Short answer: Colonoscopy prep timing depends on the specific laxative you’re using, but most people notice bowel movements starting within 30 minutes to a few hours after the first dose, and the prep is considered complete when your stool is clear or light yellow and free of solid matter. Follow your clinician’s instructions for the exact timing and dosing for your prescribed product. Details

• Typical timelines by prep type:
  • PEG-based (isomer or electrolyte-based solutions): bowel movements often begin within 1–3 hours after starting, though the full cleansing process may take several hours. Complete cleansing is usually expected by the time you go to bed or the morning of the procedure, depending on whether you’re using a split-dose regimen.

* NaP (sodium phosphate) or other stimulant/osmotic regimens: onset can be a bit longer, often 3–6 hours to start, with full clearance by the night before the procedure or the morning of. Comfort and hydration are important, and if you’re constipated or not progressing, contact your provider.

• Key signs your prep is working:
  • Continual loose stools that are clear or yellowish and free of solid stool particles. Many guidance sources emphasize a “clear” or very light-colored stool as the indicator that prep is complete.

• Common practical points:
  • If you’re on a split-dose plan, the second dose is typically taken several hours before the procedure, with the goal of having a clean colon by the time of your appointment. Sleep between doses is often possible if your schedule allows.

* Hydration matters: sip clear fluids throughout the prep period to reduce dehydration and help the laxative work more effectively.

* If the first dose doesn’t produce movement within a few hours, or if multiple doses are taken and the stool remains poorly purged, contact your healthcare provider for specific instructions.

What to do next

• Check your exact prep kit and dosing schedule from the instruction sheet provided by your clinic or pharmacy, because timing and number of doses vary by product. If you’re unsure whether your prep is complete, don’t proceed to the procedure until your stool color and consistency meet your clinician’s criteria.

• If you’d like, share which preparation kit you’re using (brand and dosing schedule), and the time you’ve started and plan to go in for the procedure. A more precise timeline can be outlined based on that information.