HCG Reconstitution Guide: IU Math, Bac Water & Storage

What You'll Need

HCG ships as a lyophilized (freeze-dried) powder in a sealed vial and is reconstituted into solution before any bench work. The single most important thing to understand before you start: HCG is a hormone that is potency-standardized in International Units (IU), not measured by mass in milligrams. There is no universal mg-to-IU conversion, because the IU reflects bioactivity rather than weight — so every calculation below is done in IU/mL, never mg/mL.

Before reconstituting, gather a clean set of supplies on a swabbed surface:

• The lyophilized HCG vial — e.g. a 5,000 IU presentation (catalogued as G5K).
• Bacteriostatic water (0.9% benzyl-alcohol "BAC water"). The benzyl alcohol is the bacteriostatic agent that supports a multi-draw refrigerated window; plain sterile water for injection (SWFI) has no preservative and is single-use-oriented.
• An alcohol prep pad to swab both rubber stoppers.
• A sterile syringe for the diluent transfer (a U-100 insulin syringe makes the IU-to-"units" math below convenient).

Step-by-Step: Reconstituting the Vial

HCG is a glycoprotein, and like any protein it is denatured by mechanical stress. The handling method below follows the FDA/USP-labeled reconstitution procedure for lyophilized chorionic gonadotropin (Pregnyl prescribing information). The watchword throughout is gentle.

1. Swab both stoppers. Wipe the rubber stopper of the HCG vial and the bacteriostatic-water vial with a fresh alcohol pad and let them dry.
2. Draw your chosen volume of BAC water. Decide your diluent volume before you draw (the math section below explains how that choice sets your final concentration), then draw exactly that amount into the syringe.
3. Run the diluent slowly down the glass wall. Insert the needle and let the bacteriostatic water trickle down the inner wall of the vial onto the powder cake — do not squirt the stream directly into the powder. A forceful jet foams and shears the protein.
4. Swirl gently — do NOT shake. Roll or swirl the vial slowly until the powder fully dissolves. Shaking and foaming denature HCG and can destroy its bioactivity. Never shake.
5. Inspect the solution. A correctly reconstituted vial is clear and colorless. Discard the vial if the solution is cloudy or shows particulates.
6. Label the vial with the reconstitution date and the calculated IU/mL concentration, then refrigerate (see Storage below).

Concentration & Volume Math

This is the whole point of an HCG guide, and it is simpler than it looks once you commit to working in IU rather than mg. Two formulas cover everything:

• Concentration (IU/mL) = total vial IU ÷ mL of BAC water added.
• Per-draw volume (mL) = target IU ÷ concentration (IU/mL).

Worked Example: the 5,000 IU Vial

The diluent volume you add is a free choice, and it sets the concentration. For a single 5,000 IU vial:

• Add 1 mL BAC water → 5,000 IU ÷ 1 mL = 5,000 IU/mL (concentrated; small draw volumes).
• Add 2 mL → 5,000 ÷ 2 = 2,500 IU/mL.
• Add 5 mL → 5,000 ÷ 5 = 1,000 IU/mL (more dilute; larger, easier-to-measure draw volumes).

To find a per-draw volume, divide your target IU by that concentration. Working at 2,500 IU/mL, a 500 IU research aliquot is 500 ÷ 2,500 = 0.2 mL. On a U-100 insulin syringe (which holds 100 "units" per mL), 0.2 mL reads as 20 units. Those figures are concentration-math illustrations for bench aliquoting only — they are not dosing guidance.

The practical rule of thumb: more diluent = lower IU/mL = a larger, easier-to-measure draw volume; less diluent = higher IU/mL = a smaller draw volume. Because there is no fixed mg-to-IU shortcut, always re-derive IU/mL from your actual diluent volume every time you reconstitute a vial.

Storage & Stability

HCG's protein nature drives every storage rule. Two failure modes — mechanical stress and freezing — both denature the molecule, so the storage discipline is about avoiding them.

• Dry powder: keep the unopened lyophilized vial cool — controlled room temperature (15–30 °C) or refrigerated per the supplier COA — and sealed until you are ready to reconstitute.
• After reconstitution: refrigerate at 2–8 °C. Per the FDA/USP Pregnyl label, the reconstituted solution is stable for 60 days refrigerated. Some research suppliers quote a more conservative window (e.g. ~30 days) — when figures differ, follow the shorter one and the vial's own COA.
• Never freeze the reconstituted solution. Freezing denatures the protein.
• Protect from light and label the vial with the reconstitution date and concentration.
• Discard at the end of the window, or immediately if the solution turns cloudy or shows particulates.

Note that the 60-day figure is the label value for a specific manufactured product, and the bacteriostatic (benzyl-alcohol) multi-draw window assumes proper aseptic technique throughout. When in doubt, defer to the more conservative number and to your vial's certificate of analysis.

Mechanism in Brief

HCG is a two-subunit glycoprotein hormone — a common alpha subunit plus a hormone-specific beta subunit, each carrying a cystine-knot fold, with the beta subunit "seat-belting" around alpha to stabilize the dimer (Lapthorn et al., Nature, 1994; PDB 1HRP). It is studied for its interaction with the luteinizing-hormone/choriogonadotropin receptor (LHCGR), a class-A G-protein-coupled receptor.

HCG and luteinizing hormone (LH) act on the same receptor, signaling mainly through Gαs → adenylyl cyclase → cAMP/PKA. Notably, the two are natural biased agonists: recombinant HCG is more potent on the cAMP arm while LH shows relatively greater beta-arrestin activity, so the receptor distinguishes the two ligands (Riccetti et al., Scientific Reports, 2017; foundational mechanism in Bahl, Federation Proceedings, 1977). The identity and receptor-sharing facts are well established; the biased-agonism nuance rests on in-vitro recombinant-system data. This is a reconstitution how-to, so the mechanism is kept brief by design.

Frequently Asked Research Questions

Why is HCG dosed in IU instead of milligrams?

HCG is a potency-standardized hormone, so it is quantified by bioactivity in International Units (IU) rather than by mass. There is no universal mg-to-IU conversion, which is exactly why all reconstitution math for HCG is done in IU/mL, not mg/mL. Most other research peptides are mass-based; HCG is the exception.

How do I calculate IU/mL for a 5,000 IU vial?

Divide the total vial IU by the millilitres of bacteriostatic water you add. A 5,000 IU vial reconstituted with 1 mL gives 5,000 IU/mL; with 2 mL it gives 2,500 IU/mL; with 5 mL it gives 1,000 IU/mL. To find a draw volume, divide your target IU by that concentration — e.g. 500 IU at 2,500 IU/mL = 0.2 mL = 20 units on a U-100 insulin syringe.

Which water should I use, bacteriostatic or sterile?

Use bacteriostatic water (0.9% benzyl alcohol) for a multi-draw vial — the benzyl alcohol is the bacteriostatic agent that supports a refrigerated multi-draw window. Plain sterile water for injection (SWFI) has no preservative and is single-use-oriented. For a deeper comparison, see our guide on bacteriostatic vs sterile vs acetic water.

How long does reconstituted HCG last, and can I freeze it?

Refrigerate at 2–8 °C; per the FDA/USP Pregnyl label the reconstituted solution is stable for 60 days refrigerated. Some research suppliers specify a shorter, more conservative window — follow that and the vial's COA. Never freeze reconstituted HCG, as freezing denatures the protein. Discard if the solution turns cloudy or shows particulates.

Why must I swirl and never shake?

HCG is a glycoprotein, and shaking or foaming a protein solution denatures it and can destroy bioactivity. Run the diluent gently down the glass wall, swirl until dissolved, and confirm the solution is clear and colorless. The same gentle handling applies to most peptides — see our step-by-step reconstitution guide for the general method.