*Yes* take it in addition to whatever else you take. Just a note though about spacing out your pills. If you take zithromax (and some other antibiotics but I'm not sure which ones) you can't take magnesium 4 hours before it or 2 hours after it because it interferes w/ the absorption of zithro. And if you are taking iron, you can't take it at the same time as calcium because calcium interferes w/ the absorption of iron. I think it is crazy that there are prenatal vitamins w/ iron that also contain calcium! (that also applies to milk or calcium fortified foods/drinks - so don't down your iron w/ a glass of milk!). Oh and your body can't absorb more than 500 mg of calcium at one time so if you are taking more than that, space out your doses. (the RDA is at least 1000 mg). I don't know how much magnesium we can absorb at once but I generally take 400 mg at a time.
I know it sounds like a lot to remember but it isn't that bad. Magnesium is something that I would recommend you take after pregnancy as well - not all our CF clinics are up to the latest research in magnesium but some of them are coming around and actually recommending it (just as they are recommending vitamin D and calcium now) to their patients - finally. Mine isn't one of them yet(UCSF) but I happen to know a few people who have done A TON of research on magnesium and I wholly trust what they say (one of them is a mom who has a grown son w/ CF). Mg is vital for something like 300 enzymatic processes and since the meds we take deplete mg from our bodies, what choice do we have but to supplement.