A co-agent is another drug, disease state, or even a procedure that would alter the efficacy, action, or applicability of the medication under consideration, and therefore would potentially cause that medication to be contra-indicated. Because of the gamut of possible concepts involved, there are many potential code systems from which the concept may be drawn. Therefore the co-agent code references a Concept Domain, not a Value-Set [Per the HL7 Binding Semantics Vocabulary project, this is called a domain binding]. For example, if a patient currently taking a drug containing nitrates, a new prescription for Cialis would be contra-indicated. Similarly, a new prescription for a beta blocker should not be given to a patient with asthma. "Identifies the co-existing agent contributing to the DUR event (drug or disease conflicting with the prescribed drug or prompting pharmacist professional service)." - NCPDP Telecommunication (Field 476-H6, Data Dictionary 201104). "Identifies the co-existing agent contributing to the DUR event (drug or disease) conflicting with the prescribed drug." - NCPDP Script (Field 7883, Data Dictionary 201104)