The prep instructions are your step-by-step guide. There are many different preps on the market and the choice is often a matter of personal preference. The traditional one gallon Nulytely prep remains the prep of choice in people who tend toward constipation because it almost always leads to excellent results. MoviPrep is a very popular alternative because it includes half the amount of prep solution but is comparable in terms of effectiveness. OsmoPrep may be appropriate in some cases but should be used with caution because it has caused kidney damage in a small number of patients and is not as effective as the other preps.