For spasticity in a few select areas, you may benefit from a targeted therapy. A doctor can inject you with botulinum toxin, phenol, or both. Botulinum treatments block the release of nerve chemicals involved in muscle contraction. Phenol injections serve as nerve blocks in affected muscles, which helps with spasticity. Effects of both treatments usually last about three to six months. Depending on the medication, side effects can include pain, swelling, soreness, fatigue, and muscle weakness.