Bottles of Miralax laxatives products for constipation relief on sale at a retail store

Answer

In your situation, it certainly would be best to discuss the symptoms you are experiencing with your doctor as there could be many factors contributing to your issue.

However, based on what you described in your question, Metamucil (psyllium fiber) most likely wouldn't be a good initial choice unless your doctor specifically recommends it as it can cause bloating, abdominal distention and bowel expansion, making it even more difficult to pass. In fact, Metamucil is contraindicated in those with fecal impaction or bowel obstruction is it could make the situation worse.

Miralax (polyethylene glycol 3350) on the other hand, retains water in the stool, does not cause abdominal distension and generally does not cause bloating either.

In a situation where you already have difficult to pass stools (so much so that they need to be physically removed), Miralax may make them easy to pass and can have a mild laxative effect.

Below, we describe in more detail how Metamucil and Miralax work.


Metamucil Info

Metamucil contains psyllium fiber and is classified as a 'bulk-forming' laxative. It works as follows:

  • Psyllium fiber (Metamucil) absorbs liquid in the GI tract, resulting in expansion of the stool. In most cases, this also softens the stool, making it easier to pass.

Metamucil is associated with more side effects than Miralax. They include:

  • Bloating
  • Abdominal distention
  • Gas
  • Fecal impaction (high doses)

While Metamucil can help to prevent fecal impaction, it should not be used to treat it in most situations as it can make symptoms worse. Most medical guidelines state that fiber products are contraindicated (i.e. should not be used) in those with a current fecal or bowel obstruction.


Miralax Info

Miralax (polyethylene glycol) is an osmotic laxative, that works by:

  • Drawing water into the intestines.

Unlike Metamucil, it doesn't typically cause fecal expansion and generally leads to more watery stools.

You can think of fiber products like Metamucil causing the stool to absorb water (and thus expanding) while Miralax simply increases water in the bowel. This is why high doses have a laxative effect and are used for colonoscopy preparation. It is also why it is most often the product of choice for fecal impaction.

Miralax generally does not cause any of the side effects fiber does, like bloating and gas.


Additional Information

As mentioned previously, you should speak with your doctor prior to making any treatment choices for your difficult to pass stools. They have your complete medical history and will be able to give you an appropriate recommendation.


Summary

  • Metamucil (psyllium fiber) is contraindicated (i.e. should not be used) in those with a fecal impaction as it could make the situation even worse.
  • Miralax (polyethylene glycol 3350) draws water into the intestines and is non-constipating. It is the better choice for those with, or susceptible to, fecal impaction.

  • References
    1. Management of Constipation in Older Adults. AAFP
    2. Efficacy and safety of traditional medical therapies for chronic constipation: systematic review. PubMed
    3. Gastrointestinal Complications (PDQ®). NCBI