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The last thing you eat before beginning a cleanse will affect your reaction to the process. Having a pizza in your colon to clean out is a challenge compared to a salad. The cheese and flour are basically like glue and since you won't have anything following it you may have a harder time getting it out.
Before beginning a fast I eat a lot of Raw veggies and fruits. Particularly cabbage and carrots because they act like a broom in your colon. One or two big salads after your pizza will help immensely before starting.
The point is we all learn from trial & error. Do yourself a favor and let your colon free itself from that pizza.