Why do I add water in two portions for oral suspensions?

Adding all the water at once before shaking tends to create a clump of dry powder at the bottom that is difficult to disperse. Adding half the water first, shaking until the powder is fully wetted and dispersed, then adding the remainder ensures complete homogeneity.

How long is a reconstituted antibiotic suspension stable?

Most oral antibiotic suspensions (amoxicillin, flucloxacillin, cefalexin) are stable for 7–14 days when refrigerated at 2–8°C. Azithromycin suspension is stable 5 days at room temperature. Always check the specific product label.

Can I keep reconstituted suspension at room temperature?

Most antibiotic suspensions must be refrigerated after reconstitution for stability. A few (azithromycin) are room temperature stable. Check the product label for specific storage requirements after reconstitution.

Reconstitution Volume — Free Pharmacy Calculator

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Amoxicillin suspension script — the patient is picking up in 5 minutes and the bottle needs reconstituting correctly to ensure the labelled 125 mg/5 mL concentration.

Reconstitution Volume

Compounding

Vial amount (mg)

Diluent added (mL)

Ordered dose (mg)

Displacement vol (mL)

From product label — 0 if not stated

Total volume = Diluent added + Displacement volume Concentration = Vial mg ÷ Total volume Volume to give = Ordered dose ÷ Concentration Example: 500 mg vial, 9.6 mL diluent, 0.4 mL displacement
Total vol = 10 mL · Conc = 50 mg/mL · For 250 mg → 5 mL

💡 Displacement volume is often omitted and causes systematic underdosing — always check the product information leaflet.

Reference: AMH · TGA Product Information

⚕️ Clinical safety: 🇦🇺 Verify with facility drug formulary and senior clinician · Meets AHPRA/ACSQHC standards

1 What this calculator does

Calculates the volume of water to add to a dry powder for oral suspension or injection to achieve the labelled concentration. Accounts for displacement volume. Works for both reconstitution to label concentration and for calculating the volume to draw up for a specific ordered dose after reconstitution.

2 Formula & professional reasoning

Volume to add = Final volume − Displacement volume
Final concentration = Label strength (mg) ÷ Final volume (mL)
Dose volume = Ordered dose ÷ Final concentration

Powder for reconstitution contains the drug plus excipients as a dry powder. When water is added, the powder itself occupies volume (displacement volume), meaning the final volume is greater than the water added. The label states either the volume to add (for oral suspensions) or the final volume after reconstitution (for injections). Getting this wrong produces an incorrect concentration — too little water gives a more concentrated suspension, too much water dilutes it.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

Amoxicillin oral suspension

Given: Label: add 68 mL water to produce 100 mL of 125 mg/5 mL

Working: Displacement: 100 − 68 = 32 mL · Conc after: 2500 mg ÷ 100 mL = 25 mg/mL = 125 mg/5 mL ✓

Answer: Add exactly 68 mL water in two portions

💡 Add approximately half the water first, shake vigorously, then add remaining water to the graduation mark.

Standard

Ceftriaxone injection — ordered dose volume

Given: Vial: 1 g · Reconstitute with 10 mL water for injection · Ordered dose: 750 mg

Working: Conc: 1000 mg ÷ 10 mL = 100 mg/mL · Volume: 750 ÷ 100

Answer: Draw up 7.5 mL

💡 Ceftriaxone — check compatibility before mixing with IV line. Do not mix with calcium-containing solutions.

Advanced

Vancomycin with displacement volume

Given: Vial: 500 mg · Add 10 mL to give 10.4 mL (0.4 mL displacement) · Ordered: 250 mg

Working: Conc: 500 ÷ 10.4 = 48.1 mg/mL · Volume: 250 ÷ 48.1

Answer: 5.2 mL

💡 Vancomycin reconstitute to ≥5 mg/mL before further dilution for infusion. Do not administer undiluted.

4 Sanity check

Oral suspensions

Add water in two portions — half first, shake, then to graduation mark

Single addition of all water before shaking makes a lump. Two-portion method ensures uniform dispersion.

After reconstitution

Shake well before each use (suspensions)

Unshaken suspension may deliver first doses at the wrong concentration.

Stability after reconstitution

Most oral suspensions: 7–14 days refrigerated

Check the label — most amoxicillin, flucloxacillin, azithromycin suspensions are stable 7–14 days at 2–8°C.

Label immediately after reconstitution

Expiry date, storage instructions, prepared by, date

Unlabelled reconstituted products must not leave the dispensary.

5 Common errors

Error	Cause	Consequence	Fix
Adding water to the graduation mark without considering displacement	Assuming displacement is negligible	For suspension: no error if following the label instruction (label already accounts for displacement). For injections: may cause error if displacement volume is significant.	For injections: check the product PI for displacement volume. For oral suspensions: always add to the graduation mark as labelled.
Using tap water instead of purified water for injections	Convenience	Particulate contamination, microbial risk	Parenteral products: always use water for injection (WFI). Oral suspensions: use cooled boiled water or freshly boiled water as per label.
Not shaking the oral suspension before dispensing dose	Omission	Top of bottle may be less concentrated than bottom — inconsistent dose	Shake vigorously for 10–15 seconds before measuring each dose. Advise patient the same.
Dispensing full bottle without advising on reconstitution	Assuming patient knows	Patient adds wrong amount of water, incorrect concentration	Always demonstrate reconstitution to caregivers. Provide written instructions with the label.

6 Reference & regulatory links

🇦🇺 Australia

🇺🇸 United States

7 Professional workflow

Common tools used alongside this one:

💊

Days Supply Calculator

After reconstitution, calculate days supply from total volume, dose volume and frequency

→

🔄

Dose Frequency Converter

Confirm frequency in doses per day before calculating days supply of the reconstituted suspension

→

Input	Result
1 g vial, 10 mL reconstitution	100 mg/mL
500 mg vial, 10 mL	50 mg/mL
Ordered 750 mg at 100 mg/mL	7.5 mL