When I first had chemo in 2006, I was given magnesium by IV when my levels were too low, but I was told to actual take magnesium pills, over the counter to supplement. Ask your oncologist about what dose you should do.
Best of luck to you.
I also took over the counter magnesium pills while on chemo.
If you want to order it on line you can get it from Swansons or maybe Puritans Pride, if you choose but you can buy it at health food stores.
Ask your doc about how much to supplement daily.