Why Do They Give Shots in the Butt?

The gluteal muscles are among the largest in the body, and that size is the main reason they became a go-to injection site. Medications that need to be absorbed slowly from a deep “depot” in muscle tissue, or that come in volumes too large for a smaller muscle to handle, end up in the buttock. But the story is more interesting than anatomy alone would suggest, because over the past few decades medicine has actually moved many injections away from the butt and into the arm or thigh, driven by research showing that gluteal shots sometimes fail to deliver medication properly and carry risks other sites avoid.

Why Muscle Size Determines Where You Get a Shot

When a drug is injected into muscle, it sits in a reservoir within the tissue and gradually enters the bloodstream. The gluteal muscles can accommodate a larger volume of fluid than the deltoid muscle in your upper arm. A standard deltoid injection tops out at roughly 1 to 2 milliliters of fluid. The gluteal site can handle up to about 4 or even 5 milliliters in a single injection, depending on the person’s body size. For medications that come in bigger volumes, like certain long-acting antipsychotics, hormonal injections, or antibiotics, the arm simply cannot hold the dose.

Blood flow matters too, and here the relationship gets counterintuitive. The deltoid muscle actually has faster blood flow than the gluteus. One study measured resting blood flow in different muscles and found the deltoid averaged about 11.6 milliliters per 100 grams of tissue per minute, roughly 19% more than the gluteus at 9.6 milliliters.¹ Faster blood flow means faster drug absorption, which is exactly what you want for a vaccine but not necessarily for a medication designed to release slowly over days or weeks. For those slow-release drugs, the slightly lower blood flow in the gluteal muscles is actually an advantage. The medication lingers in the muscle longer, extending its effect.

The Hepatitis B Wake-Up Call

For most of the 20th century, the buttock was the default site for just about every injection, vaccines included. That changed largely because of hepatitis B vaccine. When hospitals started vaccinating their staff in the early 1980s, some noticed unexpectedly low success rates. One study found that only 67% of hospital employees who received gluteal injections developed adequate antibodies, compared to 90% of employees at another facility who received the same vaccine lot in the deltoid.² The seroconversion rate dropped even further in people over 40, falling to just 54%.

A larger follow-up study confirmed these findings and dug deeper into why. Arm injections produced antibody response rates of 93%, while buttock injections lagged at 72%. Thicker skinfold measurements, older age, and smoking were all independently linked to lower antibody responses in the buttock group but had no such effect in the arm group.³ The problem was straightforward: in many people, especially older adults and those with more body fat, the needle was depositing vaccine into the subcutaneous fat layer rather than into the muscle. Fat tissue has poor blood supply and few immune cells compared to muscle, so the vaccine essentially sat there doing little. By April 1985, official guidelines changed to specify the arm as the recommended site for hepatitis B vaccination, and most other vaccines followed.

The Sciatic Nerve Problem

The other major reason gluteal injections fell out of favor for routine use is the sciatic nerve, the longest and thickest nerve in your body. It runs through the buttock on its way from the lower spine to the leg, and a misplaced injection can hit it. The sciatic nerve is the nerve most commonly injured by intramuscular injections, and the consequences can range from temporary pain and numbness to permanent foot drop and lasting disability.⁴ This remains a worldwide problem, occurring in wealthy and lower-income countries alike.

The traditional “upper outer quadrant” method that generations of nurses learned is the dorsogluteal site, located on the back of the buttock. This is the site closest to the sciatic nerve. A safer alternative within the buttock itself is the ventrogluteal site, which sits on the side of the hip rather than the rear. There are no major nerves or blood vessels at the ventrogluteal site, and the muscle there is reliably thick. Yet shifting clinical practice has been slow. One study of nurses found that 34% still frequently used the riskier dorsogluteal site.⁵ Habit and familiarity appear to outweigh the evidence for many practitioners.

Two Different “Butt” Sites That Are Not the Same

When people say “a shot in the butt,” they might mean one of two anatomically distinct locations. The dorsogluteal site is the classic one: the upper outer quadrant of the buttock, right where you would picture a cartoon syringe landing. The ventrogluteal site is more to the side and front, over the hip bone. These two spots behave differently in every way that matters clinically.

A meta-analysis comparing the two found that the ventrogluteal site produced significantly less pain, less bleeding, and smaller hematomas at 48 and 72 hours post-injection.⁶ Imaging studies help explain why: subcutaneous fat over the dorsogluteal site tends to be thick and variable, while the ventrogluteal site has thinner, more predictable fat coverage with a generous layer of muscle underneath. One systematic review and meta-analysis of tissue measurements found the ventrogluteal muscle layer averaged about 36 to 38 millimeters thick, with subcutaneous tissue around 20 millimeters, while the dorsogluteal muscle averaged about 43 millimeters but with more variable and often thicker fat over it.⁷

Even identifying the ventrogluteal site on a patient’s body has its own learning curve. The older “V method” uses anatomical landmarks felt by hand, while a newer geometric method relies on measurements between bony landmarks. The geometric approach produced thicker muscle readings and a higher rate of successful intramuscular delivery: about 75% versus 57% with the V method.⁸ The takeaway is that even within the gluteal region, not all injection sites are created equal, and technique matters as much as location.

How Body Size Complicates Gluteal Injections

One of the biggest practical challenges with buttock injections is ensuring the needle actually reaches the muscle. The gluteal region carries a substantial layer of subcutaneous fat, especially in women. If the needle is too short, the medication ends up in fat rather than muscle, which changes how the drug is absorbed and can reduce its effectiveness, as the hepatitis B vaccine debacle demonstrated.

A study measuring tissue depth in 199 subjects found a strong correlation between BMI and the thickness of subcutaneous fat at the ventrogluteal site. For women with a BMI above 30, a standard 1.5-inch needle failed to reach muscle 71% of the time. For men, the failure rate was 60% at BMIs above 35.⁹ Women also had significantly thicker subcutaneous tissue than men at comparable BMIs, a difference confirmed by the meta-analysis of tissue thickness studies.¹⁰

This creates a real clinical dilemma. Many of the medications that require gluteal injection, like long-acting antipsychotics or hormonal treatments, are used in populations where higher BMIs are common. Longer needles can solve the problem, but they increase pain and anxiety. Some clinicians have started noting that guidelines for needle length need to be revisited to account for body composition more carefully, with the ventrogluteal site recommended as the most appropriate for deep muscle injections yet underused compared to the dorsogluteal site.¹¹

Why Babies Get Shots in the Thigh

If you have ever taken an infant in for vaccinations, you know the shots go in the front of the thigh, not the arm or the buttock. The reason is developmental. Infants and toddlers have not yet built up enough gluteal muscle mass to make the buttock a reliable injection site. Their gluteal muscles are small, soft, and poorly defined compared to an adult’s. The vastus lateralis muscle on the front outer thigh, by contrast, is the largest muscle mass available in a baby and provides a safe, predictable target with no nearby major nerves.¹²

As children grow, they can transition to deltoid injections for vaccines. But for children who need intramuscular medications in larger volumes, the ventrogluteal site becomes an option once they are old enough to have developed sufficient muscle there. A randomized study in children found that ventrogluteal injections were significantly less painful than vastus lateralis (thigh) injections, with lower scores on both visual and facial pain scales immediately after the procedure.¹³ That finding has led some researchers to recommend the ventrogluteal site more broadly in pediatric practice, though the thigh remains the standard for the youngest patients.

Which Medications Still Go in the Butt

With vaccines largely migrated to the arm, what still gets injected in the gluteal region? The list is more specific than you might expect. Long-acting injectable antipsychotics like paliperidone palmitate are among the most common. These come as thick, oily suspensions designed to release slowly over weeks, and they require a large muscle with good capacity. A study comparing gluteal and deltoid injections of paliperidone palmitate found that local tolerability was slightly better with gluteal injections.¹⁴ However, research has also shown that switching to deltoid injections for some long-acting antiretroviral drugs maintained adequate blood levels, expanding site options for patients who prefer arm injections.¹⁵

Other medications commonly given in the gluteal muscles include long-acting hormonal contraceptives, testosterone injections for hormone therapy, certain antibiotics like long-acting penicillin, and some corticosteroid preparations. The common thread is volume and viscosity: these drugs are either too much liquid or too thick to work well in the deltoid. Some hormonal injections are designed for self-administration, and the thigh often wins out over the buttock in that context simply because you can reach it and see what you are doing.

Self-Injection and Site Accessibility

Whether you can easily reach and see an injection site matters a lot when you are the one holding the syringe. The buttock is essentially impossible to inject on yourself with any precision, which is why self-administered medications tend to go in the thigh or abdomen instead. But even the thigh is not always ideal. A study of patients using a sumatriptan self-injector for migraines found that the lateral thigh had very little subcutaneous tissue in men, with median depths as low as 4 millimeters, which meant the needle frequently penetrated into muscle rather than staying subcutaneous as intended. The gluteal area, with median subcutaneous depths of 45 millimeters, was actually more suitable for that particular device, though harder to reach.¹⁶

This highlights an underappreciated tension in injection site selection. The “best” site depends on who is giving the injection, what depth the medication needs to reach, how much volume is involved, and whether the patient can physically access the site. Clinic-administered injections can use the buttock because a trained professional handles the landmarking and insertion. Self-administered injections almost always go elsewhere.

The Awkwardness Factor

There is an element of gluteal injections that clinical textbooks rarely discuss but patients feel acutely: the indignity of partially undressing and presenting your backside to a healthcare worker. A mixed-methods study of participants in a long-acting injectable PrEP trial found that the intimacy and awkwardness of gluteal injections meaningfully affected patients’ experiences. It was one of five major themes that emerged from interviews, alongside pain variability and anxiety.¹⁷ For medications that require repeated visits over months or years, that discomfort adds up and can affect whether people stick with treatment.

Patient anxiety about injections in general was also notable in the study, but it tended to decrease over time with repeated exposure. The awkwardness of the gluteal site, by contrast, did not habituate as easily. This has practical implications for adherence: if an arm injection can deliver the same drug levels, offering that option may keep more patients on their medication. The growing evidence that some long-acting drugs maintain effectiveness when given in the deltoid is partly driven by exactly this concern.

What the Beef Industry Learned About Injection Sites

Humans are not the only ones who get shots in the butt, and the livestock industry’s experience offers an unexpectedly useful parallel. Cattle routinely receive intramuscular injections of vaccines, antibiotics, and vitamins, and for decades those went into the hindquarter muscles because they are large and easy to access. The problem became visible at the meat counter. A study tracking injection-site lesions in cattle found that intramuscular injections in the hindquarter muscles caused visible tissue damage in a startling proportion of carcasses: a 2-milliliter clostridial vaccine caused lesions in about 73% of cases, a 5-milliliter dose in 93%, and a long-acting antibiotic in 51%, with lesions still detectable 225 to 376 days after injection.¹⁸

The beef industry responded by moving injections to the neck, where any tissue damage would not affect valuable cuts. This shift was driven entirely by economics rather than animal welfare, but the lesson for human medicine is the same: injecting into the buttock causes local tissue reactions, and if there is a workable alternative site, there are good reasons to use it. The difference is that in humans the “cost” is not a ruined steak but pain, bruising, hematoma, and occasionally a compromised medication delivery.

The Deltoid’s Limits

Given all the problems with gluteal injections, you might wonder why the arm does not simply replace the butt for everything. The deltoid has its own vulnerabilities. Standard 25-millimeter needles may miss the deltoid muscle entirely in very small or very large patients, depositing medication either too shallowly or into the shoulder bursa, a fluid-filled sac near the joint. Bursal injections can cause persistent shoulder pain, limited range of motion, and in rare cases lasting disability.¹¹ This condition, sometimes called SIRVA (shoulder injury related to vaccine administration), has become more widely recognized as millions of vaccine doses have gone into deltoids during mass immunization campaigns.

The deltoid is also simply a smaller muscle. For a thick, oily, slow-release medication in a 3- or 4-milliliter dose, it is not built to handle the volume. Injecting too much fluid into a small muscle can cause significant pain and poor absorption. So while the arm works beautifully for vaccines and small-volume medications, the buttock remains the better tool for a specific and important subset of drugs. The real shift in practice has not been away from the butt entirely but toward using the right gluteal site when a gluteal injection is needed, and choosing the arm or thigh when the medication allows it.