GEMONIL
Clinical safety rating: caution
Comprehensive clinical and safety monograph for GEMONIL (GEMONIL).
Barbiturate that enhances GABA-A receptor activity, increasing chloride ion conductance and producing CNS depression.
| Metabolism | Hepatic via CYP2C9 and CYP2C19; induces multiple CYP enzymes. |
| Excretion | Primarily renal excretion of unchanged drug and metabolites. Approximately 60-70% excreted unchanged in urine within 24 hours; 15-20% as conjugated metabolites; fecal elimination accounts for <5%. |
| Half-life | Terminal elimination half-life is 50-70 hours in adults with normal renal function. Significantly prolonged in renal impairment, requiring dose adjustment. |
| Protein binding | Approximately 85-90% bound primarily to albumin. |
| Volume of Distribution | 0.8-1.2 L/kg. Indicates extensive tissue distribution, including brain and adipose tissue. |
| Bioavailability | Oral: 80-90% due to minimal first-pass metabolism; Intramuscular: approximately 90%. |
| Onset of Action | Oral: 30-60 minutes; Intramuscular: 15-30 minutes; Intravenous: 5-10 minutes. |
| Duration of Action | Oral: 6-12 hours; Intramuscular: 4-8 hours; Intravenous: 3-6 hours. Duration may be extended in hepatic impairment or with concurrent CNS depressants. |
| Molecular Weight | 218.25 |
Gemonil is a brand name for metharbital, a barbiturate anticonvulsant. Typical adult dose: 100 mg orally three to four times daily, adjusted based on response and tolerability.
| Dosage form | TABLET |
| Renal impairment | In renal impairment, barbiturates are excreted renally. For GFR 10-50 mL/min, reduce dose by 25-50%; for GFR <10 mL/min, avoid use or use with caution and reduce dose by 50% or more. |
| Liver impairment | In hepatic impairment, barbiturate metabolism is reduced. Child-Pugh Class A: no adjustment needed; Child-Pugh Class B: reduce dose by 25-50%; Child-Pugh Class C: avoid use or use lowest possible dose with monitoring. |
| Pediatric use | Pediatric dose for metharbital: 5-10 mg/kg/day orally in 2-3 divided doses. Alternatively, 50-100 mg two to three times daily for children 6-12 years. Titrate to seizure control. |
| Geriatric use | In elderly patients, start at lower doses (50 mg three times daily) due to increased sensitivity and reduced clearance. Monitor for sedation, ataxia, and cognitive impairment. |
| 1st trimester | Contraindicated due to teratogenic effects; associated with fetal malformations including cleft palate and neural tube defects. |
| 2nd trimester | Contraindicated due to risk of fetal malformations and developmental toxicity. |
| 3rd trimester | Contraindicated; may cause neonatal bleeding and withdrawal symptoms. |
Clinical note
Comprehensive clinical and safety monograph for GEMONIL (GEMONIL).
| Placental transfer | Readily crosses the placenta with fetal plasma levels approaching maternal levels. |
| Breastfeeding | Excreted into breast milk in potentially clinically significant amounts; has caused sedation and poor feeding in nursing infants; avoid breastfeeding. |
| Lactation Rating |
■ FDA Black Box Warning
May be habit forming; abrupt discontinuation can precipitate withdrawal seizures and status epilepticus.
| Serious Effects |
PregnancyHypersensitivity to barbituratesAcute intermittent porphyriaSevere respiratory depressionSevere hepatic impairment
| Precautions | Respiratory depression, dependence/withdrawal, paradoxical excitement, risk of suicidal ideation, use in hepatic impairment. |
| Food/Dietary | No significant food interactions reported. However, grapefruit juice may inhibit metabolism slightly, but clinical relevance is low. Maintain consistent calcium and vitamin D intake due to risk of osteomalacia with long-term use. Alcohol must be avoided due to additive CNS depression. |
Loading safety data…
| L5 |
| Teratogenic Risk | First trimester: potential teratogenicity. Second trimester: risk of intrauterine growth restriction. Third trimester: neonatal withdrawal, respiratory depression, and floppy infant syndrome. |
| Fetal Monitoring | Fetal ultrasound for growth and anatomy; maternal serum alpha-fetoprotein; nonstress test and biophysical profile in third trimester. |
| Fertility Effects | May reduce fertility by suppressing ovulation or spermatogenesis due to CNS depression. |
| Clinical Pearls |
| GEMONIL is a brand name for phenobarbital, a barbiturate used for seizure control. Monitor serum levels (therapeutic range 15-40 µg/mL) due to narrow therapeutic index. Avoid abrupt discontinuation to prevent withdrawal seizures. Can cause paradoxical excitement in children and elderly. Induces CYP450 enzymes, reducing efficacy of oral contraceptives, warfarin, and corticosteroids. Use with caution in porphyria as it may precipitate attacks. |
| Patient Advice | Take exactly as prescribed; do not stop suddenly without medical advice due to risk of seizures. · Avoid alcohol and other CNS depressants as they can increase sedation and respiratory depression. · May cause drowsiness, dizziness, or impaired coordination; avoid driving or operating heavy machinery until you know how it affects you. · Inform your doctor if you become pregnant or plan to become pregnant; phenobarbital can harm the fetus. · Use effective contraception as phenobarbital may reduce the efficacy of hormonal contraceptives. · Report any unusual bleeding, bruising, or signs of infection (fever, sore throat) as it may cause blood dyscrasias. · Do not use if you have a history of porphyria. |