Current MedicationRequest, MedicationAdministration and MedicationStatement(MedicationUsage) resources do not include any guidance about how to determine cumulative medication duration (I.e., how long a specific medication has been in use). One use case includes the ability to know that for a given diagnosis (e.g., Major Depression Disorder), the patient has been receiving anti-depressant medication for 90 days or 180 days. The evaluation needs to include the duration as prescribed allowing for skip days. However, there is yet another us case concept to consider - that of lifetime cumulative dosage for which some medications have cumulative maximum doses that lead to significant side effects.  These are two different concepts and some guidance will be very significant to allow consistent usage.