Nano, whose blood is the source of Line, is immune to Akira's blood. And Takeru didn't die from taking it. So maybe the higher percentage of Line someone is using, the higher their chances of survival are when consuming Akira's blood.
He got it all over his face, but didn't necessarily ingest enough to cause toxin to start building up in his system. As he collapsed from withdrawal, all Akira had to do to "cure" him was make sure he didn't get any more Line. Why the withdrawal period was that short, on the other hand...
It's touched on in the games. Keisuke was subjected to the same kind of research that Akira and N were but he was ultimately considered a failure, so they dropped him. The anime kind of just skirted around the issue completely.
Why is the moon always full in every episode of the anime?