Methocarbamol (Robaxin) Onset and Duration of Action
Pharmacokinetic Profile
Methocarbamol has a rapid onset of action of 1-2 hours after oral administration, with a short plasma elimination half-life of 1-2 hours in healthy adults. 1
Key Timing Parameters
- Onset of action: 1-2 hours after oral administration 1
- Peak plasma levels: Achieved within 1-2 hours 1
- Elimination half-life: 1-2 hours in healthy adults 1
- Duration of clinical effect: Approximately 4-6 hours (based on half-life and typical dosing intervals) 1
- Plasma clearance: 0.20-0.80 L/h/kg 1
- Protein binding: 46-50% 1
Metabolism and Elimination
- Methocarbamol is metabolized via dealkylation, hydroxylation, and likely conjugation 1
- Essentially all metabolites are eliminated in the urine, with small amounts of unchanged drug also excreted renally 1
Special Population Considerations
Elderly Patients
- Elimination half-life is slightly prolonged to 1.5 hours (versus 1.1 hours in younger adults) 1
- Protein binding is slightly decreased (41-43% versus 46-50%) 1
Renal Impairment
- Clearance is reduced by approximately 40% in patients on maintenance hemodialysis 1
- However, the elimination half-life remains similar (1.2 hours versus 1.1 hours in normal subjects) 1
Hepatic Impairment
- Total clearance is reduced by approximately 70% in patients with cirrhosis 1
- Elimination half-life is significantly prolonged to 3.38 hours (versus 1.11 hours in normal subjects) 1
- Protein binding is decreased to 40-45% 1
Clinical Efficacy Timeline
- In clinical trials for acute low back pain, 44% of patients achieved complete pain relief and discontinued treatment early (versus 18% with placebo) when treated for up to 8 days 2
- The typical treatment duration is 2-8 days for acute painful muscle spasm 2
Important Clinical Caveats
- The mechanism of action is not fully established but likely involves general CNS depression rather than direct muscle action 1
- Methocarbamol has no direct action on striated muscle contractile mechanisms, motor end plates, or nerve fibers 1
- Common side effects include drowsiness and dizziness due to CNS depressant effects 3
- Avoid in patients with Parkinson's disease due to potential worsening of parkinsonian symptoms from CNS depression 4