I suppose first I want to understand why you can’t take your medicine? Is this a condition of the exam? Because that doesn’t seem fair…
I’m glad you have a plan to break it up but I’ll be more blunt; you may not be able to study for 12 hours straight, I certainly can’t. And I’m fairly sure there isn’t much benefit to doing so, but it’s been a few years since I researched it… not the type of thing one generally finds consensus on anyway. But there’s a limit to what anyone’s brain can encode into new long term memory in a given amount of time.
You have to learn to work with what you’ve got, to keep adapting your methods until you find something that works. It takes time and it’s probably going to be different than what works for someone else.
It’s important to understand that the drugs don’t fix your executive function they just sort of overclock your brain, at least that’s the way I think of it.
It’s hard to give specific advice without being familiar with the exam material, but I would try to find different ways to study. Some kind of practice work that you can do in addition to the route memorization. Anything to vary the type of work and keep your mind from wandering. But more importantly if you can’t get back to studying do something else… but give your brain the rest and time it needs to absorb the material, then strengthen the memory by exercising your recall.