Microcrystalline cellulose is a widely used excipient, an inert substance used in many pill and tablet formulations. As an insoluble fiber, microcrystalline cellulose is not absorbed into the blood stream, so it cannot cause toxicity when taken orally. In fact, it is so inert it is often used as a placebo in controlled drug studies. However, some side effects have been noted in animal studies, although usually at much higher dosages than would be normal for a human subject.

Increased Bowel Movements

Because cellulose is not absorbed in the intestine, consumption of large amounts of it may increase the frequency and volume of bowel movements. However, microcrystalline cellulose has less of an impact on bowel frequency and binding than do other forms of cellulose.

Weight Loss

Some studies suggest that significant amounts of microcrystalline cellulose in the diet can promote weight loss, either by adding to a feeling of fullness or by reducing the absorption of other nutrients in the diet. However, the dosages at which these effects occurred were much higher than are found in most pharmaceuticals that use this excipient.

Allergic Reactions

Microcrystalline cellulose may create an allergic reaction in some patients, although there are few reported cases of this. Because the substance is not absorbed, symptoms of allergic reaction are likely to be limited to gastrointestinal symptoms such as diarrhea or gas.