How to Inject Testosterone in the Deltoid (Shoulder)

The shoulder is the easiest intramuscular site to reach on yourself, and it is also the site where the classic textbook landmark and the anatomy research disagree. This guide covers where the nerves under the deltoid actually run, how far down the arm to go, which needle length fits your build, and how much oil the muscle will take.

The deltoid is the muscle that caps the shoulder, and it is often the first intramuscular (IM) site people try for testosterone because you can see it and reach it with either hand. The trade-off is space. It is a smaller muscle than the hip or the thigh, and a nerve and an artery run across its underside not far below the shoulder, so where you put the needle matters more here than anywhere else. Worth knowing before you start: the prescribing information for testosterone cypionate says IM injections "should be given deep in the gluteal muscle"⁸. Using the deltoid is a choice your prescriber makes with you, not a default. This is one of three site guides linked from our injection sites hub. For the full draw-up and injection procedure, see how to inject testosterone. If you are still choosing between routes, start with SubQ vs IM testosterone.

What's under the deltoid

The deltoid is a triangle. Its broad base starts along the acromion, the flat bony ridge at the top of the shoulder, and its point ends on the outer side of the upper arm bone (the humerus), at a small ridge called the deltoid tuberosity. The thickest muscle is in the middle of that triangle. The structures that matter for injections are underneath it:

1. The axillary nerve and the posterior circumflex humeral artery. They travel together, running horizontally across the underside of the upper half of the muscle. A systematic review of 57 anatomical and clinical papers found the axillary nerve usually sits 5–7 cm below the acromion, at an average of 5.58 cm with a range of 4.34–6.39 cm across studies¹. An ultrasound study of 30 adults located the artery 7.6 ± 1.0 cm below the acromion in men and 6.2 ± 0.8 cm in women².
2. The subdeltoid (subacromial) bursa, a fluid sac near the top of the shoulder. The same review notes that bursitis and other shoulder lesions after injections are more likely to come from poor technique, meaning the wrong site, angle or needle size¹.
3. The radial nerve, further back. In the ultrasound study, the radial nerve showed up at the candidate site on the posterior (back) part of the deltoid, which is why the authors placed their safer sites on the vertical line through the middle of the acromion rather than behind it².

How to find the deltoid injection site, step by step

1. Take the sleeve off completely. A rolled-up sleeve bunches at the shoulder and pushes you to inject higher than you meant to.
2. Let the arm hang relaxed at your side. A tensed muscle is harder to landmark and usually more painful to inject.
3. Find the acromion. Run your fingers up the outer arm until you reach the flat, bony shelf at the top of the shoulder. Find the midpoint of its outer edge.
4. Picture a vertical line running straight down the outer arm from that midpoint. This mid-acromion line is where the anatomical studies placed their safer sites².
5. Find the lower tip of the muscle. Lift the arm slightly out to the side against light resistance and you will see the deltoid's V-shaped lower edge narrowing toward the middle of the upper arm.
6. Your target is the thick belly of the muscle on the mid-acromion line: well below the top third, and above the point where the muscle tapers into its V. Squeeze gently to confirm there is real muscle under your fingers.
7. Clean the site, spread the skin taut, and insert the needle at 90 degrees⁵.

Textbook landmark vs anatomy research

Standard nursing texts teach the deltoid site as the middle of the muscle, about 1–2 inches (2.5–5 cm) below the acromion process, or "three finger widths" below it^3,4. That has the advantage of being quick to teach. The problem the anatomical research raises is that the axillary nerve often sits right at the bottom of that range, around 5 cm below the midpoint of the acromion¹. The systematic review proposed a point about five fingerbreadths, or 10 cm, below the midpoint of the acromion's outer edge, which is below the nerve, the artery and the bursa¹. The ultrasound study defined its preferred site as the point where the mid-acromion line crosses the anteroposterior axillary line, an imaginary line between the tops of the front and back armpit creases. That point measured 10.5–13.8 cm below the acromion in men and 8.5–11.0 cm in women².

If you are unsure, err downward into the thick belly of the muscle rather than up toward the shoulder, and have a nurse mark your site the first time. On smaller arms everything sits closer together.

Rear delt injections: why the back of the shoulder is a poor target

People sometimes ask about injecting the rear (posterior) deltoid, because the back of the shoulder can feel meatier on a trained lifter. The anatomy argues against it. In the ultrasound study, the radial nerve appeared at the candidate site on the back of the deltoid, and the authors put their recommended sites on the mid-acromion line for that reason². Injection nerve injuries are uncommon but real. In a case series of 92 people with injection-related nerve palsy, 8 involved the radial nerve⁶. Keep the needle on the outer side of the arm, in line with the middle of the acromion.

Needle length and gauge for a deltoid injection

The needle has to pass through the fat layer and finish inside the muscle. Most published deltoid guidance comes from vaccination, and the CDC's adult table matches needle length to body weight and sex⁵:

The ultrasound study found that at its recommended deltoid site, a needle inserted 5 mm deeper than the person's subcutaneous fat thickness, at 90 degrees, was enough to reach muscle in both men and women². How long a needle you need depends on your build, not only your weight, which is one reason prescribers still pick it individually.

Gauge is a separate question. The CDC's 22–25 gauge range is for thin, water-based vaccines⁵. Nursing texts list 18–21 gauge for oil-based solutions³. Testosterone cypionate comes in cottonseed oil⁸, but a finer needle can still work because oil will pass through it, just slowly. For subcutaneous testosterone, one review notes that a 23-gauge needle "can be used without difficulty" for the long-acting esters⁹. The gauge you use is part of the prescription, so ask whoever trains you what they want you to use.

How much testosterone can you inject in the deltoid?

The deltoid holds the least volume of the three IM sites. Open RN's nursing text puts the usual limit for a single deltoid injection at about 1 mL⁴. The systematic review quotes other guidance allowing 2 mL or less¹. The ventrogluteal and vastus lateralis muscles each take up to 3 mL in an average adult⁴.

The label strengths are 100 mg/mL and 200 mg/mL⁸. At 200 mg/mL, 0.5 mL is 100 mg and 1 mL is 200 mg. At 100 mg/mL, the same doses take twice the volume. If a single dose goes well past 1 mL, it usually belongs in a larger muscle. Splitting the dose or injecting more often (see TRT microdosing) are options to discuss with your prescriber. The TRT dose calculator converts your prescribed milligrams into millilitres and syringe units, so you know the actual volume before you choose a site.

Technique details that matter for this site

1. Angle: 90 degrees to the skin, straight into the muscle⁵.
2. Skin: spread it taut rather than pinching it up. Pinching is for SubQ injections and makes an IM needle shallower⁵.
3. Speed: inject slowly. Nursing texts use about 10 seconds per mL unless told otherwise³, and thick oil may take longer.
4. Aspiration: the CDC does not recommend aspirating before vaccines⁵, while nursing texts still describe it for cases where it is required³. Testosterone is not a vaccine, so follow what your prescriber or nurse taught you.

Is there a SubQ option in the arm?

Not in the testosterone studies. The subcutaneous testosterone research describes the abdomen, about 3–5 cm to the side of the navel, and the thigh as the injection sites⁹. If your prescriber has moved you to SubQ, the deltoid is no longer your site; see SubQ vs IM testosterone for how the two routes compare and the thigh guide for the SubQ thigh technique.

Soreness, PIP and warning signs

A sore, stiff shoulder for a day or two is common after an oil injection into a small muscle. Few studies look at testosterone specifically, but a meta-analysis of IM injection technique across many drugs found that firm manual pressure over the site and rhythmic tapping (the Helfer technique) reduced injection pain. The same analysis found insufficient evidence that the Z-track technique reduced pain⁷. Keeping the arm relaxed and injecting slowly also help. The label also notes that warming and shaking the vial redissolves crystals that form in storage⁸.

Some things are not normal soreness. In the nerve palsy case series, most patients had severe pain along the nerve, with weakness or numbness, immediately after the injection⁶. If the needle produces a sharp, electric pain that shoots down the arm, stop, withdraw it and contact your clinician. The same applies to numbness or weakness in the hand or arm afterwards, or shoulder pain and restricted movement that keeps getting worse over the following days instead of better.

Common deltoid mistakes

1. Injecting too high, within a couple of fingerbreadths of the shoulder, which is the zone closest to the bursa and the top of the nerve band.
2. Drifting toward the back of the arm because the rear delt looks bigger.
3. Using a rolled-up sleeve as a guide, which moves the site upward.
4. Using a needle too short for the fat over the muscle, so the oil lands under the skin instead of in the muscle.
5. Putting a large volume into a small muscle instead of using a bigger site.
6. Using the same arm, and the same spot, every week.

Working the deltoid into a rotation

Alternate arms at the very least, and move the exact entry point about an inch within the muscle each time. Many people rotate the deltoids with the outer thighs, both of which are easy to reach alone, and add the ventrogluteal hip when someone can help, or for larger volumes. The injection sites hub has a comparison table of all three sites and a rotation plan. Peptide Me logs each shot to a point on a body map and suggests the site you have used least recently, so you do not have to remember which shoulder was last.

Disclaimer: this guide describes general anatomy and published injection guidance and is not medical advice. Testosterone is a prescription, controlled medication. Your prescriber or nurse should confirm which sites are right for you and show you how to landmark them in person before you inject a new site on your own.

FAQ

References

Primary sources — PubMed / NEJM / The Lancet.

1. Charmode S, Sharma S, Kushwaha SS, et al. Deltoid Intramuscular Injections: A Systematic Review of Underlying Neurovascular Structures to the Muscle and Proposing a Relatively Safer Site. Cureus. 2022;14(4):e24172. https://pmc.ncbi.nlm.nih.gov/articles/PMC9110073/
2. Nakajima Y, Mukai K, Takaoka K, et al. Establishing a new appropriate intramuscular injection site in the deltoid muscle. Hum Vaccin Immunother. 2017;13(9):2123-2129. https://pmc.ncbi.nlm.nih.gov/articles/PMC5612213/
3. OpenStax. Clinical Nursing Skills (senior contributor Christy Bowen). Section 12.6: Administering Intramuscular Injections. OpenStax; 2024. https://openstax.org/books/clinical-nursing-skills/pages/12-6-administering-intramuscular-injections
4. Ernstmeyer K, Christman E, eds. Nursing Skills (Open RN), Chippewa Valley Technical College. Chapter 18.6: Administering Intramuscular Medications. https://med.libretexts.org/Bookshelves/Nursing/Nursing_Skills_(OpenRN)/18:_Administration_of_Parenteral_Medications/18.06:_Administering_Intramuscular_Medications
5. Centers for Disease Control and Prevention. Chapter 6: Vaccine Administration. In: Epidemiology and Prevention of Vaccine-Preventable Diseases (Pink Book). https://www.cdc.gov/pinkbook/hcp/table-of-contents/chapter-6-vaccine-administration.html
6. Kakati A, Bhat D, Devi BI, Shukla D. Injection nerve palsy. J Neurosci Rural Pract. 2013;4(1):13-18. https://pmc.ncbi.nlm.nih.gov/articles/PMC3579035/
7. Ayinde O, Hayward RS, Ross JDC. The effect of intramuscular injection technique on injection associated pain; a systematic review and meta-analysis. PLoS One. 2021;16(5):e0250883. https://pmc.ncbi.nlm.nih.gov/articles/PMC8092782/
8. DEPO-TESTOSTERONE (testosterone cypionate) injection, prescribing information. Pharmacia & Upjohn Company LLC. Revised June 2025. DailyMed. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=735b5bf3-4464-412c-a072-1bd02f2e5380
9. Figueiredo MG, Gagliano-Jucá T, Basaria S. Testosterone Therapy With Subcutaneous Injections: A Safe, Practical, and Reasonable Option. J Clin Endocrinol Metab. 2022;107(3):614-626. https://academic.oup.com/jcem/article/107/3/614/6410585