Hi -
I use Magnesium Citrate. Magnesium Oxide is supposed to be much harder to absorb.
Mine come in powder form inside of a gel type capsule. (You can see the powder inside the clear capsule). I did a test on one - put it in a glass of luke (body temp) water and checked on it after 20 mins to make sure that the outer capsule had dissolved.
A LOT of the compressed pill forms are so compressed and have so many binding agents in them - they will pass right through some people without ever dissolving!
One more thing to consider - Magnesium and Calcium work together in the body - one needs the other to be used properly. There is a "correct" ratio of CA to Mag- which I would advise you to research or "google"....Vitamin D is very important for calcium delivery also.
Best,
Cara