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

CodeDisplayDefinition
a NoneNo reason known.
b AwayThe patient was not available when the dose was scheduled.
c AsleepThe patient was asleep when the dose was scheduled.
d VomitThe 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>