See http://en.wikipedia.org/wiki/Pill_splitting
Dosage uniformity:
In the U.S.—and for more than 130 countries which use or recognize American pharmaceutical standards—"uniformity of dosage units" is defined by the United States Pharmacopeia (USP), which describes itself as "the official public standards-setting authority for all prescription and over-the-counter medicines, dietary supplements, and other healthcare products manufactured and sold in the United States."[2]

The USP standard for dosage uniformity expresses statistical criteria in the language of sampling protocols which defies simple explanation. The pharmaceutical dosage literature sometimes boils this down as requiring a standard deviation in dosage weight of less than 6%, which roughly corresponds to the weaker rule-of-thumb offered for public consumption that the vast majority of dosage units should be within 15% of the dosage target. "Dosage unit" is a technical term which covers oral medications (tablets, pills, capsules), as well as non-oral delivery methods.[3]

A 2002 study of pill-splitting as conducted in four American long-term care facilities determined that 15 of the 22 dispensed prescriptions evaluated (68%) had fragment weight variance in excess of USP standards.[4]