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. |
| Molecular Weight | Butalbital: 224.26 Da; Acetaminophen: 151.16 Da; Caffeine: 194.19 Da |
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 | PHRENILIN contains butalbital, acetaminophen, and caffeine. Butalbital is a barbiturate; use in first trimester may be associated with neural tube defects. Avoid unless benefit outweighs risk. |
| 2nd trimester | Barbiturates may cause fetal dependence; use only if clearly needed. Acetaminophen generally considered safe in moderate doses. |
| 3rd trimester | Butalbital may cause neonatal withdrawal and respiratory depression. Avoid in late pregnancy, especially near term. |
Clinical note
Comprehensive clinical and safety monograph for PHRENILIN (PHRENILIN).
| Placental transfer | Butalbital readily crosses placenta; acetaminophen and caffeine also cross. Evidence indicates significant placental transfer. |
| Breastfeeding | Butalbital and caffeine pass into breast milk. Acetaminophen is considered compatible. Use lowest effective dose for shortest duration. Monitor infant for sedation or poor feeding. |
■ 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 barbiturates, acetaminophen, or caffeinePorphyriaSevere hepatic impairmentSevere respiratory depression
| 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. |
| Food/Dietary | Avoid or limit caffeine-containing foods and beverages (coffee, tea, cola, chocolate) due to additive caffeine content. Alcohol should be strictly avoided due to enhanced CNS depression and hepatotoxicity risk. Grapefruit juice may affect caffeine metabolism; monitor for increased caffeine effects. |
Loading safety data…
| Lactation Rating | L3 - Moderately Safe |
| 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. |
| 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. |
| Clinical Pearls | Phrenilin is a combination of butalbital, acetaminophen, and caffeine. Butalbital is a barbiturate; use with caution in patients with history of substance abuse or depression. Acetaminophen hepatotoxicity risk increases with doses >4g/day or in alcohol use disorder. Caffeine may exacerbate anxiety, insomnia, or tachyarrhythmias. Monitor for sedation and respiratory depression when used with other CNS depressants. Avoid abrupt discontinuation to prevent withdrawal symptoms. |
| Patient Advice | Take only as prescribed; do not exceed recommended dose to avoid liver damage or addiction. · Avoid alcohol while taking this medication due to increased risk of liver toxicity and sedation. · Do not drive or operate heavy machinery until you know how this medication affects you. · If you have a history of substance abuse, inform your doctor before taking this medication. · Common side effects include drowsiness, dizziness, and nausea. Report severe allergic reactions or signs of liver damage (yellowing skin/eyes, dark urine, abdominal pain). · Do not stop taking suddenly without consulting your doctor, as withdrawal symptoms may occur. · Keep out of reach of children; acetaminophen overdose can be fatal. |