A. Metamucil and similar brands are a psyllium fiber, bulk-forming powder laxative made from the husks of the Plantago Ovata plant’s seeds. There are two concerns on using it on Shabbos and Yom Tov, firstly it may be considered medicine and then, making it could be part of the prohibited melacha of losh (kneading or mixing).
Since it is used not only as a laxative but also as a diet supplement, it is not necessarily seen as a medicine that may be prohibited on Shabbos and Yom Tov, but rather as ma'achol bri'im or food that healthy people consume, and therefore permitted.
The powder is dissolved in water to drink as a liquid, similar to preparing of a cup of coffee. If not consumed when mixed, depending on the brand, composition and liquid it was dissolved in, even after a short time the mixture may solidify or become akin to a mixture of flour dissolved in water, or a somewhat thick dough-like substance. The above would amount to transgressing on the prohibited melacha of Losh.
Horav Shlomo Miller's Shlit'a opinion is that when consumed immediately after being mixed, it is permitted in need. People who drink it daily, could do so before the beginning and after the end of Shabbos.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a