Abstract: This invention relates to a prescription decision support system of the type comprising a set of variables and domains, a first constraint base comprising a plurality of logical statements, an Assess algorithm module and a Consistent algorithm module. The logical statements of the first constraint base specify the combination of disease states and treatments that are present that should not occur in a prescription. The prescription decision support system further comprises a second constraint base comprising a plurality of logical statements, the logical statements specifying the combination of disease states and treatments, which treatments should be present in the prescription. By providing such a system, it is possible for the prescription decision to reason about complex relationships including the presence or absence of treatments, or dosages, or the presence or absence of disease states. Furthermore, the system is responsive to the specific nature of the patient's case history.