PHRENILIN
Clinical safety rating: caution
Comprehensive clinical and safety monograph for PHRENILIN (PHRENILIN).
PHRENILIN is a combination of butalbital, acetaminophen, and caffeine. Butalbital is a barbiturate that enhances GABA-A receptor activity, producing sedation. Acetaminophen inhibits cyclooxygenase (COX) in the CNS, reducing prostaglandin synthesis. Caffeine is a nonselective adenosine receptor antagonist, promoting vasoconstriction and enhancing analgesic effects.
| Metabolism | Butalbital is extensively metabolized by hepatic CYP450 enzymes (especially CYP2C9) and excreted in urine. Acetaminophen is primarily conjugated in the liver via glucuronidation and sulfation, with minor CYP2E1-mediated metabolism to a toxic metabolite (NAPQI). Caffeine is metabolized predominantly by CYP1A2. |
| Excretion | PHRENILIN (butalbital/acetaminophen/caffeine): Renal excretion of metabolites; butalbital ~60-70% unchanged in urine, acetaminophen ~2-4% unchanged with majority as glucuronide and sulfate conjugates, caffeine metabolites primarily renal. |
| Half-life | Butalbital: terminal half-life ~35 hours (range 20-50 h); acetaminophen: ~2-3 hours (prolonged in hepatic impairment); caffeine: ~3-6 hours. |
| Protein binding | Butalbital: ~45% bound to plasma proteins; acetaminophen: 10-25% bound; caffeine: ~35% bound. |
| Volume of Distribution | Butalbital: Vd ~0.8 L/kg; acetaminophen: Vd ~0.9 L/kg; caffeine: Vd ~0.6 L/kg. Overall Vd for combination not established; butalbital widely distributed. |
| Bioavailability | Oral: butalbital ~90% (complete absorption); acetaminophen ~85-90%; caffeine ~100%. |
| Onset of Action | Oral: butalbital ~30-60 minutes; acetaminophen ~30-60 minutes; caffeine ~30-60 minutes. |
| Duration of Action | Butalbital: 4-6 hours (sedative effect); acetaminophen: 4-6 hours (analgesic); caffeine: 3-4 hours (stimulant). Duration may vary with dose and patient factors. |
For tension headache: 1-2 capsules (each containing butalbital 50 mg, acetaminophen 300 mg, and caffeine 40 mg) orally every 4 hours as needed, not exceeding 6 capsules per day.
| Dosage form | TABLET |
| Renal impairment | GFR 30-50 mL/min: Use with caution, maximum 4 capsules per day. GFR <30 mL/min: Avoid use due to accumulation of acetaminophen metabolites and butalbital. Not recommended in dialysis. |
| Liver impairment | Child-Pugh A: Use with caution, maximum 4 capsules per day. Child-Pugh B or C: Contraindicated due to impaired metabolism of butalbital and hepatotoxicity risk from acetaminophen. |
| Pediatric use | Not recommended in children under 12 years of age due to butalbital and caffeine content. For adolescents 12-17 years: 1 capsule orally every 4 hours as needed, not exceeding 3 capsules per day. |
| Geriatric use | Initiate at 1 capsule orally every 4 hours as needed, not exceeding 4 capsules per day. Monitor for sedation, cognitive impairment, and falls. Avoid in patients with severe hepatic or renal impairment. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for PHRENILIN (PHRENILIN).
| Breastfeeding | Butalbital: Excreted into breast milk; M/P ratio unknown. Monitor infant for sedation, poor feeding, or withdrawal symptoms. Acetaminophen: Excreted in low amounts; M/P ratio approximately 1.0; considered compatible. Caffeine: Excreted in milk; M/P ratio 0.5-0.8; moderate intake likely safe; excessive use may cause irritability in infant. |
| Teratogenic Risk | Butalbital: First trimester: Risk of major malformations (OR 1.4-2.0) including oral clefts; increased risk with prolonged use. Second and third trimesters: Avoid chronic use due to risk of neonatal withdrawal syndrome and hemorrhagic disease of newborn due to vitamin K deficiency. Acetaminophen: Considered low risk; no consistent evidence of teratogenicity. Caffeine: No increased risk of major malformations at moderate intake (<200 mg/day); high doses may be associated with growth restriction. |
■ FDA Black Box Warning
Barbiturates (butalbital) are habit-forming and may produce drug dependence. Withdrawal symptoms (e.g., anxiety, insomnia, seizures) can occur if abruptly discontinued. Use with caution in patients with a history of substance abuse.
| Serious Effects |
Hypersensitivity to any component; porphyria (butalbital); severe hepatic impairment (acetaminophen); concurrent use of MAOIs or other CNS depressants may potentiate effects.
| Precautions | Hepatotoxicity (acetaminophen) with overdose or chronic use; risk of dependence and withdrawal with butalbital; potential for caffeine-related effects (insomnia, palpitations); caution in patients with liver disease, renal impairment, or history of substance abuse. |
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| Fetal Monitoring | Monitor maternal: Liver function (acetaminophen hepatotoxicity risk), renal function, and drug abuse behavior. Monitor fetal: Ultrasonography for growth restriction if chronic use; neonatal monitoring for withdrawal syndrome (butalbital) if used near term. Assess for signs of respiratory depression in neonate if butalbital used before delivery. |
| Fertility Effects | Limited data. Butalbital may cause menstrual irregularities via enzyme induction affecting steroid metabolism; acetaminophen and caffeine at typical doses not associated with impaired fertility. Chronic use of barbiturates may reduce libido or sperm quality. |