Laurie, just tell them you refuse to start until you get answers. It's your body and you are not their ginea pig.
Actually after reading up on it, I wouldn't touch it with a 10 ft pole, due to adverse side effects.
![EEK!](images/smilies/eek.gif)
But, I wouldn't do TYsabri past 2 yrs either, because of the dangers of PML.