| Muscle relaxants: baclofen |
Baclofen is an analogue of gamma-aminobutyric acid (GABA) and may exert its effect by stimulating the GABAB receptor subtype.
The bioavailability of oral baclofen is 70% to 80%. The plasma elimination half-life averages 3 to 4 hours. Approximately 70% of the dose is excreted unchanged in the urine, necessitating dose reduction in patients with renal impairment. Baclofen passes only slowly across the blood–brain barrier. After oral administration, baclofen concentrations attained in the cerebrospinal fluid are approximately 12% of that in the plasma.
Baclofen may also be administered by continuous intrathecal infusion for intractable spasticity. The intrathecal doses are about 100 times less than oral doses.
Baclofen can cause weakness, drowsiness, dizziness, fatigue, headache, insomnia, confusion, ataxia, and respiratory and cardiovascular depression. It should not be used where there is a history of peptic ulcer. Use with tricyclic antidepressants (TCAs) can cause increased muscle weakness. Baclofen can also cause urinary frequency, dysuria, constipation and nausea. It should be withdrawn slowly as abrupt cessation can result in agitation, delirium and, occasionally, convulsions. The dose should be reduced slowly over 2 weeks.
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