[cotsuka]

Chalk?

[ahi]

It has antacid properties.

[masklinn]

Cellulose as well? Dicalcium phosphate is also a common tablet filler, though I don't know if it's biologically inactive.

[josscrowcroft]

Calcium, surely..

[masklinn]

Calcium can interact with a number of other chemicals, potentially negatively (for the surrounding organism). Although doses are probably important in judging that.

http://en.wikipedia.org/wiki/Calcium_metabolism#Interaction_...