In season 3 we learn that Cortexiphan is fatal for adults. Being as William had not had it as a kid, and the development of Cortexiphan was before the split in the timeline, how can William Bell take it without dying? The only thing I can think of is that the Cortexiphan that is fatal is a reverse-engineered version from Olivia's blood and not the original blend.
WILLIAM BELL: Yes, Walter. We cut those ideas out of your head to literally put 'the Genie' back into the bottle. Then I grew older. I grew cynical. I grew cancer. Then I realized that dosing myself with Cortexiphan would slow it down.