[athenot]

At the risk of sounding obvious, it is always a good idea to get labs drawn first and then consider supplements if needed. Blindly taking supplements can cause issues in the opposite direction.

Most of us taste our food before deciding whether to add salt, same should go with supplements.

[mypalmike]

Blood tests are not particularly useful for detecting magnesium deficiency. Deficiency is in the cells, not the blood, and blood levels are fairly transient.

That said, it is definitely something to discuss with a doctor. I've had good luck recently working with a naturopath who is highly knowledgeable about nutrition and has far more time to discuss such things as diet and supplements than any general practitioner I've ever visited.