Sure, but the supplement industry is 99% woo and most of what is sold has no proven benefit. In general people most people taking supplements don't have any known deficiency (including me, blood tests show my mag levels are fine).

"Known deficiency" is a thorny issue in itself. The various serum levels and necessary intakes vary widely by individual. And recommended daily allowances for many kinds of vitamins and minerals are set conservatively low to avoid overdoses from people chugging the bottle.

"levels are fine" can still mean fine, too low or too high for your individual need. As long as you don't overdose, trying out a supplement nonetheless can be worth a shot, but of course it can also be a waste of money.

Unfortunately, "normal values" in medicine aren't as normal as one would like.

Yep. I didn't mention it but with magnesium testing, blood tests may not catch an issue; you can be deficient in a particular organ (e.g. brain) and fine in others.

But realistically, very little of the $100 billion+ supplement industry is built on anything medically necessary. Yes, I have a waste basket diagnosis with an unclear cause that's helped by a specific supplement, but there are quite literally dozens (perhaps even hundreds) of other supplements that the snake oil merchants swear up and down are essential for the same condition, and they'd happily claim that whatever deficiency exists is undetectable. That's why I "don't believe" in supplements in general.