I have noted on this site before about my experience with such a medication/food additive/supplement.
Your bowel fills up from top to bottom, and your stomach does not mix all foods together in one mass for digestion. The bowl fills up, one bite at a time, from top to bottom. So when you take all of the metamucal (?) all at once, it will occupy only a section of the bowl (or “poo train*,” if you prefer) and not the whole thing*, which would be more advantageous.
Better to take some of the additive throughout the day, a little with each meal, until you acquire the amount prescribed . This is also a good reason to eat more fiber.
(The average adult eats less than 12 grams of fiber a day. The amount one should get is between 20 and 30 grams. Eating more fiber throughout the day will help with IBS.))