Overview
This lecture covers the Apothecary measurement system used in pharmacy, its historical context, main units, conversion methods, and practical applications in medication dosing.
Apothecary System Overview
- The Apothecary system is an old English measurement system for pharmacy.
- The grain is the standard unit for weight in this system, based on a wheat grain.
- The minim is used for very small liquid volumes, approximately a drop of water.
- The fluid dram is a larger liquid unit, and the fluid ounce is used for even larger volumes.
Key Conversions and Formulas
- 1 grain (gr) = 60 mg (milligrams).
- 1 fluid dram (fl dr) = 5 ml (milliliters).
- 1 fluid ounce (fl oz) = 30 ml.
- Some drugs (e.g., acetaminophen, iron sulfate, aspirin) use 1 grain = 65 mg.
Conversion Examples
- To convert grains to mg: multiply number of grains by 60 (or 65 for exceptions).
- Example: 4 grains of ibuprofen = 4 × 60 mg = 240 mg.
- For exceptions: 2 grains of aspirin = 2 × 65 mg = 130 mg.
- To convert fluid drams to ml: multiply number of fluid drams by 5.
- Example: 3 fluid drams = 3 × 5 ml = 15 ml.
Modern Use and Applications
- The Apothecary system is mostly replaced by the metric system in modern pharmacy.
- Still used in some compounded medications, topical formulations, and traditional/herbal preparations.
- May appear in historical texts, legal/regulatory contexts, and veterinary medicine.
Key Terms & Definitions
- Grain (gr) — Base unit of weight in the Apothecary system (usually 60 mg, 65 mg for some drugs).
- Minim — Smallest Apothecary liquid volume, about one drop.
- Fluid dram (fl dr) — Apothecary liquid volume equal to 5 ml.
- Fluid ounce (fl oz) — Apothecary liquid volume equal to 30 ml.
Action Items / Next Steps
- Practice converting Apothecary units to metric units using provided formulas.
- Review common exceptions to grain-to-milligram conversions for specific drugs.