[senkora]

gwern once ran a rigorous N=1 self-experiment on magnesium with self-blinding and week-long blocks, and concluded that it was probably helpful to him: https://gwern.net/nootropic/magnesium#conclusion

You are welcome to review his methodology and see if it still seems like the placebo effect.

[Noumenon72]

Based on his ranking of MP (mood/productivity), he concluded that it helped for the first three weeks and then got worse because he was overdosing on the magnesium. So not much there is going to apply to the general case.