Legzz,

The drug was tested in trials with every 4 week dosing. Changing the dosing schedule to longer intervals wasn't tested so efficacy on that schedule is unknown so your Doctor is being cautious - less drug from longer dosing intervals means a higher risk of relapses and disease progression. The drug doesn't disappear from your system after 4 weeks (takes up to 3 months based on studies).

I've personally set up an every 5 week dosing schedule since I tested positive for JC virus antibodies and have to balance the risk versus benefit - I want a lower concentration of Tysabri in my bloodstream just in case.

Chris