This code system http://hl7.org/fhir/reason-medication-not-given defines the following codes:

Code Display Definition
a None No reason known.
b Away The patient was not available when the dose was scheduled.
c Asleep The patient was asleep when the dose was scheduled.
d Vomit The patient was given the medication and immediately vomited it back.
<status value="active"/> <experimental value="false"/> <description value="This value set is provided as an example. The value set to instantiate this attribute should be drawn from a robust terminology code system that consists of or contains concepts to support the medication process."/> <jurisdiction> <coding> <system value="http://unstats.un.org/unsd/methods/m49/m49.htm"/> <code value="001"/> <display value="World"/> </coding> </jurisdiction> <caseSensitive value="true"/> <content value="complete"/> <concept> <code value="a"/> <display value="None"/> <definition value="No reason known."/> </concept> <concept> <code value="b"/> <display value="Away"/> <definition value="The patient was not available when the dose was scheduled."/> </concept> <concept> <code value="c"/> <display value="Asleep"/> <definition value="The patient was asleep when the dose was scheduled."/> </concept> <concept> <code value="d"/> <display value="Vomit"/> <definition value="The patient was given the medication and immediately vomited it back."/> </concept> </CodeSystem>