This is a multicenter, open-label, long-term extension study to evaluate the safety and tolerability of oral AERT in patients with spasticity due to MS. Subjects from the double blind study (Study OS440-3004) may rollover into this open-label extension study, as well as de novo subjects. The maintenance dose will be the highest tolerated dose. Subjects may choose to taper study drug after Visit 3 if they experience adverse events and in direct consultation with the investigator site. Subjects should be encouraged to titrate back to the highest dose once adverse events subside or remain on the maximally tolerated AERT dose. Once the subject has reached the maintenance dose, they will remain on that dose for approximately 1 year.