Medication Classification

If there are several meds that could be alternatives , how is that represented in FHIR

For example Med1 is identitcal to Med2 , they contain the same molecule.

Typically that’s handled in the drug terminology/knowledgebase and isn’t exposed. However, if you’re wanting to share code systems in FHIR, the relationships would be in the CodeSystem properties of the different codes. If you’re wanting to share drug knowledgebases in FHIR, you’d use the MedicationKnowledge resource - which allows you to assert relationships with other knowledgebase entries.