BACLOFEN
Clinical safety rating: caution
Animal studies have proved adverse effects but may be safe for humans
GABA-B receptor agonist; inhibits monosynaptic and polysynaptic spinal reflexes by hyperpolarizing afferent terminals.
| Metabolism | Metabolized via hepatic deamination by transaminase; primarily excreted unchanged in urine (approximately 70-80%), with minor hepatic metabolism. |
| Excretion | Renal: 70-80% unchanged; fecal: <5%; biliary: minimal. |
| Half-life | Terminal half-life: 2.5-4 hours (young adults), 4-8 hours (elderly); clinical context: requires frequent dosing for spasticity. |
| Protein binding | 30-35% bound to albumin. |
| Volume of Distribution | Vd: 0.5-0.7 L/kg; indicates distribution into total body water. |
| Bioavailability | Oral: 70-85% with high variability; intrathecal: 100%. |
| Onset of Action | Oral: 1-2 hours; intrathecal: 30-60 minutes. |
| Duration of Action | Oral: 4-6 hours; intrathecal: 4-8 hours; clinical note: continuous infusion used for sustained effect. |
Initial: 5 mg orally 3 times daily; increase by 5 mg per dose every 3 days to max 80 mg/day (20 mg 4 times daily). Intrathecal: initial test dose 50-100 mcg; for continuous infusion, daily dose typically 300-800 mcg.
| Dosage form | INJECTABLE |
| Renal impairment | CrCl 30-50 mL/min: reduce dose by 50%; CrCl <30 mL/min: avoid use or use with extreme caution, reduce dose by 75%. |
| Liver impairment | No specific guidelines; use with caution due to potential for increased sedation/neurotoxicity. |
| Pediatric use | Children 2-7 years: initial 2.5 mg orally 4 times daily, increase by 2.5 mg/dose every 3 days to max 40 mg/day; children ≥8 years: initial 5 mg orally 3 times daily, increase as in adults to max 60 mg/day. |
| Geriatric use | Start at low end of dosing range (5 mg twice daily), titrate slowly due to increased risk of sedation, weakness, and cognitive impairment. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
CNS depressants like alcohol and opioids can increase sedation Abrupt discontinuation can cause hallucinations and seizures.
| Breastfeeding | Baclofen excreted into breast milk in low concentrations (M/P ratio approximately 0.43). Relative infant dose estimated 0.9% of maternal weight-adjusted dose. Considered compatible with breastfeeding, but monitor infant for sedation and hypotonia. |
| Teratogenic Risk | First trimester: Limited human data; animal studies show increased fetal malformations (omphalocele, exencephaly) at doses equivalent to human therapeutic range. Second and third trimesters: Risk of neonatal withdrawal (hypertonia, seizures) with chronic maternal use. Avoid unless benefit outweighs risk. |
■ FDA Black Box Warning
Abrupt discontinuation may cause withdrawal symptoms including hallucinations, seizures, and life-threatening hyperpyrexia; taper dose gradually.
| Common Effects | Dizziness Convulsion Nausea Headache Sedation Hypotension low blood pressure Hypotonia decreased muscle tone Agitation Constipation Increased white blood cell count Chills Urinary retention |
| Serious Effects |
["Hypersensitivity to baclofen.","Intrathecal formulation is contraindicated in patients with active infection or bleeding disorders at lumbar puncture site.","Women who are breastfeeding (relative contraindication)."]
| Precautions | ["May cause CNS depression (drowsiness, sedation) and impair ability to drive or operate machinery.","Risk of withdrawal syndrome including fever, altered mental status, and autonomic instability upon abrupt cessation.","Use with caution in patients with renal impairment; dose adjustment required.","May exacerbate psychiatric disorders; monitor for hallucinations, confusion.","Risk of respiratory depression when combined with other CNS depressants."] |
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| Fetal Monitoring | Monitor maternal blood pressure, renal function, and signs of toxicity (sedation, respiratory depression). Fetal ultrasound for anomalies if first-trimester exposure. Neonatal monitoring for withdrawal signs post-delivery. |
| Fertility Effects | No evidence of impaired fertility in humans. Animal studies showed no adverse effects on fertility at therapeutic doses. |
| Food/Dietary | No specific food interactions. Avoid alcohol due to additive CNS depression. |
| Clinical Pearls | Abrupt withdrawal can cause severe rebound spasticity, fever, and rhabdomyolysis; taper by 5-10 mg/week. Intrathecal baclofen pumps require careful monitoring for overdose (respiratory depression) or withdrawal. Use with caution in renal impairment (dose adjust for CrCl <30 mL/min). |
| Patient Advice | Do not stop taking baclofen suddenly; sudden discontinuation can cause serious withdrawal symptoms including hallucinations, seizures, and high fever. · Avoid alcohol and CNS depressants as they increase sedation and risk of falls. · May cause dizziness or drowsiness; avoid driving or operating machinery until you know how it affects you. · Take exactly as prescribed; missed doses can lead to muscle spasms or withdrawal. · Report any unusual muscle stiffness, rapid heart rate, or dark urine immediately. |