Medication Formulary Guide

A "formulary" refers to a catalog of drugs offered by your organization. When implementing a custom EMR, Digital Health clinical administrators often curate a formulary of relevant drugs, along with relevant metadata, to assist prescribing physicians, pharmacists, and patients.

This guide covers the basics of how to use the MedicationKnowledge resource, which represents a type of medication that can be ordered. It will cover:

• How to annotate medications with the proper codes.
• How to represent physical characteristics about the medication and its package.
• How to categorize medications in the formulary.
• How to represent compounded medications, including dose variations.
• How to represent relationships between medications.

This guide is informed by the DaVinci Payer Data Exchange (PDex) US Drug Formulary implementation guide, which was authored by industry leaders, including Humana, Cigna, and Optum.

Medication Code

When defining MedicationKnowledge.code, it's highly recommended to use RxNorm as the primary coding system. RxNorm is an international standard that normalizes drug names from many different data sources.

RxNorm has codes for medications with different levels of specificity, called "term types". Refer to our RxNorm guide for a detailed understanding of the term-type hierarchy.

The Da Vinci formulary guide recommends using the following term type when creating a formulary to achieve the appropriate level of specificity:

You can use the RxNav browser to search for RxNorm codes for your formulary.

Below is an example MedicationKnowledge with both SBD and SBDG codes:

{
  resourceType: "MedicationKnowledge",
  code: {
    coding: [
      {
        system: "http://www.nlm.nih.gov/research/umls/rxnorm",
        code: "1049640",
        display: "acetaminophen 325 MG / oxycodone hydrochloride 5 MG Oral Tablet [Percocet]"
      },
      {
        system: "http://www.nlm.nih.gov/research/umls/rxnorm",
        code: "1185784",
        display: "Percocet Pill"
      }
    ]
  }
}

Branded vs. Generic Medications

RxNorm term types SBD, BPCK, and SBDG all refer to specific brand names for a given drug. Including branded drugs in your formulary indicates that physicians may only prescribe a specific brand.

In contrast, using generic term types (SCD, GPCK, SCDG) indicates that physicians may prescribe any brand of the medication.

Prescribable vs. Dispensable Medications

While most Medplum applications will include either generic or branded versions of a medication, some advanced implementations may include both. This is especially common with providers that own their own pharmacy.

In these implementations, generic codes would be used by physicians to prescribe the medication, whereas pharmacies branded medications would be used to dispense the medication. See Medication Relationships for details on how to represent the link between branded and generic MedicationKnowledge resources.

Medication Characteristics

The MedicationKnowledge resources can also be used to store details about the physical characteristics of a medication, as well as its packaging and form.

The table below summarizes the most important medication metadata elements.

Example of Drug Characteristics

{
  resourceType: 'MedicationKnowledge',
  code: {
    text: 'acetaminophen 325 MG [Tylenol]',
    coding: [
      {
        system: 'http://www.nlm.nih.gov/research/umls/rxnorm',
        code: '569998',
      },
      {
        system: 'http://hl7.org/fhir/sid/ndc',
        code: '5058049501',
      },
    ],
  },
  drugCharacteristic: [
    {
      type: {
        text: 'Color',
        coding: [
          {
            system: 'http://terminology.hl7.org/CodeSystem/medicationknowledge-characteristic',
            code: 'color',
          },
        ],
      },
      valueString: 'white',
    },
    {
      type: {
        text: 'Imprint Code',
        coding: [
          {
            system: 'http://terminology.hl7.org/CodeSystem/medicationknowledge-characteristic',
            code: 'imprintcd',
          },
        ],
      },
      valueString: 'Tylenol',
    },
  ],
  amount: {
    value: 100,
    unit: 'tablets',
    system: 'http://terminology.hl7.org/ValueSet/v3-UnitsOfMeasureCaseSensitive',
    code: '{tbl}'
  },
  packaging: {
    type: {
      text: 'Bottle',
      coding: [
        {
          system: 'http://terminology.hl7.org/CodeSystem/medicationknowledge-package-type',
          code: 'bot',
          display: 'Bottle',
        },
      ],
    },
    quantity: {
      value: 100,
      unit: 'tablets',
      system: 'http://terminology.hl7.org/ValueSet/v3-UnitsOfMeasureCaseSensitive',
      code: '{tbl}'
    },
  },
  doseForm: {
    coding: [
      {
        display: 'Tablet',
        code: '385055001',
        system: 'http://snomed.info/sct',
      }
    ]
  },
  intendedRoute: [
    {
      text: 'oral',
      coding: [
        {
          display: 'Oral use',
          code: '26643006',
          system: 'http://snomed.info/sct'
        }
      ]
    },
  ],
}

Classifications and Regulations

The MedicationKnowledge.productType field can be used to categorize the drug within the formulary, potentially along multiple dimensions. This field can also be used to describe:

• Whether a drug is generic or branded
• Whether the drug requires a prescription (see: Legal status of Supply)

Representing Controlled Substance Schedules

Certain medications, including narcotics, are classified into schedules based on the U.S. Controlled Substances Act (CSA). MedicationKnowledge.regulatory.schedule is used for specifying the regulatory schedule for any medications subject to these restrictions.

Refer to the HL7 Controlled Substances Schedule for an example valueset.

Compounded Medications

For medications that must be mixed before being dispensed, the MedicationKnowledge.ingredient field is used for listing ingredients of compounded medications, with RxNorm as the preferred code system for each ingredient.

The MedicationKnowledge.amount field indicates the total amount of the compound to be dispensed, and the strength of each ingredient is stored as a ratio of the total volume (e.g. 1g per 100g).

ingredient.isActive indicates which of the listed ingredients are active.

Dose Variations

For compounded medications with multiple dose variations, it's common to model a parent-child relationship where:

• Parent medication: Contains the core compound formula with ingredients and base concentrations
• Child medications: Represent specific dose variations of the parent compound with different volumes and administration instructions

This hierarchical structure allows for efficient management of compound families while maintaining specific dosing information for each variation.

Example: Parent Compound Medication

{
  resourceType: "MedicationKnowledge",
  status: "active",
  ingredient: [
    {
      itemReference: {
        reference: "Substance/1234",
        display: "Semaglutide"
      },
      strength: {
        numerator: {
          value: 2.5,
          unit: "mg"
        },
        denominator: {
          value: 1
        }
      }
    },
    {
      itemReference: {
        reference: "Substance/5678",
        display: "Glycine"
      },
      strength: {
        numerator: {
          value: 5,
          unit: "mg"
        },
        denominator: {
          value: 1
        }
      }
    }
  ],
  code: {
    text: "Semaglutide 2.5 mg/mL + Glycine 5 mg/mL",
    coding: [
      {
        system: "medication-coding-system",
        code: "3099",
        display: "Semaglutide 2.5 mg/mL + Glycine 5 mg/mL"
      }
    ]
  },
  productType: [
    {
      coding: [
        {
          system: "medication-type-coding-system",
          code: "compound",
          display: "Compound"
        }
      ]
    }
  ],
  doseForm: {
    coding: [
      {
        system: "http://hl7.org/fhir/administrable-dose-form",
        code: "100000073863",
        display: "Solution for injection"
      }
    ]
  },
  relatedMedicationKnowledge: [
    {
      type: {
        coding: [
          {
            system: "compounded-medication-family",
            code: "child",
            display: "child"
          }
        ]
      },
      reference: [
        {
          reference: "MedicationKnowledge/<child1-id>",
          display: "1.7mg Semaglutide 2.5 mg/mL + Glycine 5 mg/mL 3 mL"
        },
        {
          reference: "MedicationKnowledge/<child2-id>",
          display: "1mg Semaglutide 2.5 mg/mL + Glycine 5 mg/mL 1.5 mL"
        }
      ]
    }
  ]
}

Example: Child Dose Variation

{
  resourceType: "MedicationKnowledge",
  status: "active",
  amount: {
    value: 1,
    extension: [
      {
        id: "medication-days-supply",
        url: "https://www.hl7.org/fhir/datatypes.html#integer",
        valueInteger: 28
      }
    ],
    unit: "Each"
  },
  code: {
    text: "1.7mg Semaglutide 2.5 mg/mL + Glycine 5 mg/mL 3 mL",
    coding: [
      {
        system: "medication-coding-system",
        code: "3080",
        display: "1.7mg Semaglutide 2.5 mg/mL + Glycine 5 mg/mL 3 mL"
      }
    ]
  },
  administrationGuidelines: [
    {
      dosage: [
        {
          dosage: [
            {
              patientInstruction: "Inject Semaglutide 1.7mg (0.70mL or units) subcutaneously once weekly"
            }
          ],
          type: {
            coding: [
              {
                code: "maintenance",
                display: "maintenance"
              }
            ]
          }
        }
      ]
    }
  ],
  productType: [
    {
      coding: [
        {
          system: "medication-type-coding-system",
          code: "compound",
          display: "Compound"
        }
      ]
    }
  ],
  relatedMedicationKnowledge: [
    {
      type: {
        coding: [
          {
            system: "compounded-medication-family",
            code: "parent",
            display: "parent"
          }
        ]
      },
      reference: [
        {
          reference: "MedicationKnowledge/<parent-id>",
          display: "Semaglutide 2.5 mg/mL + Glycine 5 mg/mL"
        }
      ]
    }
  ]
}

Key Considerations for Parent-Child Compounds

Parent Medication Features:

• Contains the complete ingredient list with base concentrations
• Uses relatedMedicationKnowledge with type "child" to reference dose variations
• Typically lacks specific administration guidelines (handled by children)
• Serves as the master template for the compound family

Child Medication Features:

• References parent using relatedMedicationKnowledge with type "parent"
• Contains specific volume amounts and administration instructions
• Includes dose-specific coding and display names
• May include additional metadata like days supply and patient instructions

Benefits of This Approach:

• Centralizes ingredient management in the parent
• Allows for easy addition of new dose variations
• Maintains clear relationships between related compounds
• Supports efficient formulary navigation and management

Example: Traditional Single Compounded Medication

{
  resourceType: 'MedicationKnowledge',
  amount: {
    value: 200,
    unit: 'g',
    system: 'http://unitsofmeasure.org',
  },
  ingredient: [
    {
      isActive: true,
      itemCodeableConcept: {
        text: 'Baclofen powder',
        coding: [
          {
            system: 'http://www.nlm.nih.gov/research/umls/rxnorm',
            code: '1292',
          },
        ],
      },
      strength: {
        numerator: {
          value: 5,
          unit: 'g',
          system: 'http://unitsofmeasure.org',
        },
        denominator: {
          value: 100,
          unit: 'g',
          system: 'http://unitsofmeasure.org',
        },
      },
    },
    {
      isActive: true,
      itemCodeableConcept: {
        text: 'Ketoprofen powder',
        coding: [
          {
            system: 'http://www.nlm.nih.gov/research/umls/rxnorm',
            code: '6142',
          },
        ],
      },
      strength: {
        numerator: {
          value: 10,
          unit: 'g',
          system: 'http://unitsofmeasure.org',
        },
        denominator: {
          value: 100,
          unit: 'g',
          system: 'http://unitsofmeasure.org',
        },
      },
    },
    { 
      // Additional ingredients here...
    }
  ],
  packaging: {
    type: {
      text: 'Tube',
      coding: [
        {
          system: 'http://terminology.hl7.org/CodeSystem/medicationknowledge-package-type',
          code: 'tube',
          display: 'Tube',
        },
      ],
    },
  },
}

Medication Relationships

Advanced implementations might want to model structured relationships between medications. For example, administrators might want to link MedicationKnowledge resources representing generic and branded medications, to aid in fulfillment (see Branded vs. Generic Medications)

Relationships between drugs in the formulary are modeled using the MedicationKnowledge.relatedMedicationKnowledge field. The type of each relationship is specified by MedicationKnowledge.relatedMedicationKnowledge.type.

Common relationship types include:

• Parent-Child: Used for compound families where a parent contains the base formula and children represent dose variations
• Generic-Branded: Links generic and branded versions of the same medication
• Therapeutic Equivalents: Medications that can be substituted for one another

Custom relationship type systems can be defined for organization-specific needs, such as the "compounded-medication-family" system shown in the compound examples above.

Refer to the RxNORM relationship codes for an example code system of relationships between drugs.