Covered in Section 1825 of the Manual of Patent Examining Procedure (MPEP). The physical requirements for drawings are outlined in PCT Rule 11.
Many drugs and other types of pharmaceuticals are considered "compositions of matter." Follow this link for more resources for searching for chemicals / pharmaceuticals: http://clemson.libguides.com/advanced_patent_searching/chem
This is where the legal requirements are outlined in great details, in independent and dependent claims. See Section 2173 of the Manual of Patent Examining Procedure (MPEP) for further detail.