Magnesium oxide for constipation?

I don't know where to post this as there is no forum for constipation.

I was treated for prostate cancer two years ago(radiation) and it messed up an already messed up digestive system.

I have chronic constipation and I was told by an integrative medicine practitioner to try magnesium

She didn't say which kind.
I've been using magnesium oxide which I believe is the right type for constipation. Citrate is stronger but not really good for daily use for this condition.

She told me to take it at night.....but since I've read you are supposed to drink a full glass of water with it....I don't want to drink that before going to bed. I'll be up all night going to the bathroom.

So can I take it after breakfast? I also have to juggle the magnesium with several other medications and I don't want any interactions.....so around 10 am after breakfast...is that a good time?

I also take psyllium first thing in the morning.

thanks for any input!