The four pills taken before a colonoscopy are bisacodyl tablets, most commonly sold under the brand name Dulcolax. Each tablet contains 5 mg of bisacodyl, for a total dose of 20 mg. These tablets are one part of a larger bowel prep regimen that also includes a liquid laxative, typically MiraLAX mixed with a sports drink like Gatorade.
Bisacodyl is a stimulant laxative you can buy over the counter. The four tablets kick-start the emptying process so the liquid portion of your prep can finish the job. Your doctor’s office likely gave you a specific schedule showing exactly when to take them, but here’s a closer look at what the pills do, when to take them, and what to expect.
What the Tablets Actually Do
Bisacodyl works locally inside your colon rather than being absorbed into your bloodstream. Once the tablet’s coating dissolves, enzymes in your gut convert it into an active compound that triggers two things at once. First, it stimulates the muscles lining your colon to contract more forcefully, speeding up transit. Second, it changes how your colon handles water: instead of absorbing fluid back into your body, the colon retains it, which softens stool and increases its volume. The combination of stronger contractions and more water in the bowel is what gets things moving.
This is why the pills are taken a couple of hours before the liquid laxative portion. They essentially “wake up” your colon so it’s already active when the large volume of MiraLAX solution arrives.
When to Take the 4 Pills
The exact timing depends on your doctor’s instructions, and prep schedules vary between medical centers. In one common version used by the VA health system, you swallow all four bisacodyl tablets at 3 p.m. the day before your procedure with at least 16 ounces of clear liquid. Then at 5 p.m., you start drinking the first half of your MiraLAX/Gatorade solution.
Some institutions spread the pills across two days. A Durham VA protocol, for example, has patients take four bisacodyl tablets at 1 p.m. on each of the two days before the procedure, both while on a clear liquid diet. Other Cleveland Clinic prep sheets call for only three bisacodyl tablets instead of four, taken at 5 or 6 p.m. the evening before.
The point is that your specific instructions matter more than any general guide. Follow the schedule your gastroenterologist’s office provided, because it’s designed to pair the pill timing with the liquid laxative timing for your particular appointment slot.
One Important Rule: No Dairy or Antacids
Do not take the bisacodyl tablets within one hour of eating dairy products (milk, yogurt, cheese) or taking antacids. These can break down the protective coating on the tablet too early, before it reaches your colon. When the coating dissolves in your stomach instead, you’re more likely to experience stomach upset and cramping without the tablets working as effectively where they need to.
Swallow the tablets whole with water or a clear liquid. Don’t crush or chew them, for the same reason.
Common Side Effects
Bloating is often the first thing you’ll notice, sometimes before you have your first bowel movement. Stomach cramping and nausea are also common and generally don’t require medical attention. Some people experience mild rectal discomfort once bowel movements become frequent.
More serious reactions are rare but worth knowing about. Contact your care team if you develop sudden or severe stomach pain, bloody diarrhea, fever, a fast or irregular heartbeat, dizziness, or significant vomiting. Swelling in your ankles, hands, or feet can signal a fluid balance problem and should also be reported.
The Full Prep Picture
The four bisacodyl tablets are just the opening act. The bulk of the cleaning happens with the liquid laxative, which is usually a full bottle of MiraLAX (238 grams of polyethylene glycol powder) split into two doses and mixed with 64 ounces of Gatorade or a similar clear sports drink. You drink each half over about an hour, taking an 8-ounce glass every 10 to 15 minutes.
In a typical split-dose prep, you drink the first half the evening before and the second half early the morning of your colonoscopy (usually about five hours before you need to leave for the appointment). Split dosing tends to clean the colon more thoroughly than drinking everything the night before, which is why most gastroenterologists now prefer it.
Dietary Changes Before the Pills
Your prep starts before you ever take the first tablet. Most protocols ask you to switch to a low-fiber diet about three days before your procedure. That means sticking to white bread, white rice, eggs, plain pasta, peeled potatoes, and dairy while avoiding nuts, seeds, raw vegetables, leafy greens, whole grains, popcorn, and fruits with skins or small seeds like strawberries, kiwi, and watermelon.
By two days before (or at least one full day before, depending on your protocol), you’ll transition to clear liquids only: water, pulp-free juices like apple or white grape, black coffee or tea, fat-free broth, plain popsicles, sports drinks, and clear sodas. Avoid anything red or purple, as these can stain the colon lining and be mistaken for blood during the procedure. Alcohol, milk, and thick or pulpy juices are also off the list.
Staying well-hydrated throughout this process makes a real difference. Aim for at least 8 ounces of clear liquid every hour you’re awake during the clear-liquid phase. This helps the laxatives work more effectively and reduces the risk of dehydration.
These Are Not the Same as Sutab
If you’ve heard of a “pill-only” colonoscopy prep, that’s a different product called Sutab. It requires 24 tablets total (12 the night before and 12 the morning of the procedure), each taken one at a time with sips of water. Sutab is a prescription medication that works through a different mechanism than bisacodyl.
The four bisacodyl tablets in a MiraLAX prep are over-the-counter, inexpensive, and serve as just one component of the regimen. If your doctor told you to pick up “four Dulcolax tablets,” make sure you buy the laxative version, not the stool softener. Dulcolax sells both products under similar packaging, and the stool softener contains a completely different active ingredient that won’t prep your colon for a colonoscopy.