Pharmacy unit conversions: the ones that trip people up
Unit conversion errors are one of the leading causes of medication dosing mistakes. A 10-fold error from confusing mg with mcg, or mL with L, can be lethal. These are the conversions worth memorising and verifying every time.
Weight: the most common source of errors
1 gram = 1000 milligrams. 1 milligram = 1000 micrograms. So 1g = 1,000,000 mcg. A drug ordered as 0.5mg and dispensed as 0.5mcg delivers 1000 times less than intended. The reverse error (0.5mcg ordered, 0.5mg dispensed) delivers 1000 times more. Always confirm the unit before calculating. For drug doses in mcg: digoxin, fentanyl, levothyroxine, vitamin B12 injections.
Volume: household vs clinical measures
A US teaspoon = 5mL. A tablespoon = 15mL. A US fluid ounce = 29.57mL. Kitchen teaspoons vary by up to 40%, never use them to measure medication doses. Use an oral syringe or measuring cup calibrated in mL.
Concentration: percent solutions
A 1% w/v solution contains 1g per 100mL, which equals 10mg/mL. Sodium chloride 0.9% = 9mg/mL = 154 mmol/L. Glucose 5% = 50mg/mL. Adrenaline 1:1000 = 1mg/mL. Adrenaline 1:10,000 = 0.1mg/mL. The 10-fold difference between these 2 adrenaline concentrations has caused fatal errors. Always double-check the concentration on the ampoule label. For dilution and IV bag preparation calculations, use our Concentration Calculator. For IV flow rate calculations, use the IV Drip Rate Calculator.