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It is a filler/binding agent. Thus MC belongs in the context of a drug to the group of so called excipients. The study of the best suitable excipients (as a tradeoff of factors such as cost, and ease of approving the drug) is called galenics.

Methyl cellulose is also present in your toothpaste and some of the foods your eat and in the construction industry. It can be considered to be metabolically inert for humans, but can serve as a matrix for enhancing bacterial adhesion and biofilm formation.
The gradual smudging and staining of your bathroom washbowl may be largely attributable to MC and bacteria.

MC is polymer that is sold as dry powder of various mean chain lengths. It is hydrophilic and can retain large volumes of water.