How we built a medicine-substitution engine that refuses to be clever
How we built a medicine-substitution engine that refuses to be clever There is a category of bugs where the code looks perfectly correct in code review, the unit tests pass, the demo on stage goes beautifully, and a real person dies. Medicine substitution is one of them. We built Agada — point a phone camera at a medicine strip in India, and the app tells you whether the drug is registered with the regulator, what it does, and whether a chemically identical version is available at the government pharmacy for a fraction of the price. The point is real: Indians spend about ₹65,000 crore a year out of pocket on branded medicines when the same molecule sits in a Jan Aushadhi Kendra at a tenth of the cost. The Dolo-650 story (₹32 vs ₹4.90 for the same paracetamol) is the most famous example, not the only one. The hard part isn't the camera, the OCR, or the price lookup. The hard part is the substitution engine — the piece of code that decides "is this salt the same as that salt." Get that wrong in the wrong direction, and the app cheerfully tells a user that their 500mg anti-epileptic can be replaced with a 200mg one, because the strings look similar. So we built a matcher that refuses to be clever. This post is about the parts we deleted. The starting problem A user scans "Crocin 500mg Tablet IP". A Jan Aushadhi record says "Paracetamol 500 mg". A naive matcher says: both contain "500", both contain "Paracetamol" (or "Acetaminophen", which is the same thing in a different country), ship it. The user buys the generic. Savings: ₹20. Everyone wins. Now consider: Crocin 650 Advance vs Dolo 650 — same molecule, same dose, different brands. Fine. Augmentin 625 vs Augmentin 375 — same two salts, different ratio. The combo tolerance might let it through, but the clinical implication is real. Levofloxacin 500 vs Ofloxacin 400 — both fluoroquinolones, both "similar," but levofloxacin is the levo-isomer of ofloxacin and is dosed at roughly half. A "fuzzy" match here could halve so