Cal. Rev. & Tax. Code § 6358.4

There are exempted from the taxes imposed by this part, the gross receipts from the sale in this state of, and the storage, use, or other consumption in this state of, drugs or medicines administered to animal life as an additive to feed or drinking water, the primary purpose of which is the prevention and control of disease of food animals, or of nonfood animals which are to be sold in the regular course of business.