How to dose somatropin (HGH)

Prescription-only medicine. Somatropin (recombinant human growth hormone) is a medicinal product that is subject to medical prescription. This page serves exclusively as a reference for institutional research and it is not a schedule for use in humans of any kind.

Somatropin dosing: the essentials

• The pen's dose dial sets the dose directly in IU; convert IU to mg using the WHO standard: 1 mg of somatropin = 3 IU (1 IU ≈ 0.333 mg)
• Start with 1 IU once a day and hold each step for a minimum of 4 weeks before moving up, to a ceiling of 4 IU once a day
• Change injection sites every 72 hours between the abdomen, the thigh and the deltoid, in order to avoid lipohypertrophy
• Store the pens between 2 and 8 °C and protected from light; dispose of used needles in a puncture-resistant container
• In the clinical research studies, the investigators monitored serum IGF-1 and fasting glucose as their pharmacodynamic endpoints

According to the reports, somatropin (HGH) dosing within a research protocol follows this sequence:

1. Conversion of the target dose by using the WHO standard: 1 mg of somatropin = 3 IU (1 IU ≈ 0.333 mg) [3].
2. A starting dose of 1 IU once a day, set on the pen's dose dial, which carries its scale marked in IU [1].
3. Subcutaneous administration, alternating in turn between the abdomen, the thigh and the deltoid.
4. Each step is held for a minimum of 4 weeks before it is changed, with IGF-1 in the middle of the reference range as the target and a ceiling of 4 IU once a day [1].
5. Pens stored between 2 and 8 °C and needles disposed of in a puncture-resistant container [3].

This guide covers the conversion from IU to mg, the mechanics of the pen, the rotation of sites, the handling of the cold chain and the logic for dealing with missed doses. The plasma half-life of somatropin is 2–4 hours, but its biological half-life (that of IGF-1) reaches 12–24 hours, which is the reason why once-daily dosing works [3].

This guide explains how to convert between IU and mg, how to titrate somatropin in steps of 4 weeks, how to rotate the injection sites in order to avoid lipohypertrophy and how to handle missed doses without derailing the protocol along the way.

What somatropin is and why dose precision matters

Somatropin is recombinant human growth hormone, with a 191-amino-acid sequence that is identical to that of endogenous pituitary GH, and it is dosed in International Units (IU) of bioactivity rather than in units of mass [3]. The WHO international standard sets the conversion at 1 IU = 0.333 mg, that is to say, 3 IU per mg [3]. Misreading that ratio is the single most frequent protocol error in research that uses pen formats.

Precision matters for four separate reasons:

1. The pen's dose dial carries its scale in IU, but the stability data and the IGF-1 dose-response curves are published in mg [1][3].
2. A 40 IU pen contains around 13.3 mg of somatropin in total; a 100 IU pen, around 33.3 mg [3].
3. Supraphysiological deviations in IGF-1 track mg/kg and not IU/kg, so comparing across studies requires the conversion to mg [1].
4. Any in vivo work with somatropin is subject to medicines legislation and requires administrative authorisation in the form of a clinical trial [5][8].

Anyone weighing up direct GH against secretagogue combinations should review the CJC-1295 vs. ipamorelin comparison before settling on a protocol, and consult the CJC-1295 + ipamorelin dosing guide if they are running a parallel GHRH/GHRP arm.

IU-to-mg conversion table for somatropin pens

Every IU setting on the dial converts to mg by using the WHO standard of 1 IU = 0.333 mg [3]. The table below sets out the seven steps on the scale that researchers use most often [3].

The dial works in exactly the same way in both of the pen formats; what changes is the total bioactivity per cartridge and, therefore, how long the pen lasts: a 40 IU pen provides ten doses of 4 IU before it runs out, whereas a 100 IU pen provides twenty-five of them [3].

Where researchers most often go wrong with the calculation

The most frequent protocol error consists of reading a published target that is expressed in mg per day and then setting the dial to the very same number in IU. A target of 1 mg per day is equivalent to 3 IU, not to 1 IU: a threefold underdose [1]. That error usually surfaces in week 3 or week 4, when researchers compare their IGF-1 result against the published reference ranges and detect a blunted response.

Anyone with a parallel secretagogue arm should consult the CJC-1295 vs. ipamorelin comparison and the CJC-1295 + ipamorelin dosing protocol before settling on the total daily amounts.

Somatropin dosing plan: four-week steps

The standard research titration runs from 1 IU once a day up to a ceiling of 4 IU once a day. The whole of the exposure is administered in a single daily injection: the biological half-life of IGF-1, at 12–24 hours, makes any splitting of the dose unnecessary. Slow increases are the published norm, because abrupt jumps provoke fluid retention, paraesthesia and joint pain at the GH receptor sites before the down-regulation has had a chance to stabilise [1].

1. Step 1: 1 IU once a day (≈0.333 mg a day). A single subcutaneous injection, preferably taken before sleep, in order to approximate the nocturnal pulsatility [7]. Draw the baseline IGF-1 before the very first dose.
2. Intermediate steps. Move up only if the previous four weeks showed no oedema, no signs of carpal tunnel syndrome and no drift in fasting glucose. Each step is held for 4 full weeks before it is changed, even if IGF-1 happens to read low at the midpoint of the period.
3. Step 4: 4 IU once a day. This is the ceiling of the protocol. Doses above 4 IU a day are not supported by the published guidelines on GH deficiency in adults [1][3].
4. Maintenance: the lowest IU value that achieves an IGF-1 in the middle of the reference range. Titrate down one step if IGF-1 exceeds the upper reference limit for age and sex [1].

Why the titration is slow

Receptor desensitisation and the lag in IGF-1 explain why each step is held for a minimum of 4 weeks. IGF-1 does not reach a plateau until around 5–7 days after a change of dose, so assessing the response before that window has passed gives a false picture. Jumping two steps at once is the most frequent reason why researchers describe headache and transient hyperglycaemia. It also explains why so many of them are disappointed by their IGF-1 result when they did not wait long enough after an increase.

When a secretagogue arm is the better fit instead

Anyone who values an endogenous pulsatile release of GH rather than an exogenous supply should review the CJC-1295 vs. ipamorelin comparison and the CJC-1295 + ipamorelin dosing protocol instead. Direct somatropin generates a more predictable IGF-1 curve; the GHRH/GHRP combinations generate variable pulsatile responses with much scarcer comparative data behind them.

Parameters described for subcutaneous administration: step by step

The published protocols describe the correct subcutaneous administration of somatropin in ten ordered steps. The priming in step four is the step that is most often skipped in practice with a new pen, and skipping it, according to the reports, underdoses the first injection by as much as 2 IU, because the cartridge contains an air-filled dead space from the point of manufacture.

1. Pen temperature. According to the reports, taking the pen out of the fridge 30 minutes before use reduces the sting of the injection and the risk of backflow that is caused by tissue resistance. Warming it up in hot water or on a radiator is not described anywhere in the literature; room temperature only.
2. Inspect the solution. The solution in the cartridge must be clear and colourless. The protocols provide for discarding any cartridge that shows cloudiness, particles or a yellowish tint.
3. Attach the needle. The usual injection-technique guides recommend a short pen needle for subcutaneous administration [3], wiping the rubber septum with a 70% isopropyl wipe before screwing it on.
4. Prime the pen. Select an air shot of 2 IU, point the needle upwards, tap the cartridge gently so that the bubbles rise up to the tip and press the plunger until a drop appears at the tip of the needle; repeat the process if no drop forms. This step is described as essential when a new pen is being used for the first time and after every change of needle.
5. Select the dose. Turn the dial to the target dose in IU and read the dose window at eye level. Checking the figure against the research protocol before administering is the practice described.
6. Choose the site. The injection site is chosen according to the rotation scheme (abdomen, lateral thigh or upper buttock), avoiding the 5 cm zone around the navel and any site that has been used in the last 72 hours.
7. Needle angle. The protocols describe lifting the skin and inserting the needle at 45–90°: 90° at abdominal sites with short needles, 45° at lean thigh sites or with longer needles, in order to avoid intramuscular administration.
8. Speed of administration. A slow administration is described, holding the plunger down for 5 seconds before withdrawing; according to the reports, withdrawing earlier causes backflow and loss of dose.
9. Needle disposal. The needle is removed and disposed of in a puncture-resistant sharps container. Used pen needles are regarded as contaminated waste; they go into a puncture-resistant container, and not loose into the household rubbish [3].
10. Logging. The dose, the time, the site and the batch number are noted in the research log. The site data feed into the rotation scheme that is covered in the next section.

Anyone with a parallel secretagogue arm should consult the CJC-1295 + ipamorelin dosing protocol; the administration technique is the same pre-filled pen.

Injection-site rotation: avoiding lipohypertrophy

Alternate somatropin between three subcutaneous sites (the abdomen, the outer thigh and the back of the upper arm) and never repeat the same site within a period of 7 days. Injecting repeatedly into a single spot provokes lipohypertrophy, a fibrotic fatty nodule that slows down absorption in an irregular way and destabilises the IGF-1 response. Lipohypertrophy is the most frequent cause of an unexplained plateau or drop in IGF-1 midway through a protocol.

Main sites and their limits

• Abdomen: the band running 2–5 cm lateral to the navel out as far as the iliac crest. Avoid the 5 cm zone around the navel. Abdominal tissue shows the most uniform absorption kinetics for subcutaneous recombinant growth hormone and it is the standard site in once-daily nocturnal protocols.
• Outer thigh: the middle third of the lateral thigh, between the greater trochanter and the knee. In lean individuals, use a 45° angle in order to avoid intramuscular administration.
• Back of the upper arm: the fat pad that sits over the triceps, halfway between the acromion and the olecranon. In self-administered protocols it requires help from another person in order to achieve a reliable pinch technique.

Weekly rotation plan

A workable 7-day plan looks like this: Mon. left abdomen, Tue. right abdomen, Wed. left thigh, Thu. right thigh, Fri. left arm, Sat. right arm, Sun. lower abdomen (≥3 cm away from any Monday or Tuesday puncture). Record each site as per step 10 of the injection protocol. Mark the sites used on a body chart and do not return to any 1 cm² area within 72 hours or to any broader region within 7 days.

Why lipohypertrophy matters

Lipohypertrophy is a hyperplastic accumulation of adipocytes and connective tissue at sites that are injected chronically, and it reduces the capillary uptake of the peptide and makes it variable [4]. Blanco et al. (Diabetes Metab, 2013) found lipohypertrophy in 64.4% of the people who were injecting insulin, and in almost all of them the rotation of sites was either incorrect or non-existent; it is the reference figure for repeated subcutaneous injections, because there are no prevalence figures that are specific to somatropin [4]. Palpate every candidate site before injecting; any rubbery, painless nodule disqualifies that site for at least 30 days.

Anyone with a parallel secretagogue arm should review the CJC-1295 vs. ipamorelin comparison before deciding between supplementing and substituting, and then consult the CJC-1295 + ipamorelin dosing protocol for the considerations on overlapping sites.

Timing: morning dosing versus dosing before sleep

Subcutaneous somatropin before sleep is the timing that is most often cited in the research protocols from 2022–2025, because it approximates the largest endogenous pulse of GH, which occurs 60–90 minutes after falling asleep. However, the published practice reviews acknowledge that the total daily exposure and a stable IGF-1 matter more than the time on the clock does [1]. Morning dosing is a valid alternative whenever adherence tracking or daytime fasting windows take priority.

Morning dosing

Morning administration coincides with the natural rise in GH on waking and it is easier to log against a fixed daily research protocol. The plasma half-life of subcutaneous somatropin is around 2–4 hours, while the biological half-life of IGF-1 reaches 12–24 hours, so a single injection in the morning still generates a near-steady exposure to IGF-1 across the whole dosing interval [3].

Dosing before sleep

Injecting 30–60 minutes before falling asleep layers the exogenous somatropin on top of the nocturnal pulse and it is the most cited option in research models on body composition, partly because it shifts the transient insulin-antagonist effect of GH out of the daytime carbohydrate window [7]. Comparative randomised controlled trials (RCTs) that randomised morning against evening administration in 2022–2025 are scarce, so the choice is a pragmatic one rather than being evidence-led.

A single injection a day

The protocol administers the whole of the daily dose in a single injection, at either one of the two times, and it does not split it between morning and evening. The biological half-life of IGF-1 sustains the exposure across the whole interval, so splitting it merely adds needle sticks without any documented advantage [1].

Fasted state

According to the reports, injecting in a fasted state (≥3 hours after a meal) reduces the competing insulin signalling at the time of the dose, although direct comparisons of bioavailability in humans from 2022–2025 are limited.

Storage and cold chain: keeping the pen usable

Somatropin pens are kept refrigerated between 2 and 8 °C, protected from light, and they are never frozen. Freezing denatures recombinant growth hormone and renders the pen unusable regardless of how it looks; a single freeze-thaw cycle is irreversible [3]. The unopened shelf life, the in-use window after the first puncture and the tolerance to room temperature all vary from one manufacturer to another: check the product sheet for the specific product in question, because Body Pharm does not supply somatropin in Spain and these figures do not belong to any pen in this catalogue. Let the pen reach room temperature before you inject.

Transport between sites

Use an insulated bag with a gel pack that is wrapped in cloth or in bubble wrap. Direct contact between the pen and a frozen pack takes the cartridge below 0 °C and inactivates the protein. Aim for a 2–8 °C window throughout the whole journey and check it with an inexpensive digital max-min thermometer if the trip lasts for longer than two hours.

Heat excursions

Respect the room-temperature window that the manufacturer specifies and do not exceed its temperature limit either. Discard the pen if it has exceeded either one of the two. A pen that has sat in a car boot in summer or next to a south-facing window is not usable, even if the solution still looks clear.

Usability checklist before every injection

1. The solution in the cartridge is clear and colourless, with no particles or cloudiness.
2. The pen has been stored between 2 and 8 °C, or within the room-temperature window that its manufacturer specifies.
3. The date of the first puncture is noted and falls within the manufacturer's in-use window.
4. The cartridge shows no frost rings, cracks or deformation of the seal.
5. The dial moves freely and the priming drop forms at the tip of the needle.

Anyone with parallel secretagogue arms should bear in mind that they are handled in exactly the same way: they are pre-filled pens with the very same storage window. Consult the CJC-1295 + ipamorelin dosing protocol and the CJC-1295 vs. ipamorelin comparison for the differences in protocol.

Missed dose and protocol interruption

According to the reports, a missed dose of somatropin is simply skipped and the next scheduled dose is taken at the usual time, rather than doubling up in order to compensate. Subcutaneous somatropin has a plasma half-life of around 2–4 hours and a biological half-life (that of IGF-1) of around 12–24 hours [1][3], so a single missed injection only produces a passing dip in the exposure to IGF-1. An isolated missed dose does not require any change to the protocol or any re-titration at all.

Doubling the next dose generates a supraphysiological peak of GH well above the 1–4 IU a day range that this protocol covers. In research models, that raises the likelihood of fluid retention, arthralgia, paraesthesia and acute insulin resistance, without doing anything to improve the steady-state IGF-1 [1][2].

How to handle prolonged interruptions

For interruptions of more than 72 hours, restart at the previous, lower step and hold that step for a minimum of 4 weeks before moving back up again.

Use the following decision sequence:

1. Less than 12 hours late: inject as soon as it is remembered and then return to the usual plan.
2. Between 12 and 24 hours late: skip the dose and continue with the next scheduled injection.
3. 2–3 consecutive doses missed (<72 hours in total): continue at the current dose with no change to the titration.
4. Interruption of more than 72 hours: drop one step and move back up guided by IGF-1.
5. Interruption of more than 14 days: restart from the original starting dose (1 IU a day).

Anyone assessing secretagogue alternatives during the planned washout phases will find useful reference points in the CJC-1295 vs. ipamorelin comparison and in the CJC-1295 + ipamorelin dosing protocol.

Somatropin versus GH-releasing peptides: which fits the research?

Somatropin delivers exogenous recombinant GH directly and generates a predictable IGF-1 dose-response relationship. CJC-1295, ipamorelin and tesamorelin stimulate endogenous release from the pituitary with greater variability between individuals. The choice depends on whether the research model demands an exposure to GH that is precisely controlled in IU or the preservation of the pulsatile function of the hypothalamic-pituitary axis.

Comparison at a glance

When each approach fits

Somatropin is the reference standard when the protocol demands titration against a defined IGF-1 target in the middle of the reference range, moving up one step every 4 weeks within the window of 1–4 IU a day [1]. The IGF-1 response curve is well characterised and it is backed by guidelines.

Combinations of CJC-1295 plus ipamorelin suit models that prioritise pulsatile release and a lower risk of supraphysiological IGF-1 values, although no direct head-to-head RCTs against somatropin on IGF-1 endpoints were identified anywhere in the 2024–2026 literature. Consult the CJC-1295 vs. ipamorelin comparison for the receptor-level differences and the CJC-1295 + ipamorelin dosing protocol for the titration steps.

Tesamorelin has a narrower scope and it is used in research models that are aimed specifically at visceral adipose tissue.

Combination protocols

Some research models from 2022–2024 combine low-dose somatropin with a GHRP, in order to preserve the endogenous pulsatility while supplementing the basal IGF-1 at the same time. Comparative data on IGF-1 outcomes as against somatropin monotherapy remain limited for now.

Somatropin pen: the 40 IU format versus the 100 IU format

The two pen formats differ in the total bioactivity per cartridge and, therefore, in the duration of the protocol: the 40 IU pen contains around 13.3 mg of somatropin and the 100 IU pen contains around 33.3 mg, according to the WHO standard of 1 mg = 3 IU [3]. In both cases the dial sets the dose in IU and it is read in exactly the same way; the large format simply lasts for longer.

Matching the pen format to the protocol duration

Choose the format according to the daily IU target and the length of the cycle:

1. 40 IU pen, 1 IU a day: 40 days' supply. Covers the full first 4-week step with margin to spare [3].
2. 40 IU pen, 2 IU a day: 20 days. Fits short windows before the first IGF-1 check at 4–8 weeks [1].
3. 100 IU pen, 2 IU a day: 50 days. Covers a full step and part of the next one with a single cartridge.
4. 100 IU pen, 4 IU a day: 25 days. Corresponds to the ceiling of the protocol.

Reading the dose counter

The counter counts down the IU that are selected per injection, not the mg. To calculate the doses that remain, divide the counter reading by the IU per injection. A 100 IU pen that reads 28 IU still has 14 injections left in it at 2 IU a day.

Safety and monitoring considerations in research

Anyone running a somatropin protocol should monitor four parameters as a bare minimum: serum IGF-1, fasting glucose, peripheral oedema and carpal tunnel symptoms. These are research monitoring criteria and not medical advice, and all in vivo work must take place within an authorised regulatory framework.

Main biochemical markers

1. Serum IGF-1, which is the proxy value for systemic GH activity. Aim for the middle of the reference range for age and sex; hold each step for a minimum of 4 weeks before moving up to the next one [1].
2. Fasting glucose and HbA1c, since somatropin acts in a counter-regulatory way against insulin and chronic dosing reliably reduces insulin sensitivity, a stable finding throughout the older literature by Møller and Jørgensen and confirmed in paediatric and adult reviews from 2020 [2].
3. IGFBP-3, a secondary marker that tracks the biological half-life of 12–24 hours of subcutaneous somatropin.

Clinical observation parameters

Keep track of peripheral oedema (ankles, hands), arthralgia and paraesthesia in the territory of the median nerve. Carpal tunnel symptoms and fluid retention are the dose-limiting effects that are most frequently described in GH replacement in adults, and they usually resolve on titrating down [1].

Regulatory and ethical framework

All use of somatropin in humans is regulated as a medicinal product subject to medical prescription and, in a research context, it requires administrative authorisation as a clinical trial, an ethics opinion and compliance with Good Clinical Practice (GCP) and with Good Manufacturing Practice (GMP) [5][6][8]. Somatropin also appears on the WADA list of prohibited substances, which is relevant for any cohort that is close to sport. In vitro and analytical work is only subject to medicines legislation once the molecule is actually administered.

Anyone weighing up direct GH against endogenous pulse approaches should review the CJC-1295 vs. ipamorelin comparison and the CJC-1295 + ipamorelin dosing protocol before settling on a monitoring plan.

Frequently Asked Questions

1. How many IU does a beginner's somatropin research protocol cover? Research protocols in adults start at 1 IU once a day, which according to the WHO standard of 1 mg = 3 IU equates to around 0.333 mg a day [1][3]. Each step is held for a minimum of 4 weeks and the titration is guided by IGF-1 up to a ceiling of 4 IU a day [1].

2. Can somatropin be injected intramuscularly? Subcutaneous administration is the validated route in adult and paediatric protocols and generates a plasma half-life of 2–4 hours and a biological half-life (that of IGF-1) of 12–24 hours [3]. Intramuscular injection is not provided for in the current SmPCs and falls outside the characterised pharmacokinetics [3].

3. How long does a 40 IU pen last at 2 IU a day? A 40 IU cartridge dosed at 2 IU a day gives 20 days of administration, not counting the loss to priming after the initial 1–2 IU prime on first use. The cumulative priming at pen changes shortens it slightly in workflows that use several pens.

4. What happens if somatropin is left out of the fridge overnight? One night out of the fridge does not render it unusable provided it stays within the room-temperature window that its manufacturer specifies; it goes back in the fridge and the cumulative time is noted. Once that window or its temperature limit has been exceeded, the protocols provide for discarding it [3].

5. Can somatropin be combined with CJC-1295 or ipamorelin in research? No randomised study from 2024–2026 directly compares or combines somatropin with CJC-1295/ipamorelin on IGF-1 endpoints. Anyone modelling combined protocols should review the CJC-1295 vs. ipamorelin comparison and the CJC-1295 + ipamorelin dosing guide before designing a combined arm.

6. Is somatropin the same thing as HGH? Somatropin is recombinant human growth hormone with the same 191-amino-acid sequence as hGH of pituitary origin. The terms are used as synonyms in the clinical and research literature, although "HGH" colloquially also covers non-recombinant references.

Next steps

Research protocols usually confirm the target IGF-1 range and the baseline value of serum IGF-1 first of all, and then choose a pen format (40 IU or 100 IU) according to the expected duration of the protocol. An injection-site rotation scheme is drawn up from the 7-day plan set out above, with calendar reminders for the step changes at weeks 4, 8 and 12 and with IGF-1 checks on the very same dates. If a parallel secretagogue arm is being run, it is worth consulting the CJC-1295 vs. ipamorelin comparison and the CJC-1295 + ipamorelin dosing guides before settling on the protocol.

Sources

1. Molitch ME, et al. Evaluation and treatment of adult growth hormone deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011. pubmed.ncbi.nlm.nih.gov/21602453
2. Møller N, Jørgensen JOL. Effects of growth hormone on glucose, lipid, and protein metabolism in human subjects. Endocr Rev. 2009. pubmed.ncbi.nlm.nih.gov/19240267
3. EMA, EPAR and summary of product characteristics (SmPC) for Omnitrope (somatropin): IU/mg equivalence (1 mg = 3 IU), half-life of around 3 hours after subcutaneous administration, adult posology, pen instructions for use and storage. ema.europa.eu
4. Blanco M, et al. Prevalence and risk factors of lipohypertrophy in insulin-injecting patients with diabetes. Diabetes Metab. 2013. pubmed.ncbi.nlm.nih.gov/23886784
5. AEMPS, Centro de Información online de Medicamentos (CIMA): register of medicinal products authorised in Spain and their prescription status. cima.aemps.es — consulted in October 2026.
6. ICH, quality guidelines Q1A(R2) (stability) and Q7 (good manufacturing practice for active substances). ich.org
7. Van Cauter E, et al. Age-related changes in slow wave sleep and REM sleep and relationship with growth hormone and cortisol levels in healthy men. JAMA. 2000. pubmed.ncbi.nlm.nih.gov/10938176
8. Regulation (EU) No 536/2014 of the European Parliament and of the Council on clinical trials on medicinal products for human use: prior authorisation of every clinical trial; text on EUR-Lex. eur-lex.europa.eu