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name: langcare-drug-interactions description: > Checks for clinically significant drug-drug interactions across active medications using CYP450 metabolism pathways, pharmacodynamic interactions, and contraindicated combinations. Use when asked to check drug interactions, medication safety check, CYP interactions, or before adding a new medication to an existing regimen.

Drug Interaction Checker

When to Use This Skill

Use when a clinician needs to evaluate the active medication list for drug-drug interactions, particularly before prescribing a new medication or during medication reconciliation.

Clinical Workflow

1. Use fhir_search to pull all active MedicationRequest and MedicationStatement resources
2. Use fhir_search to pull active AllergyIntolerance for drug allergy cross-check
3. Use fhir_search to pull relevant Observation resources (renal function, hepatic function) for dose adjustment context
4. Map each medication to its CYP450 substrate/inhibitor/inducer profile (see references/cyp450-table.md)
5. Identify pharmacokinetic interactions (CYP inhibition/induction affecting drug levels) and pharmacodynamic interactions (additive/synergistic effects: QT prolongation, serotonin syndrome, bleeding risk, CNS depression)
6. Classify interactions by severity: contraindicated, major, moderate, minor
7. Present findings with mechanism, clinical significance, and recommended action

FHIR Resources

• MedicationRequest -- Active prescriptions with medication codes
• MedicationStatement -- Patient-reported medications including OTC and supplements
• AllergyIntolerance -- Drug allergy cross-reference
• Observation -- Renal function (CrCl/eGFR), hepatic function (ALT, AST) for dose adjustment context

FHIR Query Examples

Pull All Active Medications

fhir_search(resourceType="MedicationRequest", queryParams="patient=[patient-id]&status=active&_count=100")

Pull Patient-Reported Medications (OTC, supplements)

fhir_search(resourceType="MedicationStatement", queryParams="patient=[patient-id]&status=active&_count=100")

Pull Renal Function for Dose Context

fhir_search(resourceType="Observation", queryParams="patient=[patient-id]&code=33914-3&_sort=-date&_count=1")

Clinical Guidelines

• FDA drug interaction guidance for labeling
• AHA/ACC guidelines on QT-prolonging drug combinations
• Beers Criteria for potentially inappropriate drug combinations in elderly

Interpretation Guide

• Classify by severity: Contraindicated (avoid combination), Major (modify therapy), Moderate (monitor closely), Minor (be aware)
• For CYP interactions, specify the pathway (CYP3A4, CYP2D6, etc.), whether the interacting drug is an inhibitor or inducer, and the expected effect on the substrate drug levels
• For pharmacodynamic interactions, specify the shared mechanism (QT prolongation, serotonin, bleeding, CNS depression, anticholinergic burden)
• Provide actionable recommendations: alternative medications, dose adjustments, monitoring parameters

Safety

• Never fabricate clinical data -- only report what FHIR returns
• Flag critical/abnormal values immediately
• Scope all FHIR queries to the authenticated patient
• Use standard terminology (LOINC, SNOMED CT, RxNorm, ICD-10)
• Present data in clinician-friendly format with reference ranges