Earlier quoted context omitted.
I would absolutely be willing to bet that a small dose of Benadryl won't make someone hallucinate "a small amount". Put it another way if makes you feel better -- a little bit of D3 via direct sunlight and D3 rich foods will improve your mood and health outcomes in a variety of ways. An excessive amount of D3 will give you kidney stones and harm your bones.
Would you notice it if you became slowly, progressively, stupider over several years? Maybe, maybe not.
It's one thing to be worried about subtle effects from medication, but I already know what the long term health and brain effects are from constant insomnia and regularly forgetting to eat. Neither of those are good for brain function.
So yeah, seems like a pretty reasonable risk to take, particularly given the fact that I have a psychiatrist, therapist, and multiple friends and family monitoring me.
Let me flip the question back to you. Would you notice if D3 or iron or protein intake was subtly harming you over the course of multiple years? Is that something that terrifies you?