Baclofen
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baclofen mechanism of action pharmacology GABA
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baclofen clinical uses indications spasticity overdose side effects
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Baclofen
Class: Skeletal muscle relaxant / GABA-B agonist
Generic name: Baclofen | Trade names: Lioresal, Gablofen (intrathecal)
Mechanism of Action
Baclofen is a structural analogue of gamma-aminobutyric acid (GABA) that acts as a GABA-B receptor agonist. It facilitates GABAergic inhibition of motor reflex arcs at the level of the spinal cord, reducing excitatory neurotransmission to alpha motor neurons. It also acts on GABAergic interneurons in the sacral intermediolateral cell column, contributing to relaxation of the striated urethral sphincter. (Management of Non-Neurogenic Female Lower Urinary Tract Symptoms, p. 79)
Clinical Indications
| Indication | Notes |
|---|---|
| Spasticity (primary use) | MS, spinal cord injury, cerebral palsy, ALS |
| Trigeminal neuralgia | Second-line adjunct |
| Alcohol use disorder | Off-label; studied for craving reduction |
| Hiccups (singultus) | Off-label |
| Urethral sphincter spasm | Facilitates voiding in lower urinary tract dysfunction |
Dosing
Oral:
- Start: 5 mg TID, titrate gradually
- Effective range: 15–80 mg/day in divided doses
- Maximum: up to 240 mg/day for spasticity (Harrison's, p. 12556)
Intrathecal (ITB pump):
- Used in refractory spasticity via implanted programmable pump
- Delivers drug directly to CSF, allowing much lower doses with fewer systemic side effects
- Requires pump implantation by neurosurgery/pain management
Adverse Effects
| System | Effects |
|---|---|
| CNS | Sedation, drowsiness, dizziness, confusion, fatigue |
| Muscle | Weakness (can worsen function in patients who rely on spasticity to ambulate) |
| GI | Nausea, constipation |
| GU | Urinary retention or incontinence |
| Endocrine | Sexual dysfunction |
Important: Some patients with spasticity actually depend on increased muscle tone to stand, transfer, or walk — aggressive dosing can reduce functional independence (Harrison's, p. 12556).
Baclofen Withdrawal (Critical)
Abrupt discontinuation — especially of intrathecal baclofen — can cause a life-threatening withdrawal syndrome:
- Early: pruritus, anxiety, increased spasticity
- Severe: hyperthermia, rhabdomyolysis, autonomic instability, seizures, multi-organ failure, death
Management: reinstate baclofen immediately; benzodiazepines (diazepam or lorazepam) as bridge; cyproheptadine may have a role in ITB withdrawal.
Baclofen Toxicity / Overdose
Clinical features (dose-dependent):
- Excessive sedation → coma
- Hypotonia, areflexia
- Respiratory depression
- Bradycardia, hypotension
- Seizures (paradoxically, especially in ITB overdose)
Management: Supportive — no specific antidote. Airway management, mechanical ventilation if needed. Physostigmine has been used anecdotally for CNS depression but is not standard. Hemodialysis can enhance elimination in severe oral overdose.
Comparisons with Other Antispastics
| Drug | Mechanism | Notes |
|---|---|---|
| Baclofen | GABA-B agonist | Spinal, oral & intrathecal |
| Diazepam | GABA-A (benzodiazepine) | Useful for nocturnal leg spasms (2–4 mg at bedtime) |
| Tizanidine | α₂-adrenergic agonist | Presynaptic inhibition; 2–8 mg TID |
| Dantrolene | Direct muscle inhibitor (ryanodine receptor) | For non-ambulatory patients; risk of hepatotoxicity |
(Harrison's, p. 12556)
Special Considerations
- Renal impairment: Baclofen is renally cleared — reduce dose significantly in CKD; accumulation causes prolonged toxicity
- Elderly: Higher sensitivity to CNS/sedative effects; start low, titrate slowly
- Pregnancy: Category C; use only if clearly needed
- Abrupt discontinuation: Never stop suddenly, especially at high doses or with ITB pumps
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