Reconstitution guide

What reconstitution actually is

Many compounded preparations are supplied as a lyophilized cake: the drug substance was dissolved, frozen, and then dried under vacuum so that the ice sublimed away without ever passing through a liquid phase. What remains is a porous plug of solid material sitting in a sealed, often partially evacuated vial. Removing the water removes the main driver of chemical degradation, which is why the dry form tolerates far longer storage than the solution does.

Reconstitution reverses that. Adding a defined volume of sterile fluid dissolves the cake and produces a solution of known concentration. Two things follow from that sentence and they matter more than anything else on this page: the concentration is a consequence of the volume you add, and the stability clock starts the moment fluid enters the vial.

Choosing a diluent

The dispensing pharmacy specifies the diluent. Where a label or prescription names one, that instruction governs; the notes below explain the distinctions you will encounter rather than authorising a substitution.

Bacteriostatic water for injection

Sterile water containing a preservative — most commonly benzyl alcohol at around 0.9%. The preservative suppresses microbial growth introduced by repeated stopper punctures, which is what makes multi-dose use of a vial plausible at all. It is the usual choice when a vial is intended to be entered more than once. Preservative-containing diluents are not appropriate for every patient population, and neonatal use in particular is contraindicated.

Sterile water for injection

The same fluid without any preservative. It contains nothing to inhibit microbial growth, so a vial reconstituted with it is properly treated as single-use with immediate administration, regardless of how much solution is left behind. It is also hypotonic, which is a consideration for larger-volume preparations.

Sodium chloride 0.9% (normal saline)

Isotonic and, in the plain formulation, unpreserved — so the single-use logic applies just as it does to sterile water. Bacteriostatic saline exists as a separate, preserved product. Saline is not universally compatible: some preparations are formulated for water and can behave differently in an electrolyte solution, which is exactly why the diluent is specified rather than left open.

Aseptic technique

Every subsequent step assumes the fluid path stayed clean. Work on an uncluttered surface away from air currents and foot traffic. Perform hand hygiene and assemble everything you need before opening any packaging, so you are not reaching across a prepared field mid-procedure.

Check the vial before touching it: identity, strength, expiry, and the integrity of the seal and the glass. Confirm that what is in front of you matches the prescription. A discrepancy discovered now is administrative; discovered later it is a wasted vial at best.

Remove the flip-off cap and wipe the exposed stopper with a fresh alcohol pad using firm, unidirectional strokes. Do the same for the diluent vial with a separate pad. Then let both dry. Alcohol disinfects as it evaporates, so a stopper punctured while still wet has not had the contact time it needed, and residual alcohol can be carried into the vial on the needle. Once dry, treat the stopper surface as clean and do not touch it again.

Avoiding coring

Coring is when the needle punches out a fragment of the rubber stopper and carries it into the vial, where it becomes particulate contamination. It is a function of angle and technique. Enter through the centre of the stopper, where the septum is designed to be pierced, with the bevel facing up; press down while applying slight rotational pressure so the needle cuts rather than punches. Use the same puncture site as little as possible across repeated entries, and use the smallest gauge that will do the job. Larger-bore and blunted needles core more readily. If you see a fragment floating in the solution, the vial is not usable.

Adding the diluent

Draw the specified volume of diluent, keeping the needle tip below the fluid surface so you are not pulling air, and expel any bubbles before you move to the product vial. Verify the volume against the graduations at eye level.

Insert the needle into the product vial and angle it so the tip rests against the inside glass wall. Depress the plunger slowly and let the fluid run down the wall and pool underneath the cake. The point is to avoid a jet of fluid striking the lyophilized plug directly: a hard stream drives air into the solution, produces foam, and mechanically stresses the material. Foam is not merely cosmetic — it makes accurate measurement difficult and it takes time to dissipate.

Watch the plunger while you work. Lyophilized vials are often under partial vacuum, which will draw fluid in and can pull the plunger from your hand; conversely, once the vacuum is satisfied, the vial pressurises as you add volume and will push back. Equalise as you go by letting the syringe find its balance rather than forcing it, and vent gently if the vial becomes positively pressured before you withdraw the needle. Withdrawing from a pressurised vial sprays solution back through the stopper.

Dissolution

Once the fluid is in, remove the needle and let the vial sit. Most cakes begin dissolving immediately and finish within a few minutes. Encourage it by swirling the vial gently or rolling it between your palms.

Do not shake. Vigorous agitation introduces an air-liquid interface across which many molecules — peptides and proteins especially — will denature or aggregate, and it produces foam that traps drug substance. Patience costs nothing here. If a cake is stubborn, warming the vial in the hand for a minute is usually enough; if material still will not dissolve after a reasonable interval, treat that as a signal to stop and consult the dispensing pharmacy rather than escalating the agitation.

Visual inspection

Before the solution is used for anything, inspect it. Hold the vial up against a well-lit white background and then against a dark one; the two backgrounds reveal different things. You are looking for a clear solution, free of visible particulates, fibres, crystals, or rubber fragments, with no cloudiness, haze, or colour that the label does not describe. Anything that fails inspection is not administered — see the storage and handling page for the discard criteria in full.

Labeling

A reconstituted vial that is not labeled is an unknown. At minimum, record on or with the vial:

• The date and time reconstitution was performed.
• The diluent used, including whether it was preserved.
• The volume of diluent added.
• The resulting concentration.
• The beyond-use date assigned per the pharmacy's labeling and your policy.
• The initials of the person who prepared it.

The diluent volume matters as much as the date, because it is the only way anyone else can reconstruct the concentration. Two vials of identical product reconstituted with different volumes are different concentrations and will require different draw volumes for the same mass.

The concentration arithmetic

Three relationships cover everything. All examples below use generic vials with no compound named and no clinical target implied.

Concentration

Concentration equals the drug mass in the vial divided by the volume of diluent added. A 5 mg vial reconstituted with 2 mL yields 2.5 mg/mL. The same 5 mg vial reconstituted with 5 mL yields 1 mg/mL. Nothing about the vial changed — only the arithmetic.

Volume per dose

Volume equals the mass you want divided by the concentration. From that 2.5 mg/mL solution, measuring 0.5 mg requires 0.2 mL. From the 1 mg/mL solution, the same 0.5 mg requires 0.5 mL.

Syringe units

On a U-100 insulin syringe, 1 mL is marked as 100 units, so multiply the millilitre figure by 100: 0.2 mL is 20 units, 0.5 mL is 50 units. U-50 and U-40 barrels use different calibrations and the multiplier changes accordingly — the syringe guide covers this in detail, and the calculator will do the conversion for whichever barrel you are holding.

Drawing subsequent doses

For a vial that is genuinely multi-dose, each later entry repeats the hygiene of the first: disinfect the stopper with a fresh alcohol pad every time, allow it to dry, and use a new sterile needle and syringe for every draw. Never re-enter a vial with a used needle.

Removing solution leaves negative pressure behind, which makes the next draw harder and encourages bubbles. Injecting a volume of air roughly equal to the volume you intend to withdraw, before drawing, equalises the vial and gives a cleaner draw. Draw with the vial inverted and the needle tip well below the fluid level, then tap out bubbles and expel them before you record the volume. Confirm the measurement at eye level with the syringe vertical; reading a barrel at an angle is a reliable way to be a unit or two off.

Re-inspect the solution on every entry, not just the first. Changes in clarity or colour can develop over the life of a vial, and the whole point of inspection is to catch them before administration.