The asNeeded structure can be used to define a simple “as required” (or “pro re nata” expressed as “PRN” for those who still prefer to use Latin abbreviations) instructions. In this case use the asNeededBoolean option. The absence of asNeededBoolean is equivalent to where asNeededBoolean has a value of “false”.
The asNeededCodeableConcept option is used to bound the dosage instruction to a coded term. Any number of coded terms can be used from the SNOMED-CT hierarchy as a descendant of 404684003 Clinical Finding (finding). An extensive medication-as-needed-reason value-set is defined within FHIR which is a subset of the SNOMED-CT clinical findings hierarchy.