My daughter's doc at first said take Half of the Nulytely prep plus 4 tablets (5 mgs each) bisacodyl, which is the generic form of Ducolax. BUT I read that a clinical trial showing 4 tablets posed more issues in terms of side effects than the 2 tablets. Also, 4 tablets didn't work any better than 2.
The (9/2007) instructions from the manufacturer of both Nulytely and Halflytely recommend only 2 tablets of Bisacodyl, saying, "The HalfLytely and 20 mg Bisacodyl Tablets Bowel Prep Kit does not provide any additional efficacy benefits over the HalfLytely and 10 mg Bisacodyl Tablets Bowel Prep Kit." The manufacturer's recommendation referred to results from a clinical trial showing a higher incidence of side effects, but no added benefits, from the higher dosage of 4 tablets Bisacodyl.
I asked the doc, considering the higher safety profile of the lower dosage, whether it would be possible to reduce the Bisacodyl dosage to only 2 tablets (5 mg each) instead of 4. He said it was OK for my daughter to take just the 2.
The notes of the colonoscopy include the gastroenterologist's notes on what a great job the prep did - just the 2 tablets plus half the Nulytely.