Dang asked
I take it that Metamucil can"t be used as a colonoscopy prep instead of Miralax? My wife bought the Metamucil for her prep.
Answer
Yes, you are correct that Metamucil (psyllium fiber) cannot be used as a substitute for other pre-colonoscopy bowel preparations such as Miralax. Although they share a similar name, they work differently and you will not get the expected results if you use Metamucil instead.
In fact, using Metamucil instead of Miralax may cause fecal impaction (since you would be using more than the recommended dosage) and you will have to re-schedule your colonoscopy appointment since you won't have had an appropriate bowel evacuation prior.
How Miralax Works
Miralax, also known as polyethylene glycol, is classified as an osmotic laxative. It is non-absorbable and works by binding and retaining water into the stool. When not used in volumes needed for colonoscopy preparation, it can be used for constipation as it helps to soften the stool.
It has advantages over other bowel preparations as it generally does not deplete electrolytes, can be used in those with contraindications to other bowel preps (e.g. renal disease) and doesn't interfere with diagnosis of certain GI diseases, like inflammatory bowel disease.
The main disadvantage of Miralax is that large volumes of liquid need to be swallowed to achieve the appropriate amount of cleansing. In fact, one study reported that a large percentage of patients (> 5%) do not finish polyethylene glycol preparations due to the large volume needed or other issues, like taste.
How Metamucil Works
Metamucil contains psyllium fiber, which is bulk forming fiber. It absorbs liquid in the GI tract and causes expansion of the stool. While effective, using too much can create a stool that has expanded too much, making it difficult to pass. Metamucil also does not effectively evacuate the bowels like Miralax does.
While it seems like Miralax and Metamucil work in similar way, it is important to think of Miralax as retaining water in the colon (which produces a watery stool) while Metamucil absorbs water and causes an overall expansion of the stool.
Additional Information
It is important to use the bowel preparation prescribed to you by your doctor. Every option comes with certain considerations. If you accidentally have used the incorrect bowel prep product, reach out to your doctor for guidance.
