Many times, effects on spasticity are longer lasting. It is not clear if this is due to breaking down patterned movements (many muscles contracting together rather than singly) or from allowing weak muscles to get stronger over time (that were overpowered before by more spastic muscles pulling against them). It is vital to have close follow-up after the injections to figure out the best course of treatment.
Spread of toxin effects. The effect of botulinum toxin may affect areas away from the injection site and cause serious symptoms including: loss of strength and all-over muscle weakness, double vision, blurred vision and drooping eyelids, hoarseness or change or loss of voice, trouble saying words clearly, loss of bladder control, trouble breathing, and trouble swallowing.