What Is Somapacitan?

1. How does somapacitan work, and why only once a week?

Somapacitan is made in E. coli bacteria. Its protein chain has 191 amino acids, matching natural GH. At position 101, a leucine is swapped for a cysteine, and a side chain is attached there. That side chain combines an albumin-binding piece with a water-loving spacer, and it adds about 1,191 g/mol to a total molecular weight of 23,305 g/mol (FDA prescribing information).

Albumin is the most abundant protein in blood. Because over 99% of somapacitan is bound to it, the drug is shielded from rapid clearance and lingers for days. That is what allows a weekly schedule, the same injection rhythm many readers already know from weekly GLP-1 drugs, even though the two act on unrelated receptors.

Once in circulation, somapacitan binds the same GH receptor as natural GH. Part of its effect is direct, and part runs through IGF-1 (insulin-like growth factor 1), which the liver releases in response. That is why IGF-1 is the main blood test used to guide the dose.

This is different from drugs that prompt the pituitary to release more of the body’s own GH, such as tesamorelin and sermorelin. Somapacitan supplies the hormone itself.

2. How long does somapacitan last in the body?

The label’s pharmacology data show how differently it behaves from daily somatropin (Sogroya label):

The IGF-1 response follows its own clock. It peaks 2–4 days after each injection, and in adults it rises less than proportionally as the dose goes up. Compared with daily GH, the weekly product produces larger swings between the IGF-1 peak and the low point before the next dose.

A human radiolabel study by Helleberg and colleagues gave seven healthy men a single 6 mg dose and tracked it for 28 days. They recovered 94.0% of the dose, 80.9% of it in urine, and found no intact somapacitan in urine or feces. Breakdown happens by splitting the linker that joins the side chain to the protein.

Exposure varies between people. The label reports lower levels in women, particularly those taking oral estrogen, and in people with higher body weight. Levels were higher in adults over 65, about 4.7-fold higher with moderate liver impairment, and about 1.6–1.75-fold higher with severe kidney impairment or hemodialysis.

A half-life chart built from these figures shows how drug levels fall. It cannot show your IGF-1 level, which depends on the person and the dose.

3. What did the clinical trials find?

Adults with GH deficiency: REAL 1

Johannsson and colleagues’ REAL 1 trial enrolled 301 previously untreated adults with GH deficiency in 17 countries. Participants received weekly somapacitan or weekly placebo (both double-blind), or open-label daily GH, for 34 weeks, with a 52-week extension afterward. The main outcome was the change in trunk fat percentage on a DXA scan, an x-ray test of body composition. The average somapacitan dose was 2.56 mg per week.

Somapacitan beat placebo on the main outcome. Daily GH lowered trunk fat more, while visceral fat and lean mass changed by similar amounts in both active groups. The changes held through week 86, according to the trial and the Sogroya label.

IGF-1 SDS, which expresses IGF-1 as the number of standard deviations from the average for age and sex, rose from −2.54 to −0.17 on somapacitan and stayed near −2.6 on placebo. In other words, the drug brought a very low level close to normal.

Children with GH deficiency: REAL 4

Miller and colleagues’ REAL 4 trial randomized 200 prepubertal children aged 2.5–11 years, in a 2:1 ratio, to somapacitan 0.16 mg/kg per week or daily Norditropin 0.034 mg/kg per day for 52 weeks. Children on somapacitan grew 11.2 cm per year and those on daily GH grew 11.7 cm per year. The difference of −0.5 cm per year (95% CI −1.1 to 0.2) met the trial’s noninferiority margin, meaning the weekly drug was shown not to be meaningfully worse.

Other causes of short stature

A phase 3 basket trial, one protocol covering several conditions, compared somapacitan 0.24 mg/kg per week with daily somatropin for 52 weeks. The label reports these height velocities:

The label cautions that some comparator doses were below the highest daily dose approved in the US for these conditions, so the gaps favoring somapacitan should be read with that in mind. The same program included 105 girls with Turner syndrome, who grew 9.0 cm per year on somapacitan versus 9.5 cm per year on daily GH, a result that met noninferiority (Mauras and colleagues, 2026). Turner syndrome is not on the US label.

Pooled results

A 2026 meta-analysis by Azhar and colleagues combined 10 randomized trials with 920 children and adults comparing somapacitan with daily Norditropin. Height velocity did not differ (mean difference 0.12 cm/year, 95% CI −0.65 to 0.89), and neither did adverse events (risk ratio 1.03, 95% CI 0.88 to 1.20). IGF-1 SDS was slightly higher on somapacitan, by 0.24 (95% CI 0.02 to 0.46).

4. Who is Sogroya approved for, and is it legal?

The FDA first approved Sogroya on August 28, 2020, to replace GH in adults with GH deficiency (FDA approval letter). On April 28, 2023, it added children 2.5 years and older with growth failure due to GH deficiency (FDA supplement letter). The label revised in February 2026 also lists children 2.5 years and older with short stature after SGA birth without catch-up growth by age 2, Noonan syndrome or ISS.

The label specifically excludes adults who do not have GH deficiency. In the EU, the European Medicines Agency authorized it on March 31, 2021, for adults with GH deficiency, and it is also used for GH-deficient children from age 3.

In the US it is a prescription biologic and not a controlled substance. In sport, the WADA 2026 Prohibited List bans GH and its analogs in and out of competition, and class S2.2.3 names somapacitan as an example alongside lonapegsomatropin and somatrogon.

5. How is Sogroya dosed?

Sogroya comes in prefilled pens holding 5 mg, 10 mg or 15 mg in 1.5 mL. The doses below are the label’s, and adjustment is the prescriber’s job; the general idea behind stepwise increases is covered in the dose titration guide.

Use is not recommended with severe liver impairment at any age, or with moderate liver impairment in children. It calls for an eye exam of the retina before treatment starts and for supervision by a clinician experienced with GH deficiency.

6. What are the side effects and warnings?

Sogroya carries no boxed warning, and most of its warnings come from long experience with daily GH. Over the 34-week adult trial, these reactions occurred in more than 2% of somapacitan users and at least 1 percentage point more often than with placebo (Sogroya label):

Lab shifts from normal to high were more common for phosphate (17.5% vs 4.9%) and creatine kinase, a muscle enzyme (9.2% vs 6.6%). These shifts came and went and did not worsen. REAL 1 reported injection-site reactions in 6.7% of the somapacitan group, 5.9% of the daily GH group and 9.8% of the placebo group. Serious adverse events were 5.8%, 9.2% and 8.2%. Both rates were lower on somapacitan than on placebo.

In children with GH deficiency, common reactions included nasopharyngitis, a common cold (16.7% vs 16.2% on daily GH), headache (12.1% vs 8.8%), fever (9.1% vs 11.8%) and limb pain (9.8% vs 2.9%). No adults developed antibodies to the drug. About 12.1% of children tested positive at some point in the first year, usually once, with no neutralizing antibodies and no clinically significant effect.

Other labeled warnings include:

• higher death rates when GH is given during acute critical illness after heart or abdominal surgery, major trauma or acute respiratory failure;
• possible tumor progression, and second tumors in childhood cancer survivors who had head radiation;
• raised pressure inside the skull, with headache and vision changes, usually in the first 8 weeks;
• fluid retention and carpal tunnel symptoms, usually short-lived and dose-related;
• falling cortisol or thyroid function once GH is started;
• in children, hip growth-plate slippage, worsening scoliosis and pancreatitis;
• serious allergic reactions, and fat lumps or dents from reusing one injection site.

The label lists active cancer, active proliferative or severe non-proliferative diabetic eye disease, closed growth plates in children, and severely obese or breathing-impaired children with Prader-Willi syndrome among its contraindications. No human pregnancy data exist. In rats and rabbits, it did not cause birth defects at exposures about 12 times the highest human dose.

7. Blood sugar, interactions and GLP-1 drugs

Growth hormone works against insulin. The label warns that GH can reduce insulin sensitivity, particularly at higher doses, notes that new type 2 diabetes has been reported with somatropin, and advises periodic glucose checks.

Takahashi and colleagues’ post hoc analysis pooled three adult phase 3 trials and found no new diabetes cases on somapacitan. In previously untreated adults, fasting glucose, insulin and insulin resistance rose more on daily GH than on somapacitan at 34 weeks, but the difference was gone by 86 weeks, and HbA1c did not differ. The manufacturer sponsored that analysis.

The label’s interaction list:

GLP-1 drugs are not named on the label, and no trial has paired somapacitan with semaglutide or tirzepatide. For someone with diabetes on a GLP-1 drug plus insulin or another glucose-lowering medicine, the label’s diabetes-drug warning is the relevant one. Starting both at the same time also makes it hard to tell which drug caused a change in glucose, weight or swelling.

8. Can somapacitan offset muscle loss on GLP-1 drugs?

There is no evidence on this. Lean-mass loss during weight loss is a real concern, covered in the GLP-1 and muscle loss guide, but no somapacitan trial enrolled people taking GLP-1 drugs or people with normal GH levels.

The REAL 1 gain of about 1.1 kg of lean mass over placebo was in adults with a measured GH deficiency, whose IGF-1 started far below normal. Strength and physical function were never outcomes; the trial tracked scans and IGF-1. Healthy-volunteer studies of somapacitan looked only at how the drug is absorbed and cleared.

For someone with normal GH, the label’s risks are the risks of GH excess, and the HGH guide reviews what daily GH has and has not shown in healthy adults. The label’s overdose section mentions low and then high blood sugar and fluid retention, and signs of gigantism or acromegaly with long-term excess.

9. What can you measure or track on weekly GH?

The weekly schedule makes timing matter more than with daily GH. An IGF-1 result only makes sense if you know how many days it was drawn after the last injection. What the label asks clinicians to check:

A useful personal record keeps the injection day, the dose, and the date and time of each IGF-1 draw together, so a clinician can see where in the weekly cycle each result fell. Swelling, joint pain, hand numbness, headaches with vision changes and glucose readings belong in the same log. The peptide tracker vs spreadsheet guide covers ways to keep those records in one place.

Scale weight and waist size are less informative. In the adult placebo-controlled trial, weight gain was a reported side effect while visceral fat fell, so a rising scale number does not show whether fat, lean tissue or fluid changed.

10. Somapacitan FAQ

• Is somapacitan the same as HGH?

  It is a modified form of human growth hormone. The protein is the same length as natural GH, with one amino acid swapped and an albumin-binding side chain attached at that spot. It acts on the same GH receptor, but its half-life is far longer, so it is given weekly instead of daily.

• What is Sogroya approved for?

  In the US, Sogroya is approved for adults with growth hormone deficiency and for children 2.5 years and older with growth failure from GH deficiency, short stature after being born small for gestational age, Noonan syndrome or idiopathic short stature. Adults without GH deficiency are outside the label.

• How long does somapacitan stay in your system?

  The label gives an elimination half-life of about 2–3 days in adults with GH deficiency and about 34–37 hours in children. It reports that the drug is cleared within one week of stopping. Steady state is reached after one or two weekly doses.

• What are the side effects of Sogroya?

  In the adult placebo-controlled trial, back pain, joint pain, indigestion, sleep problems, dizziness, swelling and higher blood pressure were more frequent than with placebo. Labeled warnings shared with daily GH include reduced insulin sensitivity, fluid retention, raised pressure inside the skull and concerns about tumor growth.

• Can somapacitan be used with semaglutide or tirzepatide?

  No published trial has studied that combination, and the Sogroya label does not name GLP-1 drugs as an interaction. It does say growth hormone lowers insulin sensitivity and that doses of insulin and other diabetes drugs may need adjusting, so any combination belongs under a prescriber's supervision.

• Does somapacitan build muscle in healthy people?

  There is no controlled evidence on that. The lean-mass gain in the REAL 1 trial, about 1 kg more than placebo, was measured in adults who lacked growth hormone. The trials did not measure strength or performance, and none enrolled people with normal GH levels.

• Is Sogroya a controlled substance?

  No. In the US it is a prescription biologic that is not scheduled as a controlled substance. In sport, WADA prohibits it at all times and names somapacitan among the GH analogs in class S2.2.3.

• When should IGF-1 be checked on Sogroya?

  The label says to draw IGF-1 3–4 days after the previous dose, because IGF-1 peaks 2–4 days after each weekly injection. A sample taken at another point in the week can give a misleading picture of the dose.

11. Sources