I'm not sure about this. I know it sounds awesome and all, but you are forcing yourself to have hallucinations. That can't be good for your mind. Now of course you can always "dismiss" your Tulpa, but what if you are in a weak state of mind, like a depression? It sounds like something cool, but what if you are more vulnerable for a multiple personality disorder thingy? Won't your Tulpa REALLY get a mind on its own?
No your medication is probably not getting in the way. All the medicine does is making you more focused, by for example making you less impulsive. Your medicine has nothing to do with Tulpas at all.
When I take my ADD medicine I just get a high blood pressure and I hate that so I usually "forget" to take it. :shrug: