Generic Dose Calculators
The screenshot on the left stems from MedCalc and features the formula that triggered Apple's rejection of our update. The one on the right belongs to Infusion - Drug Dose Calculator, one of the many generic drug dose calculators that used to be available on the App Store.
As one can see, neither formulas actually provide medicinal dosage. They apply a simple calculation to a dosage that is provided by the user, most likely after looking it up in some reference material of their choice. This is a calculation performed multiple times during routine medical care, and as such falls under FDA's category 3B, apps that the FDA does not intend to enforce requirements.
Still, this operation is enough for the application to fall under the 22.9 rule and not pass validation. This is a very broad interpretation of the “Apps that calculate medicinal dosages” directive that virtually kills an entire niche of apps available on the App Store today. Such apps can't be updated without being rejected and will likely be taken down eventually.