Cystic Fibrosis and Gut Dysmotility: The Effect of Polyethylene Glycol (PEG) on Intestinal Transit

Inclusion Criteria: * previously diagnosed with CF confirmed with genetic mutations consistent with CF * greater than 18 years old who chose to participate in the research study * have symptoms of constipation (constipation will be defined as stool frequency less than three times per week, Bristol stool scale form 1-2 in \>25% of stools, and/or the sensation of incomplete evacuation, manual maneuvers to facilitate stools, and the sensation of blockage) The patients enrolled are allowed to be on CFTR modulating drugs and/or receiving enteral feeding. Exclusion Criteria: * minors (\<18 years old) * active nicotine use or patients on nicotine replacement * history of abdominal surgery (surgeries involving gastrointestinal luminal resection) ie small bowel or colonic resection that increases risk of post-operative strictures or narrowing of the lumen. Surgeries such as Nissen fundoplication, gastrostomy tube placement, gynecologic surgeries, appendectomy, and/or cholecystectomy are not excluded from this study. * history of lung transplantation or pancreas transplant * BMI \>40 * pregnancy (this will be screened via urine pregnancy test) * incarcerated persons * patients with DIOS (distal intestinal obstructive syndrome) * patients with known hypersensitivity to PEG * persons unable to remain off the contraindicated medications.