Subcutaneous injection sites: where weekly injections go, and what the trials specified

"Subcutaneous" means under the skin. The target is the subcutaneous fat, which has a smaller blood supply than muscle and releases medicine into the circulation more gradually. The labels for weekly GLP-1 pens describe the route in those terms: Zepbound's instructions say to inject under the skin (subcutaneously) only ^[2], and the Wegovy label says to inject subcutaneously in the abdomen, thigh or upper arm ^[1].

The same route goes by several names. SC, SQ, subQ and subcut all mean subcutaneous; the CDC uses "SUBCUT" on its own vaccine sheets ^[4]. A common search pairs subQ with "subdermal", but they are not the same word: subdermal is used for things placed just beneath the skin, such as implants, and is not the name of the injection route.

Subcutaneous is one of several injection routes. Intradermal injections go into the skin itself, intramuscular injections go deeper, into muscle, and intravenous injections go into a vein. Which route a medicine uses is fixed by its label, not chosen at the time of injection.

The arm row is the one that catches people out. The Zepbound instructions say the user or another person can inject into the stomach or thigh, but that another person should give the injection in the back of the upper arm ^[2], because the back of one's own arm is hard to reach at the right angle.

Other subcutaneous sites appear in some patient leaflets for other medicines, but for the weekly incretin pens these three are the whole list. A subcutaneous injection sites diagram on a product leaflet shows the same shaded areas: the belly around, but not at, the navel, the front of the thighs, and the back of the upper arms.

The belly is the most-used site because it has the most accessible fat and is easy to see. The useful boundary is the one MedlinePlus gives: below the ribs, above the hip bones, and at least 2 inches (5 cm) from the belly button ^[3]. Within that zone, no position has been shown to be the best; the labels treat the whole abdomen as one site ^[1].

What the labels do specify is rotation. The Wegovy label says to rotate injection sites with each dose ^[1]. The Zepbound instructions say to change the injection site each week, and that the same area of the body can be used as long as a different spot within it is chosen ^[2].

Moving between sites does not change the dose. The Wegovy label states that the time of day and the injection site can be changed without any dosage modification ^[1].

Weekly pens. Prefilled single-dose pens hide the needle, and their instructions are short. The Zepbound pen instructions, as an example, set out these steps ^[2]:

1. Check the pen and the medicine: not frozen, not cloudy, colorless to slightly yellow, no particles.
2. Wash hands and choose a site: stomach, thigh, or the back of the upper arm if someone else is injecting.
3. Pull off the base cap and place the clear base flat against the skin at the chosen site, then unlock.
4. Press and hold the injection button for up to 10 seconds; a first click marks the start and a second click the end.
5. Confirm the gray plunger is visible, then put the used pen in a sharps container.

The Wegovy label adds the same visual check: use the injection only if the solution is clear, colorless and free of particles ^[1]. Because the pen sits flat, the needle enters at a right angle to the skin, and its length is fixed by the device.

Syringes. With a needle and syringe the angle and depth are set by hand. The CDC's sheet for subcutaneous vaccines specifies a 23- to 25-gauge needle, 5/8 inch long, inserted into the subcutaneous tissue at a 45-degree angle ^[4]. Other syringe medicines set their own needle and angle in their instructions, which is why a single "correct" angle does not exist across products.

The difference is depth. A subcutaneous injection stops in the fat; an intramuscular injection passes through the fat into muscle. Muscle has more blood flow, so intramuscular injections usually reach the bloodstream faster, and they generally need longer needles. Using the wrong route changes how a drug is absorbed, which is why each label names one.

"Degree of injection" searches are really asking about angle. For subcutaneous vaccines given by syringe, the CDC's angle is 45 degrees with a 5/8-inch needle ^[4]. Weekly pens are designed to be held flat against the skin ^[2], so the angle question does not arise in the same way. Intramuscular vaccines use a different technique, covered in the CDC's separate intramuscular guidance rather than on this page.

Every cagrilintide trial in this series used the subcutaneous route, once weekly. The phase 1b trial gave subcutaneous cagrilintide alongside subcutaneous semaglutide 2.4 mg ^[5], and REDEFINE 5 describes once-weekly subcutaneous injections of the combination ^[6].

The devices changed over time. In the phase 2 trial in type 2 diabetes, semaglutide and cagrilintide were given as separate injections from two different pens, and three participants reported injection-site reactions ^[7]. The fixed-dose CagriSema product uses a single-dose, single-use dual-chamber pen; in a 2026 usability study, 150 adults completed simulated injections after a median of 3 minutes of training, taking 15 seconds to prepare and inject ^[8].

The published trial reports in this series specify the route and the device, but their abstracts do not name the body sites used. The site rules for approved weekly pens, the abdomen, thigh and upper arm with rotation each week ^[1][2], are the closest reference until CagriSema has a label of its own. The trial doses given by this route are on the CagriSema dosing schedule page, the product itself on the CagriSema record, and the volume arithmetic for cagrilintide's trial doses on the cagrilintide calculator. For combinations no trial has injected, see cagrilintide with retatrutide.