Q. What brocho do you make when drinking unflavored Metamucil?

A. Metamucil is one of the brand names for a psyllium fibre compound, used as a bulk-forming laxative and cholesterol-lowering medication.
Poskim rule that one does not recite a brocho when eating or drinking something that one does not derive any pleasure when consuming it. The classic case is drinking water just to swallow a pill (Binyan Tziyon 1: 10, Yad Halevy 35 et. al.).
Therefore Horav Shlomo Miller’s Shlit’a opinion is not to recite a brocho unless it is a flavored compound. (See question 1217 in regards to using Metamucil on Shabbos and Yom Tov).

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a