The buttocks house some of the largest muscles in the body, and large muscles with a rich blood supply are exactly what you want when a medication needs to be absorbed steadily into the bloodstream. That combination of sheer muscle mass and vascularity is the core reason clinicians have been injecting drugs into the gluteal region for over a century. But the story is more complicated than “big muscle, good target,” because the buttocks also carry a thick layer of fat, a major nerve trunk, and enough anatomical variation from person to person that intended muscle injections frequently miss the muscle entirely.
Why Muscle Tissue Is the Target
When a drug is injected into muscle, it enters a tissue that is dense with blood vessels. That vascular network picks up the medication and carries it into the general circulation at a predictable rate. This is especially important for medications that need to be released gradually, like long-acting antipsychotics or hormonal preparations, which are formulated as thick, oily depots designed to sit inside muscle fibers and slowly dissolve over weeks or even months.1PubMed Central. A narrative review of the success of intramuscular gluteal injections and its impact in psychiatry If that depot ends up in the fat layer instead, the drug absorbs unpredictably because subcutaneous fat has far fewer blood vessels. You might get too little medication reaching the bloodstream, or it might trickle in erratically, undermining the whole point of the injection.
The gluteal muscles are attractive for these depot injections precisely because they can accommodate a relatively large volume of fluid. You can inject two to five milliliters into the gluteal region, whereas a smaller muscle like the deltoid in your upper arm typically handles only about one to two milliliters comfortably. For drugs that come in thick, viscous formulations, that extra capacity matters.
Two Spots on the Buttock, and They Are Not Equal
When people picture a “shot in the butt,” most imagine the upper outer quadrant of the buttock, the area healthcare workers call the dorsogluteal site. For decades this was the default location, and it is still widely used in many countries. But there is a second site on the side of the hip, the ventrogluteal area, that sits over the gluteus medius and minimus muscles. Research increasingly favors this spot, and understanding why requires knowing what lies beneath each one.
The dorsogluteal site sits over the gluteus maximus, which is padded by a variable and often thick layer of subcutaneous fat. One ultrasound study in older adults found the muscle layer at the dorsogluteal site averaged only about 26 millimeters thick, while the ventrogluteal site averaged around 40 millimeters of muscle.2PubMed. Examining the Safety of Dorsogluteal and Ventrogluteal Sites for Intramuscular Injection in Older Adults That thicker muscle at the ventrogluteal site gives the needle more room to land where it should. The dorsogluteal site also has thinner total tissue overall and carries a higher risk of the needle contacting bone or depositing medication into fat instead of muscle. Perhaps most critically, the dorsogluteal region is closer to the sciatic nerve, which we will get to shortly.
Despite the evidence in its favor, the ventrogluteal site remains underused. Surveys of nurses and nursing students suggest that many still default to the dorsogluteal site because that is what they were taught, and the ventrogluteal landmark can be harder to locate by feel alone.3PubMed Central. Are nursing students safe when choosing gluteal intramuscular injection locations? Old habits in clinical practice die hard.
The Surprisingly Low Success Rate
Here is something that catches most people off guard: studies using CT scans to check whether gluteal intramuscular injections actually reached the muscle have found success rates between roughly 32 and 52 percent. In one study of 50 patients, only about a third of intended intramuscular injections were confirmed to have reached muscle tissue. When the results were broken down by sex, the gap was stark: around 56 percent of men received true intramuscular delivery, compared to just 8 percent of women.1PubMed Central. A narrative review of the success of intramuscular gluteal injections and its impact in psychiatry In another study of 115 patients, the rate was only about half.
The reason for these failures is straightforward: the fat layer over the gluteal muscles is thicker than clinicians often expect, and standard needles are not always long enough to get through it. One CT-based study found that a standard 37-millimeter needle would fail to reach muscle fibers in roughly 55 percent of women and 14 percent of men.4Canadian Association of Radiologists Journal. Computed Tomographic Measurement of Gluteal Subcutaneous Fat Thickness in Reference to Failure of Gluteal Intramuscular Injections That is not a rounding error. It means that for a substantial share of patients, what was charted as an intramuscular injection was actually a subcutaneous one.
Body Size, Fat Distribution, and Needle Length
Body mass index turns out to be a strong predictor of whether a gluteal injection will succeed. Research measuring the subcutaneous fat thickness at the ventrogluteal site found a tight correlation with BMI, with correlation coefficients above 0.8 for both men and women. The practical cutoff was striking: for women with a BMI above 30 and men with a BMI above 35, a standard 1.5-inch (3.75 centimeter) needle had an expected failure rate of about 71 percent in women and 60 percent in men.5American Journal of Therapeutics. Body Mass Index: A Reliable Predictor of Subcutaneous Fat Thickness and Needle Length for Ventral Gluteal Intramuscular Injections
Women tend to carry more subcutaneous fat in the gluteal region than men of the same BMI, which is why the sex difference in injection success is so pronounced. This is not about overall body size so much as about where fat gets deposited, a difference driven largely by hormones. For patients with higher BMI or gynoid fat distribution patterns, some clinicians now use ultrasound to measure the fat layer before selecting a needle length.6PubMed Central. Ultrasound for the determination of appropriate needle length for intragluteal injection of long-acting rilpivirine-cabotegravir That kind of individualized approach is especially relevant for long-acting injectable medications where accurate muscle delivery is critical for the drug to work as intended over weeks.
Why Vaccines Moved Away from the Buttocks
If you have gotten a flu shot or a COVID vaccine in recent years, it almost certainly went into your upper arm, not your buttock. This shift happened for good reasons. Vaccines deposited into the thick fat layer of the buttocks can produce weaker immune responses because the drug does not reach the well-vascularized muscle tissue efficiently. The BMJ noted that injecting vaccines into the buttocks risks lower seroconversion rates, meaning fewer people develop adequate antibody levels, specifically because of inadvertent deposition into subcutaneous fat.7BMJ. The importance of injecting vaccines into muscle: Different patients need different needle sizes
The deltoid muscle in the upper arm is now the standard site for most adult vaccinations. It is easy to access without requiring the patient to undress, the fat layer is thinner, and the muscle is close to the surface. For infants and toddlers who have not developed enough deltoid muscle mass, the anterolateral thigh (the outer front of the upper leg) is preferred. The buttock still has a role for certain medications, but for vaccines, the evidence pointed clearly toward better sites, and practice changed accordingly.
The Sciatic Nerve Problem
The sciatic nerve is the largest nerve in the body, running from the lower spine down through the buttock and into the leg. Its path takes it through the gluteal region, and when an intramuscular injection is placed too far medially or inferiorly on the buttock, the needle can hit or inject medication directly into or around this nerve. The consequences range from temporary pain and numbness to permanent foot drop, chronic pain, and paralysis of muscles below the knee.8PubMed Central. Iatrogenic Injury to the Sciatic Nerve due to Intramuscular Injection: A Case Report
This is not a theoretical concern. Sciatic nerve injury from misplaced gluteal injections has been documented across healthcare systems worldwide, in wealthy countries and poorer ones alike, and it remains a persistent problem despite being entirely preventable with correct technique.9PubMed. Sciatic nerve injury from intramuscular injection: a persistent and global problem The ventrogluteal site carries a lower risk of sciatic injury because the nerve does not pass through that area. This is another reason clinical guidelines increasingly push practitioners toward the ventrogluteal site or away from the buttock altogether for routine injections.
When Fat Becomes Dangerous
Beyond reduced drug absorption, injecting into fat instead of muscle can sometimes cause serious local tissue damage. Nicolau syndrome is a rare but severe complication in which the skin and underlying fat tissue die from loss of blood supply after an injection. A case series of 17 women who developed gluteal necrosis traced the problem to misplaced injections that deposited non-steroidal anti-inflammatory drugs into fat instead of muscle, compounded by needles that were too short for the thickness of the fat layer.10PubMed Central. Nicolau syndrome after intramuscular injection of non-steroidal anti-inflammatory drugs (NSAID) The result was ischemic necrosis, tissue death from blocked blood flow, leaving wounds that required extensive treatment. All 17 patients were women, consistent with the broader pattern of thicker gluteal fat layers in female patients raising the risk of subcutaneous misdelivery.
This is an extreme outcome and quite rare, but it underscores a broader point: the gluteal injection site is less forgiving of error than it appears. The fat pad is invisible under the skin, its thickness varies enormously between individuals, and the consequences of getting it wrong range from a medication that simply does not work well to tissue destruction.
Injection Techniques That Help
Clinicians have developed specific techniques to improve the reliability and comfort of gluteal injections. The Z-track method involves pulling the skin and subcutaneous tissue to one side before inserting the needle, then releasing it after the injection is complete. This creates a zigzag path through the tissue layers that helps prevent medication from leaking back out through the needle track. A randomized trial found that the Z-track technique significantly reduced drug leakage and inflammation at the injection site compared to standard technique.11International Journal of Research in Medical Sciences. Z-track technique reduces pain at the injection site, drug leakage, post-injection gluteal inflammation in Pritchard regimen for severe pre-eclamptic patients: findings from a randomized controlled trial Another study confirmed that drug leakage was reduced with the Z-track method when administering intramuscular diclofenac, though pain reduction was not always significant.12PubMed. The Effect of the Z-Track Technique on Pain and Drug Leakage in Intramuscular Injections
Another longstanding practice, aspiration, which means pulling back the syringe plunger before injecting to check for blood, has largely fallen out of favor. A systematic review concluded there is no clinical reason to aspirate when using the deltoid, ventrogluteal, or vastus lateralis sites for vaccination.13PubMed. Aspirating during the intramuscular injection procedure: a systematic literature review A more recent meta-analysis confirmed that aspiration increases pain and procedure time without reducing complication risk, supporting international guidelines that now discourage it as routine practice.14PubMed. Aspiration in intramuscular injection: a meta-analysis of pain, duration, and clinical outcomes
What About Children
For infants and young children, the preferred intramuscular injection site is the anterolateral thigh, the fleshy outer portion of the upper leg. Young children have relatively undeveloped gluteal muscles, making the buttock a less reliable target. However, the gluteal sites are not completely off the table for pediatric patients. Research on gluteal subcutaneous and muscle thickness in infants and children found that both the ventrogluteal and dorsogluteal sites can serve as alternatives when the anterolateral thigh is problematic.15PubMed Central. Analysis of gluteal subcutaneous and muscle thickness in infants and children for application to intramuscular injection, autologous fat grafting, and gluteal artery perforator flaps
Pain is a real concern in pediatric injections, and site selection affects it. A randomized controlled trial comparing intramuscular injections in the ventrogluteal region versus the outer thigh in children found that children who received injections in the ventrogluteal area reported significantly less pain immediately after the procedure.16PubMed Central. Comparison of pain levels developed during intramuscular injections to laterofemoral and ventrogluteal regions in children: a randomized controlled study That finding is worth noting because the outer thigh is the current default for most pediatric vaccinations, and children are notoriously vocal about injection pain. Whether the ventrogluteal site gains traction in pediatric practice will depend on clinician comfort with locating it on smaller bodies.
When the Buttock Is Still the Right Choice
Despite the shift away from the buttock for vaccines, gluteal injections are not disappearing. They remain the standard route for a specific category of medications: long-acting injectables. Monthly or every-other-month injections for conditions like schizophrenia, bipolar disorder, and HIV rely on the gluteal muscles to serve as a slow-release depot. These oily, viscous formulations need a large muscle mass to hold the volume and release the drug steadily. Moving them to the deltoid is sometimes possible but is not always practical for the volumes and formulations involved.
For these patients, getting the injection right is especially high-stakes. A long-acting antipsychotic that ends up in subcutaneous fat instead of muscle may produce subtherapeutic blood levels for weeks, potentially leading to a relapse of psychotic symptoms. The same logic applies to long-acting antiretroviral injections for HIV, where maintaining adequate drug levels is essential to keep the virus suppressed. This is why the research into ultrasound-guided needle length selection, BMI-based predictions, and ventrogluteal site preference is not academic hairsplitting. For patients on these medications, accurate muscle delivery is the difference between a treatment that works and one that fails silently.
The Landmark Problem
One reason gluteal injection technique varies so much in practice is that clinicians rely on surface landmarks to find the right spot. For the dorsogluteal site, the traditional method involves mentally dividing the buttock into quadrants and aiming for the upper outer one. For the ventrogluteal site, practitioners palpate the bony landmarks of the hip (the anterior superior iliac spine and the iliac crest) and place the injection within a triangle defined by these structures and their own hand position. Studies measuring tissue depth at these landmark-defined sites have found meaningful differences. At ventrogluteal landmarks, the distance from skin surface to bone was shorter (about 53 to 55 millimeters) compared to the dorsogluteal site (about 62 millimeters), reflecting the thinner fat layer and more accessible muscle at the ventrogluteal location.17Open Journal of Psychiatry. Comparison of Gluteal Muscle Intramuscular Injection Sites of Japanese Healthy Subjects: Considerations for Optimal Insertion of Injection Needle Length
But landmarks assume a degree of anatomical consistency that does not always hold. People vary in bone structure, fat distribution, and muscle development. An experienced nurse can usually find the right spot quickly, but under time pressure or with an unfamiliar patient body type, errors happen. This is part of why some researchers advocate for routine ultrasound guidance for gluteal injections in patients who are obese or who have unusual fat distribution patterns, turning what has traditionally been a by-feel procedure into something more precise.