Drug interactions: induces CYP3A4, inhibits CYP2C9/2C19 |
| Food/Dietary | Grapefruit juice increases carbamazepine levels and should be avoided. High-fat meals may alter absorption; take consistently with meals. Avoid alcohol due to additive CNS depression. |
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| Lactation Rating | L3 (Moderately Safe) |
| Teratogenic Risk | Pregnancy Category D. First trimester exposure increases risk of neural tube defects (spina bifida, 1%), craniofacial anomalies, and congenital heart defects. Second and third trimester exposure may cause fetal anticonvulsant syndrome (growth retardation, developmental delay) and neonatal hemorrhage due to vitamin K deficiency. Risk of major malformations is 2-3 times baseline. |
| Fetal Monitoring | Maternal: serum carbamazepine levels (target 4-12 mcg/mL), CBC, liver function tests, sodium levels. Fetal: high-resolution ultrasound at 18-20 weeks to detect neural tube defects; consider maternal serum alpha-fetoprotein screening. Neonatal: monitor for withdrawal symptoms, coagulation abnormalities (administer vitamin K at birth). |
| Fertility Effects | Carbamazepine may reduce efficacy of oral contraceptives due to CYP3A4 induction, leading to unintended pregnancy. No direct impairment of fertility in females; in males, has been associated with reversible decreases in sperm motility and count. |
| Clinical Pearls | Carbamazepine extended-release (Tegretol-XR) is an enzyme inducer of CYP3A4, CYP2C9, and CYP2C19, leading to reduced efficacy of oral contraceptives, warfarin, and many antiretrovirals. Monitor serum levels (therapeutic range 4-12 mcg/mL) due to autoinduction and variable absorption. Avoid concurrent use with grapefruit juice. Use with caution in patients with HLA-B*1502 allele (Asian ancestry) due to risk of Stevens-Johnson syndrome. Do not crush or chew tablets; swallow whole to maintain extended-release properties. |
| Patient Advice | Swallow Tegretol-XR tablets whole; do not crush, chew, or split them. · Take the medication at the same times each day with or without food. · Report any signs of serious skin reactions (rash, blisters, mouth sores) immediately. · Do not stop taking abruptly as it may cause seizures. · Avoid grapefruit and grapefruit juice while on this medication. · Carbamazepine can reduce effectiveness of birth control pills; use additional non-hormonal contraception. · May cause dizziness or drowsiness; avoid driving until you know how it affects you. · Notify your doctor if you experience fever, sore throat, or unusual bleeding/bruising. |