Reconstitution: Does Adding Water Change the Amount of Medication in My Vial?
Let’s bust a big myth. If you add more bacteriostatic water to your vial, you are not adding more medication. You’re just changing how concentrated that medication is in each millilitre (mL). The total amount of medication in the vial stays exactly the same. 💡
Why This Matters
Most people dose by drawing up a certain volume (e.g., “0.2 mL” or “20 units” on an insulin syringe). If you change the amount of water in the vial but keep drawing the same volume, you’ll accidentally change the dose. That’s why understanding concentration is everything. 🎯
What Changes vs What Doesn’t
- ✅ Doesn’t change: total medication in the vial (the mg you started with).
- 🔄 Does change: concentration (mg per mL) and therefore how much volume you need to draw for a given mg dose.
The Simple Maths (Promise)
Concentration = Total medication (mg) ÷ Total liquid (mL)
Dose volume (mL) = Target dose (mg) ÷ Concentration (mg/mL)
• Add 2 mL water → concentration is 5 ÷ 2 = 2.5 mg/mL.
• Later add 2 mL more (now 4 mL total) → concentration is 5 ÷ 4 = 1.25 mg/mL.
If your target dose is 0.25 mg:
• At 2.5 mg/mL, draw 0.10 mL (that’s 10 units on a U-100 insulin syringe).
• At 1.25 mg/mL, draw 0.20 mL (20 units).
Same medication amount, different volume because the concentration changed. 🧪
“Units” vs mL (The Classic Confusion)
On a standard U-100 insulin syringe, 100 units = 1.0 mL.
So: 10 units = 0.10 mL, 20 units = 0.20 mL, and so on.
If you change concentration, the units you draw for the same mg dose will change too. 🔢
Common Mistakes (and Easy Fixes)
- ❌ Mistake: “I topped up with more water so it’s ‘weaker’—I’ll just keep drawing the same 10 units.”
✅ Fix: Recalculate the volume using the new concentration. - ❌ Mistake: Guessing volumes instead of measuring at eye level.
✅ Fix: Take 10 extra seconds and read the line properly. - ❌ Mistake: Forgetting what you diluted to.
✅ Fix: Label the vial with date + total mL after mixing. 🏷️
Quick Sanity Checks
- Did I change the total mL in the vial? → Yes → Recalculate concentration.
- Am I dosing by volume (mL/units)? → Yes → Recalculate the volume for my mg target.
- Did I label the vial? → Do it now. 📝
Final Word
Adding more water never adds medication—it only spreads the same amount over more liquid. Respect the maths, label your vial, and you’ll keep your dosing consistent without the guesswork. 🚀
P.S. Want zero maths? Use our Reconstitution Calculator: plug in vial mg and total mL, and it tells you the concentration (and the volume you’d need for a target mg).
Brisbane & Sydney based. Clear guidance, quick shipping, no-BS support.
