While this is a wonderful idea in theory, a key reason it's often not used for experiments is that there are just too many variables to control for (including, for example, the ongoing effects of spending a month wondering if you're taking something that's affecting you... which affects you in and of itself).

I agree it's not perfect, you're right to cite the Hawthorne effect. The vitamin D one for example would be affected by weather and seasonality. But I still think it would be really cool and a good product.

Don't those effects cancel with the effects on those participants who take the same 2 treatments but in the reverse order?

That’s why you need multiple participants and why this idea for doing experiments on just yourself doesn’t work well

Yeah, even watching the news can make your stress levels go up.