First of all, if oral medications like mag citrate and lactulose is not effective, it is time to use rectal medications like fleets enema and glycerin suppositories. If regular fleets doesn't work, try glycerin enema which is usually effective,
Secondly, you can eat. Food consumption actually triggers the intestines to move. The reason mag citrate should not be taken with food is because food can potentially neutralize the effects of the medication. If you have already taken the mag citrate, you can eat.
I hope this is useful.