alt Hacker NewsThere was some article about it on HN a while ago. If I remember right the problem was that its bioavailability is super low. You can take all you want, but only a tiny percent makes it through to get absorbed. In theory you could increase the the dose a lot but I imagine that might have other issues.
I believe the problem is that the oral route causes most of it to be broken down in the digestive system. Nasal spray seems to be more effective because it has a chance to directly enter the blood stream rather than go through the gut.