You can give crushed granite but charcoal and oystershell should be kept out until a couple days after the medicine is through. Some medicines are effected a lot by calcium and it can cause the body not to absorb it properly. Charcoal also does the same. Adsorbs to toxins (includes drugs) and neutralizes them to where they're harmless/non-effective.
When a DRUG is spelled with the last few letters are >>> MYCIN....NO GRIT IS ALLOWED for the birds...The MYCIN drug is not going to do any good,because the mineral contents of grit wipe out the drug....So you would be medicating for nothing...After a day when the drug is no longer being used,give them the grit...When in DOUBT...NO GRIT should be given.......Alamo