Temporal relationship of muscle weakness and pain reduction in subjects treated with botulinum toxin A.

Botulinum toxin A has demonstrated efficacy in relieving pain in spastic and nonspastic muscle conditions. This analgesic property has generally been attributed to muscular relaxation. This study demonstrates initial muscular relaxation and concomitant pain relief in a masticatory muscle model. However, as muscle power returns to normal, pain relief is still very evident. This result suggests that the analgesic effect attributed to botulinum toxin is more complex than simple muscular relaxation.