Editorial note: this article summarises published research and regulatory documentation on somatropin. Baclabs does not sell somatropin, HGH or any peptide; we supply bacteriostatic water and reconstitution consumables. Nothing here is medical advice.
Somatropin is recombinant human growth hormone: a 191-amino-acid protein identical in sequence to the pituitary hormone, and a prescription-only medicine in the UK with a full Summary of Product Characteristics (SmPC). That makes it the one compound in the muscle-growth category where dosing is not guesswork, the licensed doses are published, small, and titrated against a biomarker.
This article does two things. It sets out what somatropin is licensed for and dosed at in adult growth hormone deficiency, and it works through the reconstitution arithmetic for the 10 IU and 12 IU lyophilised vials that generate most “how to mix HGH with bacteriostatic water” searches, including the international unit to milligram conversion that trips people up, and what each mark on a U-100 insulin syringe actually delivers.
What somatropin is licensed for
In the UK, somatropin is authorised in children for growth hormone deficiency, Turner syndrome, chronic renal insufficiency, short stature in children born small for gestational age, and Prader-Willi syndrome. In adults, it is authorised as replacement therapy for pronounced growth hormone deficiency, whether adult-onset or persisting from childhood [1].
It is not licensed for body composition, athletic performance or anti-ageing in people with normal pituitary function. Adult GH deficiency is a diagnosed endocrine condition, generally following hypothalamic-pituitary disease, cranial irradiation or traumatic brain injury [2].
Licensed adult dosing
The doses are far smaller than most people expect.
- Genotropin. Adult-onset GH deficiency: 0.15–0.3 mg per day, titrated against IGF-1 concentrations. Continuing from childhood-onset disease: 0.2–0.5 mg per day. Maintenance rarely exceeds 1.0 mg/day; patients over 60 start at 0.1–0.2 mg/day [1].
- Norditropin. Adult-onset GH deficiency: start low at 0.1–0.3 mg/day, increasing monthly on clinical response. Childhood-onset restart: 0.2–0.5 mg/day, then adjusted to IGF-1 [2].
Titration to an IGF-1 target, rather than a fixed dose, is the defining feature of proper replacement.
Undesirable effects are dose-related. Peripheral oedema is very common; headache, paraesthesia, arthralgia, joint stiffness and myalgia are common; carpal tunnel syndrome, pruritus, muscle stiffness and type 2 diabetes mellitus are uncommon [2]. In healthy adults, a systematic review of 27 randomised trials in 440 participants found lean body mass rose by about 2.1 kg with no significant improvement in quadriceps or biceps strength, no gain in VO2max, and far more adverse events in the GH arms, soft tissue swelling in 44% versus 1%, fatigue in 35% versus 0% [3][4].
The IU to mg conversion
Licensed somatropin is dosed in milligrams. Lyophilised vials are usually labelled in international units. The Norditropin SmPC states the conversion directly: 1 mg of somatropin corresponds to 3 IU [2].
Everything else follows from that:
| Vial label | Somatropin mass | Equivalent in mcg |
|---|---|---|
| 1 IU | 0.333 mg | 333 mcg |
| 2 IU | 0.667 mg | 667 mcg |
| 10 IU vial | 3.33 mg | 3,333 mcg |
| 12 IU vial | 4.0 mg | 4,000 mcg |
A licensed adult starting dose of 0.15–0.3 mg/day is therefore 0.45–0.9 IU/day, so a 10 IU vial holds roughly eleven to twenty-two days of replacement.
How to mix HGH with bacteriostatic water: the arithmetic
The formula is the same one used for every lyophilised vial:
Concentration = total vial content ÷ volume of diluent added. On a U-100 insulin syringe, 100 units = 1 mL, so each unit is 0.01 mL and delivers one hundredth of the per-mL concentration.
Worked through for the two common vial sizes:
| Vial | Bac water added | Concentration | Per U-100 unit | Units for 2 IU |
|---|---|---|---|---|
| 10 IU (3.33 mg) | 1 mL | 10 IU/mL, 3.33 mg/mL, 3,333 mcg/mL | 0.1 IU, 33.3 mcg | 20 units |
| 10 IU (3.33 mg) | 2 mL | 5 IU/mL, 1.67 mg/mL, 1,667 mcg/mL | 0.05 IU, 16.7 mcg | 40 units |
| 12 IU (4.0 mg) | 1 mL | 12 IU/mL, 4.0 mg/mL, 4,000 mcg/mL | 0.12 IU, 40 mcg | 16.7 units |
| 12 IU (4.0 mg) | 2 mL | 6 IU/mL, 2.0 mg/mL, 2,000 mcg/mL | 0.06 IU, 20 mcg | 33.3 units |
Worked example. A 10 IU vial reconstituted with 2 mL of bacteriostatic water holds 3.33 mg in 2 mL, so 1.67 mg/mL, or 1,667 mcg/mL. Each U-100 syringe unit is 0.01 mL and therefore delivers 16.7 mcg, which is 0.05 IU. To measure a 0.2 mg (200 mcg, 0.6 IU) licensed adult replacement dose you would draw 200 ÷ 16.7 = 12 units.
Second example. A 12 IU vial with 1 mL gives 4,000 mcg/mL, so 40 mcg per unit. A 0.2 mg dose is then 5 units, at the bottom of a U-100 syringe’s usable scale where measurement error is proportionally largest. Using 2 mL halves the concentration and doubles the units drawn, improving precision. That is the practical argument for the larger diluent volume.
| Target dose | 10 IU + 1 mL | 10 IU + 2 mL | 12 IU + 1 mL | 12 IU + 2 mL |
|---|---|---|---|---|
| 0.1 mg (0.3 IU) | 3 units | 6 units | 2.5 units | 5 units |
| 0.2 mg (0.6 IU) | 6 units | 12 units | 5 units | 10 units |
| 0.3 mg (0.9 IU) | 9 units | 18 units | 7.5 units | 15 units |
| 0.5 mg (1.5 IU) | 15 units | 30 units | 12.5 units | 25 units |
| 1.0 mg (3 IU) | 30 units | 60 units | 25 units | 50 units |
Technique and stability
Somatropin is a protein and denatures with shear stress. The Genotropin SmPC is explicit: do not shake vigorously, as this might cause denaturation of the active substance [1]. Add the diluent slowly down the inside wall of the vial rather than directly onto the lyophilised cake, then swirl or roll gently until dissolved. The solution should be clear; discard anything cloudy or containing particles.
Reconstituted licensed somatropin is stored refrigerated at 2–8 °C and not frozen. Genotropin’s reconstituted cartridge is stable for 28 days at 2–8 °C [1]; Norditropin FlexPro after first use may be kept up to 4 weeks refrigerated or 3 weeks below 25 °C [2]. Note that licensed presentations supply their own solvent, and the stability data was generated with it, where a diluent is provided in the pack, that is the one to use.
Diluent choice and reconstitution
A vial punctured repeatedly over several weeks needs a preserved diluent, that is the entire reason bacteriostatic water exists. Baclabs supplies UK-stocked bacteriostatic water with 0.9% benzyl alcohol in 30 mL vials; the reconstitution guide sets out the general maths, bacteriostatic vs sterile water explains when a preservative is and is not appropriate, and storage and shelf life covers the 28-day in-use limit after first puncture. Reconstituting hormone products specifically is covered in reconstituting hCG, HGH and fertility medications.
For how somatropin compares with the unlicensed compounds in the same category, see the muscle growth peptides hub and IGF-1 LR3 vs IGF-1 DES.
Legal status and anti-doping
Somatropin is a prescription-only medicine in the UK. Unlicensed HGH sold online is not somatropin with a marketing authorisation, whatever the box says, and supplying it for human use engages the Human Medicines Regulations 2012 and MHRA enforcement. In sport, growth hormone and its analogues and fragments are prohibited at all times under WADA section S2 [5][6].
Frequently asked questions
How many IU are in 1 mg of somatropin?
Three. The Norditropin SmPC states that 1 mg of somatropin corresponds to 3 IU [2]. So 1 IU is 0.333 mg or 333 mcg, a 10 IU vial holds 3.33 mg, and a 12 IU vial holds 4 mg.
How much bacteriostatic water should be used for a 10 IU HGH vial?
Either 1 mL or 2 mL works chemically; the choice is about measurement precision. With 1 mL you get 10 IU/mL, so each U-100 syringe unit is 0.1 IU (33.3 mcg). With 2 mL you get 5 IU/mL, so each unit is 0.05 IU (16.7 mcg) and doses are drawn at twice the number of units, which reduces proportional error at small doses.
How many units on an insulin syringe is 2 IU of HGH?
It depends entirely on the concentration. From a 10 IU vial reconstituted with 1 mL, 2 IU is 20 units; with 2 mL, it is 40 units. From a 12 IU vial with 1 mL, 2 IU is about 16.7 units; with 2 mL, about 33.3 units. Always recalculate from the vial and diluent volume actually used.
What is the licensed somatropin dose for adults?
For adult-onset growth hormone deficiency, 0.15–0.3 mg per day titrated against IGF-1 (Genotropin) or 0.1–0.3 mg per day increasing monthly on clinical response (Norditropin), with maintenance rarely exceeding 1.0 mg/day [1][2]. These are replacement doses for a diagnosed deficiency, not performance doses.
Can reconstituted HGH be kept at room temperature?
Licensed products specify refrigeration at 2–8 °C after reconstitution, with Genotropin stable for 28 days at that temperature [1]. Norditropin FlexPro allows a maximum of 3 weeks below 25 °C as an alternative to 4 weeks refrigerated [2]. Somatropin is a protein and degrades faster warm; refrigeration is the default.
References
- Genotropin 12 mg powder and solvent for solution for injection, Summary of Product Characteristics. electronic medicines compendium
- Norditropin FlexPro 10 mg/1.5 ml solution for injection in pre-filled pen, Summary of Product Characteristics. electronic medicines compendium
- Systematic review: the effects of growth hormone on athletic performance (DARE structured abstract). NIHR Centre for Reviews and Dissemination
- Liu H et al. Systematic review: the effects of growth hormone on athletic performance. Annals of Internal Medicine, 2008
- WADA Prohibited List S2: Peptide hormones, growth factors, related substances and mimetics. Drugs.com summary of the WADA list
- The Prohibited List. World Anti-Doping Agency
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.