Centrally Acting Skeletal Muscle Relaxant Pregnancy: Avoid — limited safety data; insufficient human evidence; animal studies show no teratogenicity

Methocarbamol

Brand names: Robaxin, Robaxisal

Adult dose

Dose: 1.5 g four times daily for first 48–72 hours; then 750 mg–1.5 g four times daily

Route: Oral

Frequency: Four times daily

Max: 6 g/day (first 48–72 hours); 4.5 g/day maintenance

Short-term use only (typically 5–7 days); IV available for severe spasm at 1–3 g IV at maximum 300 mg/min; take with food to reduce GI upset

Paediatric dose

Route: N/A

Frequency: N/A

Max: Not recommended under 16 years

IV formulation used in tetanus management under specialist supervision

Dose adjustments

Renal

IV formulation: avoid if eGFR <30 (polyethylene glycol vehicle potentially nephrotoxic); oral form: use with caution

Hepatic

Use with caution in hepatic impairment — reduced metabolism; no specific dose recommendation

Clinical pearls

Mechanism: centrally acting CNS depressant with no direct peripheral neuromuscular effect — likely depresses polysynaptic pathways in spinal cord and brain; reduces muscle spasm without causing flaccid paralysis
Acute musculoskeletal back pain: Cochrane 2003 review supports short-term (5–7 day) muscle relaxant use (including methocarbamol) as adjuncts to NSAIDs and physiotherapy for acute low back pain; not recommended beyond 2 weeks
Urine discolouration: methocarbamol metabolites excreted as dark brown or black pigment — warn patients proactively; harmless but alarming if unexpected; no organ toxicity associated
Tetanus (IV): methocarbamol IV adjunctively used for tetanic muscle spasm — historical indication that distinguishes it from other oral muscle relaxants; specialist use only
Preferred over diazepam for short-term spasm in non-anxious patients: no dependence potential, no controlled drug status, no euphoria — safer for acute prescribing in orthopaedic settings; NICE CG88 supports short-term muscle relaxant use for acute back pain

Contraindications

Renal impairment (IV formulation — PEG vehicle)
Myasthenia gravis (potential worsening)
Hypersensitivity to carbamate derivatives

Side effects

Sedation
Dizziness
Nausea
Urine discolouration (dark brown or black — harmless)
Headache
Blurred vision
Hypotension (IV)

Interactions

CNS depressants — additive sedation
Alcohol — enhanced CNS depression
Pyridostigmine — may antagonise cholinergic effect in myasthenia

Monitoring

Sedation level
Functional improvement in spasm
Hepatic function if prolonged use
Renal function if IV formulation used

Reference: BNFc; BNF 90; SPC Robaxin; Cochrane Review (Muscle relaxants for acute low back pain, 2003); NICE CG88 (Low Back Pain); Toth and Urtis, Am J Phys Med Rehabil 2004. Verify against your local formulary and the latest BNF before prescribing.

Curated clinical cross-links plus same-class fallbacks.

Pathways