What a subcutaneous injection is

What “subcutaneous” means

Subcutaneous means beneath the skin. MedlinePlus, the US National Library of Medicine’s health site, defines a subcutaneous injection as one given into the fatty tissue just under the skin. It notes that this is the usual way people give themselves certain medicines, such as insulin, blood thinners and fertility drugs ^[1].

GLP-1 injections belong to the same group. The Ozempic® injection, Mounjaro® and Trulicity® labels each describe the product as an injection for subcutaneous use ^[2][4][7]. A brand can cover more than one route, though: the Wegovy® label covers an injection given under the skin and tablets taken by mouth ^[3].

Three layers, three routes

Think of three layers: the skin on top, a layer of fat beneath it, and muscle below that. A subcutaneous injection goes into the fat. An intramuscular injection goes into the muscle, and an intravenous one directly into a vein.

The route is part of the approval. The Ozempic® Medication Guide says not to inject into a muscle or a vein ^[2], and the European product information for Ozempic® says the same ^[10]. An FDA and ISMP presentation on pen injectors lists the wrong route, such as by mouth, into a vein or into a muscle, among errors reported with insulin pens, and reminds health professionals that those pens are labelled for subcutaneous use only ^[12].

From the fat layer into the blood

An injection under the skin does not put the whole dose into the blood at once. The medicine has to move out of the fat layer first, and how much arrives, and how fast, differs by product. Section 12.3 of each label gives the figures.

Swallowing semaglutide is a useful contrast: the Wegovy® label estimates that only about 1% to 2% is absorbed from a tablet, against 89% after an injection ^[3].

Why the trip from the fat layer is slow

Some of the reasons are written into the labels. The Saxenda® label says liraglutide lasts long enough for daily use partly because its molecules cling together, which delays absorption ^[6]. The European product information for Trulicity® says the large size of dulaglutide slows its absorption and reduces how fast the kidneys clear it ^[11].

Research adds a second layer. In a study in sheep, the share of a dose that left the injection site through lymph rose with the size of the molecule, and molecules above about 16,000 daltons were absorbed mainly through the lymph vessels ^[13]. A 2014 review of medicines given under the skin describes absorption as relatively slow and mostly incomplete, with proteins above about 20,000 daltons taken up mostly by the lymphatic system, and says the reasons part of a dose is lost before it reaches the blood are still poorly understood ^[14].

Size varies a lot among these medicines. The semaglutide and tirzepatide labels give molecular weights of about 4,100 and 4,800 daltons ^[2][4], while dulaglutide, built on part of an antibody, is about 63,000 ^[7]. The labels do not say which path each medicine takes, and the animal findings may not match people exactly.

Where it can go

MedlinePlus describes the usual areas as the side or back of the upper arm, at least 3 inches (7.6 cm) below the shoulder and above the elbow; the outer side of the upper thigh; and the belly between the ribs and the hip bones, at least 2 inches (5.1 cm) from the belly button ^[1].

The GLP-1 labels name the abdomen, thigh and upper arm ^[2][4][7]. Both tirzepatide labels add that the back of the upper arm is for another person to inject ^[4][5]. The European Ozempic® information says the injection area can be changed without changing the dose ^[10]. People who also use insulin are told to give the two as separate injections and never mix them; the same body area is acceptable, but not right next to each other ^[2][4][7]. Our article on injection areas covers each label in more detail.

Healthy skin and rotation

MedlinePlus says the chosen spot should be healthy, without redness, swelling, scarring, bruising or other damage. It also advises moving each injection at least 1 inch (about 2.5 cm) from the last one, to keep the skin healthy and help the body absorb the medicine well ^[1].

The GLP-1 labels ask for a different spot with each dose ^[2][4]. The Saxenda® label gives a specific reason: rotating spots within the same area lowers the risk of cutaneous amyloidosis, an abnormal protein deposit in the skin that appears among the reactions reported after approval ^[6]. Over months, keeping all of this in your head is hard, which is one reason people track injection sites.

Short needles, used once

MedlinePlus describes subcutaneous needles as very short and thin and says a needle and syringe are not used more than once ^[1]; the FDA presentation notes that insulin pens use short needles for this route ^[12]. The pen labels add a new needle for every injection and never sharing a pen with another person ^[2][4].

Why can a short needle work? An ultrasound study of 388 US adults with diabetes measured the skin at about 1.9 to 2.4 mm thick on average at common injection areas, while the fat layer beneath averaged about 10 to 15 mm and varied far more between people ^[15]. The pramlintide label reports similar absorption with 6.0 mm and 12.7 mm needles ^[8]. Which needle your device uses, and how to hold it, is part of your training, not something to choose from an article.

Training comes first

MedlinePlus lays out each step of giving an injection ^[1]. This article deliberately does not, because the labels put training first. The Ozempic® Medication Guide says your health care provider should show you how to use the pen before the first dose ^[2], and the Wegovy® instructions say not to use the pen without proper training ^[3].

The tirzepatide labels are the most detailed. They ask that patients and caregivers be trained for the exact presentation they receive, whether a vial, a single-dose pen or a KwikPen®, and trained again if it changes. They say the KwikPen® is not recommended for self-injection by people with impaired vision, and the Mounjaro® label adds children. For vials, they call for a syringe suited to the dose and a new syringe and needle every time ^[4][5].

Drawing medicine from a vial into a syringe is an extra skill; MedlinePlus notes that it needs its own proper technique ^[1]. A product with no FDA label has no approved instructions at all, so a clinician’s guidance matters even more.

After the injection

MedlinePlus says some soreness at the site is expected, and to contact your health care provider if you notice signs of infection, such as redness, swelling, red streaks or a fever ^[1].

Reactions at the site were counted in the trials. In the Mounjaro® placebo-controlled pool, they were reported in 3.2% of people on the medicine and 0.4% on placebo, and they were more common in people who developed antibodies to tirzepatide (4.6%) than in those who did not (0.7%) ^[4]. Serious allergic reactions are rarer, and the Medication Guide lists the signs that need urgent help ^[2]. The table below sums up when to get help.

Ozempic®, Wegovy® and Saxenda® are registered trademarks of Novo Nordisk A/S. Mounjaro®, Zepbound®, Trulicity® and KwikPen® are registered trademarks of Eli Lilly and Company. Symlin® is a registered trademark of the AstraZeneca group of companies; the FDA lists its US presentations as discontinued ^[9]. PepLabo is not affiliated with or endorsed by Novo Nordisk, Eli Lilly and Company or AstraZeneca.

When to get help