Searches for HCG mixing water, HGH mixing water and fertility medication mixing water come from the same situation: a prescribed lyophilised hormone, a solvent ampoule that may or may not have come in the box, and uncertainty about whether bacteriostatic water can be used instead. The answer depends on the product, because licensed medicines come with a specified solvent and a summary of product characteristics (SmPC) that states how long the mixed solution lasts.
This article goes through the UK and US product information for human chorionic gonadotropin (hCG), somatropin (recombinant human growth hormone) and the follitropin and menotrophin fertility medicines, where bacteriostatic water is specified and where it is not, and how the maths works when a product is dosed in International Units (IU). These are prescription-only medicines in the UK, and the prescriber’s instructions and the product leaflet take precedence over anything here.
Why the solvent matters more for hormones than for research peptides
A licensed medicine has been stability-tested with a specific solvent, and the SmPC’s storage statement is only valid for that combination. Swapping diluent changes the preservative, the tonicity and sometimes the pH, and the manufacturer’s data no longer applies. That is why the same hormone carries different instructions in different markets: the preservative in the solvent, not the hormone, sets the storage period. The general principles are in bacteriostatic water vs sterile water.
hCG mixing water: what the labels say
The UK Pregnyl (Organon) leaflet for the 1,500 IU and 5,000 IU vials describes a solvent of sodium chloride 9 mg and water for injection to 1.0 mL. The instruction is that the powder must be dissolved in the liquid from the solvent ampoule, that the solution “should be used immediately after reconstitution”, and that any unused solution is discarded [1]. The UK product is single-dose: one vial, one injection, no storage.
The US Pregnyl label is different. Its diluent is “water for injection with 0.56% sodium chloride and 0.9% benzyl alcohol”, and the label states that the “reconstituted solution is stable for 60 days when refrigerated”; the instructions are to add 1 to 10 mL of diluent to the powder and agitate gently until dissolved [2]. That is a multi-dose product built around a benzyl alcohol-preserved diluent, and the label warns that the benzyl alcohol is “not for use in newborns” [2].
So the honest answer to “can I use bacteriostatic water for hCG” is that a licensed hCG product in one major market is supplied with exactly that, and the UK product is not. If a UK prescriber intends a multi-dose regimen from a single 5,000 IU vial, ask which diluent and storage period they mean; do not assume the 60-day US figure transfers to a different formulation.
hCG concentration maths in IU
The formula is the same as for any peptide, with IU replacing milligrams: concentration = IU in vial ÷ mL of diluent. On a U-100 insulin syringe each unit is 0.01 mL.
| Vial | Diluent | Concentration | IU per U-100 unit | Units for 250 IU | Units for 500 IU |
|---|---|---|---|---|---|
| 5,000 IU | 1 mL | 5,000 IU/mL | 50 IU | 5 | 10 |
| 5,000 IU | 2 mL | 2,500 IU/mL | 25 IU | 10 | 20 |
| 5,000 IU | 5 mL | 1,000 IU/mL | 10 IU | 25 | 50 |
| 10,000 IU | 10 mL | 1,000 IU/mL | 10 IU | 25 | 50 |
Licensed hCG doses are large. The US label gives 500 to 1,000 USP units three times a week in one hypogonadotropic hypogonadism regimen, and 5,000 to 10,000 USP units as a single ovulation-triggering dose [2]. An ovulation trigger uses the whole 5,000 IU vial, which is why the UK single-dose format exists.
HGH mixing water: somatropin comes with its own solvent
Licensed somatropin is not reconstituted with bacteriostatic water in the UK; it comes as a two-chamber cartridge or vial-and-solvent pack with a manufacturer solvent that already contains a preservative. Genotropin (Pfizer) 12 mg is supplied with a solvent containing metacresol, mannitol and water for injections. The SmPC states that once mixed the product is stable for 28 days at 2–8°C, and instructs users to dissolve the powder “by gently tipping it back and forth” and not to shake vigorously because this “might cause denaturation of the active substance” [3].
Two things carry over to any growth hormone product. The 28-day refrigerated window depends on the metacresol in the solvent, which does the job benzyl alcohol does in bacteriostatic water. And somatropin is a 191-amino-acid protein, far more sensitive to shear than a short peptide; swirling, not shaking, is a stability requirement.
Licensed somatropin dosing is set by the prescriber. The Genotropin SmPC gives 0.025 to 0.035 mg/kg/day for children with growth hormone deficiency, and for adult-onset deficiency a starting dose of 0.15 to 0.3 mg daily, titrated against IGF-I, with maintenance rarely exceeding 1.0 mg daily [3]. Growth hormone mixing water questions from people holding lyophilised somatropin without a solvent are a different category: the product is prescription-only in the UK, and there is no SmPC-backed stability figure for somatropin in bacteriostatic water. The 28-day multi-dose convention from USP <797> applies to the preserved diluent, not to the protein [4]. See the somatropin reconstitution article.
Fertility medication mixing water: follitropin and menotrophin
The fertility gonadotropins split the same way hCG does, between single-dose and preserved multi-dose formats.
- Gonal-f 75 IU (follitropin alfa), UK. Supplied with its own solvent, to be reconstituted by swirling gently, not shaking, and labelled “for immediate and single use following first opening and reconstitution” [5].
- Gonal-f Multi-Dose, US. Reconstituted with “Bacteriostatic Water for Injection (0.9% benzyl alcohol), USP”: 1 mL for the 450 IU vial or 2 mL for the 1,050 IU vial, giving 600 IU/mL in both cases. The label instructs users to discard unused reconstituted solution after 28 days, refrigerated or at room temperature [6].
- Menopur 1200 IU (menotrophin), UK. Supplied with a metacresol-preserved solvent; the SmPC allows storage for a maximum of 28 days at not more than 25°C, for individual patient use, with multiple withdrawals [7].
The pattern is consistent. Where a product is meant for multiple withdrawals, its solvent contains a preservative, benzyl alcohol or metacresol, and the storage period is 28 days (60 for US Pregnyl). Where the solvent is unpreserved, the product is single-use.
Gonadotropin concentration example
Gonal-f Multi-Dose 1,050 IU in 2 mL gives 600 IU/mL, so each U-100 unit holds 6 IU and a 150 IU dose is 25 units (0.25 mL). The label’s starting doses are 75 IU daily for ovulation induction and 150 IU daily for assisted reproduction, adjusted by the clinic [6]. Menopur 1200 IU reconstituted as directed also gives 600 IU/mL; its SmPC gives 75 to 150 IU daily as the initial anovulation dose [7].
Reconstitution technique for hormone vials
The procedure is the same as for any lyophilised vial and is set out in the reconstitution guide. For hormones the priorities are: bring vial and solvent to room temperature; run the solvent down the glass wall; swirl, never shake; use only a clear solution; refrigerate a multi-dose vial at 2–8°C and label it with mix and discard dates. Storage detail is in how to store bacteriostatic water.
Where a prescriber has specified a preserved diluent for a multi-dose regimen, Baclabs supplies UK-stocked bacteriostatic water in 30 mL vials; the product is labelled for use only as a diluent or solvent [8]. It is not a substitute for the solvent a manufacturer packages with a licensed product.
Frequently asked questions
Can I use bacteriostatic water for HCG?
The US Pregnyl product is supplied with a benzyl alcohol-preserved diluent and is stable for 60 days refrigerated, so the combination is well established. The UK product comes with unpreserved sodium chloride and is single-use. If your regimen involves multiple doses from one vial, ask the prescriber which diluent and storage period apply.
What is the mixing water for HGH?
Licensed somatropin products such as Genotropin come with a metacresol-preserved solvent in a two-chamber cartridge, and the SmPC-backed storage period of 28 days at 2–8°C applies only to that solvent. There is no manufacturer stability data for somatropin mixed with bacteriostatic water; the 28-day multi-dose convention covers the diluent, not the protein.
How much bacteriostatic water for 5000 IU HCG?
It depends on the dose to be measured. With 1 mL, each unit on a U-100 syringe holds 50 IU; with 2 mL, 25 IU; with 5 mL, 10 IU. A 500 IU dose is therefore 10, 20 or 50 units respectively. Choose a volume that makes the dose easy to read, and follow the prescriber’s instructions.
Why does UK Pregnyl say use immediately but the US version lasts 60 days?
Because the solvents differ. The UK solvent is unpreserved sodium chloride 9 mg/mL, so nothing inhibits bacterial growth after the first puncture. The US diluent contains 0.9% benzyl alcohol, which supports multi-dose use, and the manufacturer holds 60-day refrigerated stability data for that combination.
References
- Pregnyl 1500 IU and 5000 IU powder and solvent for solution for injection, UK package leaflet (Organon). UK patient information leaflet
- Pregnyl (chorionic gonadotropin for injection, USP), US prescribing information. DailyMed, US National Library of Medicine
- Genotropin 12 mg powder and solvent for solution for injection, Summary of Product Characteristics. electronic medicines compendium (emc), Pfizer
- Crosswalk of Guidances and Standards for Managing Single- and Multi-Dose Vials (USP <797>, CDC, TJC). American Society of Health-System Pharmacists
- GONAL-f 75 IU (5.5 micrograms) powder and solvent for solution for injection, Summary of Product Characteristics. electronic medicines compendium (emc), Merck Serono
- Gonal-f Multi-Dose (follitropin alfa for injection), US prescribing information. EMD Serono
- Menopur 1200 IU powder and solvent for solution for injection, Summary of Product Characteristics. electronic medicines compendium (emc), Ferring
- Bacteriostatic Water for Injection, USP, prescribing information (Hospira). DailyMed, US National Library of Medicine, current label
Disclaimer: This content is for educational purposes only and does not constitute medical advice. Bacteriostatic water is supplied for reconstitution of substances intended for research and for use as directed by a healthcare professional. Peptides and medicines discussed here may be unlicensed in the UK or prescription-only; consult a qualified clinician before using any medicine. Baclabs does not sell peptides.