Does Metamucil Interact With Benadryl Or Prednisone?

In our latest question and answer, our pharmacist discusses whether or not Metamucil interacts with Benadryl or prednisone.


I am taking Metamucil and am scheduled for a cat scan any day now. Can my doctors mix Metamucil with prednisone and Benadryl for my cat scan? I have had a bad reaction a few years ago so now the doctors always use prednisone and Benadryl for the scan. Google says I should not mix Metamucil with these other 2 drugs.

Asked by Maxine On Aug 07, 2022

Answered by
Medical Content Reviewed By HelloPharmacist Staff

Published Aug 09, 2022
Last updated Aug 09, 2022

Key points

  • Benadryl and prednisone are common medications to take before a CT scan.
  • Metamucil, in general, should not be taken at the same time as other drugs since it can decrease both their rate, and extent, of absorption.

Thanks so much for your question Maxine!

I'm sorry to hear you've had problems in the past getting a CT scan (also known as a 'CAT' scan). It's not uncommon to pre-medicate with certain drugs, like Benadryl and prednisone, to reduce the risk of a reaction to the contrast media used for the scan. This is especially true if you have had a reaction in the past.

Metamucil With Benadryl And Prednisone

In regard to taking both of these drugs (Benadryl and prednisone) with Metamucil (a psyllium fiber product), you should not take them at the same time.

This is a general rule of thumb for fiber products in general.

Metamucil, being a fiber, can significantly affect the absorption of drugs if taken at the same time. Generally, fiber decreases both the rate and extent of absorption of other drugs given with it since fiber forms a gel-like substance in the stomach when ingested. This may make these medications less effective.

You should, ideally, take any drugs at least 30-60 minutes before taking a fiber product like Metamucil or at least 2 hours after.

If you are scheduled for a CT scan on a certain day, I would just take your pre-medication regimen of Benadryl and prednisone as directed by your doctor and simply take your Metamucil dose later in the day to avoid an interaction.

Final Words

Thank you so much again for reaching out! Please contact us again if anything else comes up!


  • Fiber supplements and clinically proven health benefits: How to recognize and recommend an effective fiber therapy, PubMed
  • Dietary fiber and its interaction with drugs, PubMed

About the Pharmacist

Dr. Brian Staiger, PharmD

Dr. Brian has been practicing pharmacy for over 11 years and has wide-ranging experiences in many different areas of the profession. From retail, clinical and administrative responsibilities, he's your knowledgeable and go-to source for all your pharmacy and medication-related questions! Feel free to send him an email at! You can also connect with Dr. Brian Staiger on LinkedIn.

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