Consider Updating Dosage to Allow Multiple PRN Reasons

Providers can specify multiple PRN reasons/indications on medication orders in our system (e.g. take as needed for nausea and vomiting). Currently, there's not a great way to represent multiple PRN reasons in the Dosage datatype/in MedicationRequest.dosageInstruction (and equivalents). While the dosage.asNeededBoolean can convey some detail, the reasons themselves are hard to transmit clearly.

We've looked at multiple options:

1) Multiple dosageInstruction repetitions, each with an asNeededCodeableConcept representing one of the reasons - this may cause confusion as it's unclear what dose + timing specified in each Dosage element is for/needs agreement on what dose + timing specified in each Dosage element is for (i.e. is it every 6 hours as needed for nausea and also every 6 hours as needed for vomiting (two doses allowed every 6 hours), or every 6 hours as needed for nausea or vomiting (one dose allowed every 6 hours)).

2) Multiple MedicationRequest resources, each representing a PRN reason - similar confusion about what the dose + timing is for.

3) New 0..* codeableConcept element representing PRN reasons (either in place of or in addition to asNeededBoolean) - this seems like the most straightforward option (and easiest for backporting to previous versions as extensions).