Overview
This lecture covers how to calculate days supply for various medication dosage forms, including tablets, liquids, inhalers, eye/ear drops, and injectables, using real prescription examples and key conversion factors.
Introduction to Days Supply
- Days supply means how many days a prescribed amount of medication will last.
- If days supply is given, calculate how much medication to dispense by multiplying daily use by number of days.
- If days supply needs calculation, divide total amount dispensed by daily use; units must match, so convert as needed.
Solid Dosage Forms (Tablets, Capsules)
- For solid forms, days supply = total dispensed ÷ number used per day.
- Example: Metformin 500mg, 1 tablet BID for 30 days → 2 x 30 = 60 tablets.
- Example: Amoxicillin 500mg, 1 TID, #21 caps → 21 ÷ 3 = 7 days.
Oral Liquids (Suspensions, Syrups)
- Common conversions: 1 tsp = 5 mL, 1 tbsp = 15 mL, 1 oz = 30 mL, 1 mL = 20 drops.
- To find how much to dispense: daily amount x days of use.
- Example: Amoxicillin 250mg/5mL, 1 tsp BID x 10 days → 5mL x 2 x 10 = 100mL.
- For days supply: total mL dispensed ÷ daily use in mL.
- Example: Robitussin AC, 1 tsp Q4-6h PRN, 4 oz dispensed, max 6 doses/day → 120mL ÷ 30mL = 4 days.
Inhalers & Nasal Sprays
- Check product label for total metered doses.
- Days supply = total number of doses per device ÷ max daily doses used.
- Example: Albuterol inhaler, 200 doses, max 8/day → 200 ÷ 8 = 25 days.
- Example: Flonase, 120 sprays, 2 sprays/nostril QD = 4/day → 120 ÷ 4 = 30 days.
Eye & Ear Drops
- Conversion: 1mL = 20 drops.
- Calculate total drops dispensed and drops per day for days supply.
- Example: Xalatan, 2.5mL, 1 drop OU QPM (2 drops/day) → 2.5mL x 20 = 50 drops, 50 ÷ 2 = 25 days.
- For bottle size: total drops needed ÷ 20 = mL; choose closest bottle size.
Injectables (Insulin)
- U-100 insulin = 100 units/mL; 10mL vial = 1000 units.
- Days supply = total units dispensed ÷ units used daily.
- Insulin expires 28 days after opening, so max supply is 28 days.
- Example: Lantus 10mL vial, 5 units QHS → 1000 ÷ 5 = 200, but expires after 28 days.
- Example: Humalog pens, 5 x 3mL = 15mL, 100 units/mL = 1500 units; 60 units/day → 1500 ÷ 60 = 25 days.
Key Terms & Definitions
- Days Supply — Number of days a prescription’s quantity will last at prescribed use.
- BID/TID/QID — Twice/three/four times daily.
- OU/OS/OD — Both eyes/left eye/right eye.
- PRN — As needed.
- U-100 Insulin — Insulin concentration of 100 units per mL.
- Metered Dose — Pre-set amount delivered per actuation (inhalers/nasal sprays).
Action Items / Next Steps
- Memorize common conversion factors (tsp, tbsp, mL, drops, etc.).
- Practice example calculations for each dosage form.
- Review insulin concentrations and expiration after opening.