In my history of MS I've been on three different disease-modifying therapies in order to stabilize. Generally you change when your condition is declining, and generally it's when it's declining rapidly that a neurologist would make that decision.
It's difficult to give you a broad answer, because it's different for everyone. MS is a disease that's individual, so clearly a drug or any type of disease-modifying therapy is going to interact. I think the evidence with all disease-modifying therapies shows that the old ones, which initially came to market in 1998, were about 33% effective. The last one, Tysabri, has a lot of controversy around it as well, and I have had 32 infusions of that drug. My neurologist determined that it was time to come off that drug because of the side effects that are a potential with long-term use. That drug is a very effective drug, with a 70% reduction of attacks. Of course, attacks create cumulative disability; that's basically what I'm saying.