[Backend] Implement Drug Interaction Checker in Prescriptions Module

[llinsss]

⚠️ This is a backend issue — work is done inside the backend/ folder

Description

PrescriptionsService imports and injects DrugInteractionService from backend/src/modules/prescriptions/services/drug-interaction.service.ts, but this file contains only a stub. When a new prescription is created, no interaction check is performed, meaning dangerous drug combinations can be silently saved.

Current State

• backend/src/modules/prescriptions/services/drug-interaction.service.ts — stub/empty
• PrescriptionsService.create() calls this.drugInteractionService.check() but receives no real result
• No interaction data source is defined

What Needs to Be Built

1. Interaction Data Source

• Define a static JSON dataset of known drug interactions (common veterinary medications)
• Or integrate with an external API (e.g. RxNorm / custom vet drug DB) via HTTP
• Store interaction rules in a drug_interactions DB table with severity levels: MILD, MODERATE, SEVERE, CONTRAINDICATED

2. DrugInteractionService

@Injectable()
export class DrugInteractionService {
  async check(
    newMedication: string,
    existingMedications: string[],
  ): Promise<InteractionResult[]>
}

export interface InteractionResult {
  drug1: string;
  drug2: string;
  severity: 'MILD' | 'MODERATE' | 'SEVERE' | 'CONTRAINDICATED';
  description: string;
}

3. Integration in PrescriptionsService

• On create(), fetch all active prescriptions for the pet
• Run interaction check against new medication
• If CONTRAINDICATED or SEVERE: throw BadRequestException with interaction details
• If MODERATE: save prescription but attach a warning in the response
• If MILD: save silently, log warning

4. New Endpoint

POST /prescriptions/check-interactions
Body: { petId, medication }

• Allows frontend to check interactions before submitting a prescription form

Acceptance Criteria

• CONTRAINDICATED combinations are blocked with a descriptive error
• MODERATE interactions return a warning alongside the saved prescription
• POST /prescriptions/check-interactions returns interaction results without saving
• Interaction data covers at least 20 common veterinary drug pairs
• Unit tests cover all severity levels and the no-interaction case

Files to Create / Modify

• backend/src/modules/prescriptions/services/drug-interaction.service.ts (implement)
• backend/src/modules/prescriptions/prescriptions.service.ts (integrate check)
• backend/src/modules/prescriptions/prescriptions.controller.ts (add check endpoint)
• backend/src/modules/prescriptions/dto/check-interaction.dto.ts (new)
• backend/src/modules/prescriptions/services/drug-interaction.service.spec.ts (new)

Priority

High — patient safety feature

Estimated Effort

2–3 days

[pharuq411]

@pharuq411 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

i will like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @pharuq411 to this issue.

[drips-wave]

Congratulations, @pharuq411! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @pharuq411: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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