Extract medications from this medical document. Return a JSON array or null.

Each entry:
- type: "prescription"
- value: (empty)
- summary: med name + dose, e.g. "Metformin 500mg"
- timestamp: "YYYY-MM-DD" if start date mentioned
- data: {"medication": "...", "dosage": "...", "frequency": "...", "prescriber": "..."}

Extract each distinct medication as a separate entry.

Document:
{{MARKDOWN}}