I have been on for about 5 weeks, and have "cheated" twice, each time I had NO ill effects, and it actually boosted my weight loss........and I had no stomach upset or anything like that, it actually helped me get motivated.
This is something I want to do my whole life, but I am beginning to think about once a month (not planned, only when I see something I want) I go off of it for a day (I figure knocking myself out of ketosis for only a snack or a meal doesn't make me happy..........I make it a whole day!!!)
One thing that I certainly do when I go "off" I drink several glasses of OJ and several glasses of milk (two things I miss at meal times: OJ for breakfast, and milk for every other meal) and that also replenishes vitamins.
By the end of the ONE DAY I feel rejuvinated and WANT to go back on for several weeks.

This is your body, this choice is yours, good luck!!!
Heidi