The liquid that a patient drinks before a colonoscopy is known as a bowel preparation agent, according to a report published by the National Institutes of Health. Common bowel preparation agents include polyethylene glycol, sodium phosphate and magnesium citrate, states Harvard Health Publications.
Bowel preparation agents cause diarrhea in order to clean the colon, notes Harvard Health Publications. A clean colon improves the effectiveness of a colonoscopy by making it easier for medical professionals to detect colon cancer, polyps and lesions. Bowel preparation agents may cause side effects such as nausea, cramps, electrolyte imbalances, dehydration and kidney damage. A patient should discuss the different solutions with his doctor so as to choose the most suitable option.