Constipation is one of the most prevalent gastrointestinal disorders in patients with chronic kidney disease (CKD) and has been associated with their adverse kidney and cardiovascular outcomes; however, little is known about the effects of constipation treatment on clinical outcomes nor on outcome-related biochemical and microbiological parameters in patients with CKD. The investigators aim to test the feasibility of delivering an intervention with constipation treatment and determine its effects on changes in clinical, biochemical, and microbiological parameters in patients with CKD and constipation.

Inclusion Criteria:

1. Male or female patients >18 years old.
2. Patients able and willing to provide written informed consent and HIPAA authorization.
3. CKD stages G3-G5 without kidney replacement therapy with estimated glomerular filtration rate (eGFR) <60 mL/min/1.73m2 calculated using the Chronic Kidney Disease-Epidemiology Collaboration (CKD-EPI) 2021 creatinine equation.
4. Functional or opioid-induced constipation based on the Rome IV criteria.
5. Patients must refrain from taking any laxatives for at least one week prior to screening, although rescue with one bisacodyl 5 mg tablet will be allowed when a patient does not have a bowel movement for ≥3 consecutive days or when symptoms become intolerable.
6. A negative pregnancy test in female patients prior to enrolment, unless the patient is two years postmenopausal or has had a documented tubal ligation or hysterectomy.

Exclusion Criteria:

1. History of drug abuse, anorexia nervosa, or bulimia.
2. History of irritable bowel syndrome, inflammatory bowel disease, or Clostridium difficile bacterial infection (ever).
3. Liver cirrhosis or chronic active hepatitis (patients with treated hepatitis can be included).
4. Galactosemia.
5. Allergies to lactulose.
6. History of gastrointestinal surgery except appendectomy.
7. Use of antibiotics or immunosuppressants within 30 days prior to the enrollment in the study.
8. Use of pre- or probiotics within 30 days prior to enrollment in the study.
9. Women who are pregnant, capable of becoming pregnant and not participating in an acceptable form of birth control, or who are breast feeding.
10. Patients currently participating in another interventional study.
11. Major surgery within one month prior to enrollment in the study or planned surgery while the patient is in the study, other than dialysis vascular access surgery.
12. Patients who the investigator determine have a medical status that would preclude the patient's participation.
13. Patients on hemodialysis or peritoneal dialysis.
14. Patients with a functional kidney transplant.
15. Patients with past lactulose use.