Where should I put a medication effectiveness in FHIR

Our practitioners go back after a med admin and chart the effectiveness of the medication. Right now I am putting this as an event in the admin event history. Is this correct?

Part of me thinks this is an observation, but then that gets away from the medication admin object that it’s referring to.

Just curious to get another perspective.

Thanks!

Also, I am working in FHIR 3.

As per http://wiki.hl7.org/index.php?title=FHIR_Rules_for_asking_questions, questions should only be asked in one place. Please see response here: http://stackoverflow.com/questions/41572123/where-should-i-put-a-medication-effectiveness-in-fhir