Subcutaneous (SubQ) Injection

Also known as: SubQ · SC injection · subcutaneous injection

A subcutaneous (SubQ) injection delivers a drug into the fat layer between the skin and the muscle, where it is absorbed gradually.

Subcutaneous means under the skin. In a SubQ injection the needle passes through the skin, which is about 2 mm thick at common sites, into the fatty tissue beneath it. In an ultrasound study of 388 adults with diabetes, mean fat-layer thickness was about 10.8 mm at the arm, 10.4 mm at the thigh, 13.9 mm at the abdomen and 15.4 mm at the buttocks, with wide variation by sex and body size: women had on average 5.1 mm more fat at these sites, and a 10-point difference in BMI accounted for about 4 mm.

From the injection site, a drug has to travel through the tissue and reach either small blood vessels or the lymphatic system. How fast that happens depends on the molecule's properties, such as its size and charge, and some breakdown can happen at the injection site before absorption. That is why the same drug can behave differently by route, and why approved labels specify one.

Why it matters

Insulin is injected SubQ, and approved peptides such as tesamorelin are labeled for SubQ use. Technique matters because missing the fat layer changes absorption. The same ultrasound study concluded that needles of 8 mm or longer inserted straight in may often enter muscle in the limbs of men and of people with a BMI under 25, while 4 to 5 mm needles reach the fat layer in almost all adults.

The injection site itself also matters. Injecting repeatedly into thickened, lumpy tissue (lipohypertrophy) blunts and destabilizes absorption, which is why site rotation is part of SubQ technique. Moving between abdomen, thighs and arms is fine for rotation, but the fat layer averaged about 10 to 11 mm on the limbs versus about 14 mm on the abdomen, so needle length matters more on the limbs.

How to do it

Use the shortest needle available; in the ultrasound study, 4 to 5 mm needles reached the fat layer with minimal risk of entering muscle in nearly all adults.
Choose a site such as the abdomen at least 2 inches (5 cm) from the navel, and avoid scars, bruises and hard bumps from earlier injections.
Clean the skin with an alcohol swab and let it dry.
Pinch a fold of skin, insert the needle straight in, then release the pinch.
Push the plunger fully, withdraw the needle, and apply light pressure with gauze if it bleeds.
Use a different spot next time and dispose of the syringe in a sharps container.

Common mistakes

Using a long needle straight in on a lean thigh or arm, which can reach muscle.
Injecting into lumps, scars or bruised skin.
Using the same spot every time instead of rotating.
Injecting before the alcohol on the skin has dried.
Assuming SubQ and intramuscular injection give the same absorption.

Sources

ReferenceSkin and subcutaneous adipose layer thickness in adults with diabetes at sites used for insulin injections: implications for needle length recommendations. Curr Med Res Opin, 2010. PMID 20429833.
ReviewMechanistic determinants of biotherapeutics absorption following SC administration. AAPS J, 2012. PMID 22619041.
Prescribing informationEGRIFTA WR (tesamorelin) for injection - prescribing information and instructions for use. DailyMed, U.S. National Library of Medicine, 2025.

Page updated October 3, 2026. How we grade evidence: methodology.

Related Terms

Intramuscular (IM) Injection

An intramuscular (IM) injection delivers a drug into a muscle, using a longer needle than a subcutaneous injection, at sites such as the deltoid.

Injection Site Rotation

Injection site rotation means moving each injection to a different spot so the same patch of tissue is not injected repeatedly.

Insulin Syringe

An insulin syringe is a small, fine-needle syringe marked in insulin units; on the common U-100 type, 100 units equals 1 mL.

Subcutaneous Absorption Rate

Subcutaneous absorption rate is how fast a drug injected into the fatty layer under your skin moves into your bloodstream.

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