There is no drug interaction between Metamucil and Benadryl. These medications are also not known to cause an increase in side effect severity or frequency if they are taken together.


However, fiber supplements can very often decrease, or at the very least, slow the absorption of medication. As a general rule of thumb, medication is best taken at least 30 minutes before taking fiber or 2 to 3 hours after taking fiber to ensure optimal absorption.


Metamucil

Metamucil is a plant derivative known as psyllium which is used to draw water into the colon to help with mild constipation symptoms. This medication is very benign as it is essentially a condensed form of fiber to help move stool through the intestines. Adequate fiber intake is important for optimal health.


The main concern with this medication is if you are seriously constipated, it will likely cause more harm than actually be helpful in inducing a bowel movement. This type of medication is also not helpful in patients who have reduced movement in their bowels, like in those who are taking opioids. Like with other stomach medication, this can cause some stomach cramping and potentially diarrhea.


Benadryl

Benadryl is a "first-generation" antihistamine and is most commonly used to treat:

  • Allergy symptoms
  • Allergic reactions
  • Motion sickness
  • Insomnia


The onset of action of Benadryl is around 30 minutes and will generally last around 4 to 6 hours per dose. Benadryl has significant sedative effects and is often used a sleep aide.