Penicillin shots are most commonly given as intramuscular injections into the upper outer quadrant of the buttock (the gluteal muscles), the front of the thigh (the vastus lateralis), or the upper arm (the deltoid muscle). The specific site depends on the patient’s age, body size, and which penicillin formulation is being used. For the thick, slow-release suspension known as benzathine penicillin G, which treats syphilis and prevents rheumatic fever recurrence, the gluteal region remains the most frequently chosen site in adults because the large muscle mass can accommodate the viscous medication and the slow, steady absorption it requires.
Why Penicillin Needs to Go Into Muscle
Not every penicillin formulation comes as a shot. Oral penicillin tablets work fine for many everyday infections. But certain formulations, especially benzathine penicillin G, are designed as depot injections: the drug forms a reservoir inside the muscle that slowly releases penicillin into the bloodstream over days or weeks. That slow release is the entire point. If benzathine penicillin is accidentally given intravenously instead of intramuscularly, the depot mechanism is completely lost, and sustained therapeutic drug levels are never achieved.1Clinical Infectious Diseases. Intramuscular versus Intravenous Administration of Benzathine Penicillin This is why route matters so much with penicillin injections: putting the drug in the right tissue is not just about convenience or comfort, it determines whether the treatment actually works.
Because the medication must sit inside muscle tissue and release gradually, clinicians choose injection sites with enough muscle bulk to hold the volume of drug and enough blood flow to carry it steadily into the circulation. Thin, superficial muscles or fatty tissue above the muscle would trap the drug in the wrong layer and compromise absorption.
The Three Main Injection Sites
In practice, three muscle groups handle the vast majority of penicillin injections. Each has trade-offs in safety, comfort, and suitability for different patients.
- Gluteal muscles (buttock): The most common site for adult benzathine penicillin injections. The gluteal region offers a large, deep muscle mass that handles the thick suspension well. Historically, clinicians aimed for the upper outer quadrant of the dorsogluteal area (the back of the hip). More recently, best practice has shifted toward the ventrogluteal site (the side of the hip), which has a thicker muscle layer, less overlying fat, and is farther from major nerves and blood vessels.
- Vastus lateralis (outer thigh): The preferred site in infants and young children, because their gluteal muscles are not yet well-developed enough for safe deep injections. The outer thigh provides a reliable, accessible muscle even in very small patients. It is also used in adults when the gluteal site is unavailable.
- Deltoid (upper arm): Suitable for smaller-volume injections in adults. The deltoid is often chosen for vaccines and for penicillin formulations that do not require a large injection volume. For benzathine penicillin G, which typically involves a relatively large, viscous dose, the deltoid is less ideal because it is a smaller muscle and the injection can be more painful.
The choice among these three sites is not arbitrary. Clinicians match the site to the patient’s muscle development, the volume and viscosity of the drug, and the risk profile of the surrounding anatomy.
Why the Preferred Buttock Site Has Changed
For decades, the standard teaching was to inject into the dorsogluteal area, the fleshy part at the back of the buttock. Nurses and doctors learned to mentally divide the buttock into four quadrants and aim for the upper outer one, which was considered safest. The problem is that the sciatic nerve runs through the lower half of the buttock, and even the upper outer quadrant carries some risk of nerve damage if the injection lands too medially or too deep.
Animal research has confirmed just how damaging a misplaced injection near the sciatic nerve can be. In a controlled experiment with rats, injections of penicillin G near the sciatic nerve caused some of the most severe nerve injuries among several drugs tested, and the damage was statistically significant compared to control groups.2PubMed Central. All aspects of sciatic nerve injection injury: an experiment with 78 rats While rat anatomy is not identical to human anatomy, the findings reinforce why clinicians have moved toward the ventrogluteal site. The ventrogluteal area sits on the side of the hip rather than the back, well away from the sciatic nerve’s path, and the muscle there is consistently thick even in patients who carry less body fat on their buttocks.
This shift has been gradual. Many older practitioners were trained on the dorsogluteal approach, and in some clinical settings it remains common out of habit. But nursing and medical guidelines increasingly recommend the ventrogluteal site as the default for adult intramuscular injections, including penicillin.
A Rare but Serious Vascular Complication
Beyond nerve injury, penicillin injections carry a small risk of a condition called Nicolau syndrome, sometimes described as livedo-like dermatitis. This occurs when the injection inadvertently damages local blood vessels, leading to skin discoloration, pain, and in severe cases tissue death around the injection site. The exact mechanism is not fully understood, but theories include spasm of nearby blood vessels triggered by the injection, small emboli blocking blood flow, direct mechanical injury to the vessel wall, or an inflammatory response to the drug.3PubMed Central. Nicolau Syndrome due to Penicillin Injection: A Report of 3 Cases without Long-Term Complication
Nicolau syndrome is genuinely rare, and in a series of three reported cases following penicillin injection, none resulted in long-term complications.3PubMed Central. Nicolau Syndrome due to Penicillin Injection: A Report of 3 Cases without Long-Term Complication Still, it is one reason clinicians aspirate the syringe before injecting (pulling back slightly on the plunger to check for blood return) and choose sites with well-known vascular anatomy. Proper technique at the right site keeps this risk vanishingly small.
Reducing the Pain of Penicillin Shots
If you have ever received a benzathine penicillin injection, you probably remember the pain. The drug is suspended in a thick, viscous solution that does not flow easily through the needle, and the injection itself takes longer than a typical shot. Pain during and after the injection is one of the most common reasons people dread follow-up doses or skip them altogether, which is a real problem when a course of monthly injections is needed to prevent rheumatic fever relapse.
One well-studied approach is mixing the benzathine penicillin with a small amount of lidocaine, a local anesthetic, before injecting it. A systematic review pooling data from multiple trials found that lidocaine mixed with benzathine penicillin significantly reduced immediate post-injection pain by nearly four points on a ten-point scale, and the pain reduction persisted at five and twenty minutes after injection.4PubMed Central. Local anaesthetic to reduce injection pain in patients who are prescribed intramuscular benzathine penicillin G: a systematic review and meta-analysis No severe adverse reactions were reported with the lidocaine approach. In children with rheumatic heart disease specifically, a randomized crossover trial confirmed that lidocaine as the diluent produced a significant drop in pain at five minutes, though by sixty minutes the pain scores between lidocaine and standard diluent had evened out.5PubMed Central. Utility of lidocaine in reducing pain of intramuscular benzathine penicillin G in children with rheumatic heart disease: A randomized, double-blind crossover study
Technique also matters. One method used in clinical trials is the Z-track technique: before inserting the needle, the clinician pulls the skin and underlying tissue to one side by a couple of centimeters, inserts the needle, injects the medication slowly, then releases the skin after withdrawing the needle. As the tissue slides back into its natural position, it seals the injection tract and prevents the medication from leaking back out along the needle path. The needle is inserted about two-thirds of its length, and the clinician aspirates before injecting to confirm the needle is not in a blood vessel.6PubMed Central. Evaluation of Combined Strategy to Reduce the Pain of Penicillin G Benzathine Injection in Primary Syphilis Slow injection speed and keeping the patient relaxed both help as well. None of these steps eliminates pain entirely, but combining lidocaine with good technique makes a meaningful difference.
When Body Size Affects the Injection Site
A standard intramuscular injection needle is about 25 millimeters (one inch) long. That length works well for average-sized adults, but it may not be long enough to reach actual muscle tissue in everyone. Research evaluating deltoid injection sites found that a standard one-inch needle may fail to reach the muscle in a substantial proportion of adults with obesity, effectively depositing the medication into subcutaneous fat rather than muscle.7Medical Research Archives. Intramuscular Injection Guideline Revisions are Needed Based on Body Mass Index, Needle Length, Sex, and Skin to Muscle Depth When the drug ends up in fat instead of muscle, absorption can be slower, less predictable, or incomplete, potentially undermining the treatment.
The problem is more pronounced in women than in men, because women tend to carry more subcutaneous fat over the deltoid area. A combined analysis from New Zealand found that the arm circumference and BMI thresholds at which a longer needle becomes necessary were lower for women than for men. For women, arm circumferences as modest as 30 centimeters could already mean a standard needle would not reliably reach the muscle, while for men the threshold was closer to 35 centimeters.8PubMed. When should a longer needle be used for intramuscular injection in obese patients? A combined analysis of New Zealand data This is one reason clinicians sometimes switch to the gluteal or thigh site in larger patients: the muscle is deeper there too, but the available muscle mass is greater and a longer needle (typically 38 mm or 1.5 inches) can be used more easily without the anatomical constraints of the deltoid.
If you are receiving a penicillin injection and you are in a larger body, it is worth making sure your clinician has considered needle length. A shot that does not reach the muscle is not just less effective; for a depot formulation like benzathine penicillin, it could mean the slow-release mechanism never works properly.
Where These Shots Happen Beyond the Clinic
Penicillin injections are traditionally given in clinical settings: doctor’s offices, outpatient clinics, emergency departments, and hospitals. For one-time treatments like a single benzathine penicillin injection for syphilis, a clinic visit is usually straightforward. But for people who need monthly injections over years to prevent rheumatic fever recurrence, getting to a clinic every four weeks is a substantial burden, especially for those in remote or underserved communities.
Stakeholder consultations have explored alternatives. In one workshop gathering perspectives from clinicians, patients, and health system leaders, the large majority agreed that healthcare workers should deliver benzathine penicillin injections, but half of the respondents also felt patients should be able to self-administer if feasible, and about 40 percent supported parents giving injections to children at home. More than three-quarters wanted flexibility in location, with home, school, workplace, or another patient-chosen setting offered as alternatives to traditional clinics.9PubMed Central. Stakeholders Want a Menu of Choices: Findings from a Consultation Workshop on Improving Access to Secondary Prophylaxis of Rheumatic Fever and Rheumatic Heart Disease These ideas remain aspirational in many resource-limited settings, but they reflect a genuine shift in thinking about how and where long-term injectable treatments can be delivered. Some programs in Australia and New Zealand have already moved toward community-based delivery, particularly for Aboriginal and Torres Strait Islander populations who bear a disproportionate burden of rheumatic heart disease.
What Happens When Penicillin Shots Are Not Available
Benzathine penicillin G has faced recurring global supply shortages, driven by supply-and-demand imbalances and a fragile manufacturing base. Few pharmaceutical companies produce the drug, and production disruptions can ripple worldwide.10PubMed Central. Securing the supply of benzathine penicillin: a global perspective on risks and mitigation strategies to prevent future shortages This is not a theoretical concern. In April 2023, a critical shortage in the United States led the CDC to recommend reserving remaining benzathine penicillin doses for pregnant individuals and newborns with syphilis, two populations for whom no good alternative exists.11Open Forum Infectious Diseases. P-724. Benzathine Penicillin G Shortage and Effects on Syphilis Patient Outcomes
For everyone else, clinics had to pivot. At the Tampa VA Medical Center, for example, oral doxycycline became the preferred syphilis treatment for fourteen months while the shortage persisted. Although the FDA temporarily allowed importation of a European benzathine penicillin product, federal healthcare facilities faced ongoing procurement restrictions and continued to ration supply.12Open Forum Infectious Diseases. P-738. Doxycycline for Syphilis Treatment at A VA Medical Center During a Benzathine Penicillin Shortage Doxycycline works for syphilis in many situations, but it requires two weeks of twice-daily pills rather than a single injection, and adherence to an oral regimen is never as reliable as a one-shot treatment administered by a clinician.
These shortages highlight a tension at the heart of penicillin injection therapy. The drug itself is old, cheap, and effective, but its supply chain is fragile precisely because it is not a high-profit product. When shortages strike, the populations who need benzathine penicillin most, including people with syphilis and those on long-term rheumatic fever prophylaxis, are the ones who lose access first.
Conditions That Call for Penicillin Injections
Benzathine penicillin G is the treatment of choice for syphilis at all stages, and it is the standard prophylaxis for preventing recurrent episodes of acute rheumatic fever and the progression of rheumatic heart disease. In syphilis treatment, a single intramuscular dose can cure primary and secondary infections; later stages require three weekly injections. For rheumatic fever prophylaxis, patients receive an injection every three to four weeks, sometimes for a decade or longer.
Procaine penicillin G, another injectable formulation, has been used historically for infections requiring daily intramuscular dosing over shorter courses, such as certain forms of pneumonia or soft-tissue infections. It releases penicillin more quickly than benzathine penicillin but still provides a depot effect lasting roughly a day. In modern practice, however, procaine penicillin has become less commonly used as oral and intravenous alternatives have expanded.
Adherence is a persistent challenge for any treatment requiring repeated injections. Studies of children on oral penicillin prophylaxis for rheumatic fever have consistently shown poor compliance with daily pill-taking, which is one of the reasons monthly injections remain the preferred approach despite the pain and inconvenience: a healthcare worker confirms the dose was given, removing the uncertainty of self-administered oral therapy. Patient preference research in Australia, where rheumatic heart disease disproportionately affects Aboriginal and Torres Strait Islander communities, has underscored the need to better understand what drives treatment completion and to design delivery models that respect patient choice.13BMJ Paediatrics Open. Patient preferences for prophylactic regimens requiring regular injections in children and adolescents: a systematic review and thematic analysis
Penicillin Allergy and Pre-Injection Testing
About one in ten people report a penicillin allergy, but the real rate of true allergy is far lower. Many reported allergies are based on childhood reactions that were never confirmed, side effects that were not actually allergic in nature, or family history rather than personal experience. Before administering a penicillin injection, especially benzathine penicillin for syphilis in someone with a reported allergy, clinicians may perform skin testing to determine whether the allergy is genuine.
Penicillin allergy skin testing is done on the forearm, not at the injection site itself. Tiny amounts of penicillin derivatives are placed just under the skin (intradermally) or on the skin surface (by prick test), and the site is observed for a wheal-and-flare reaction over about fifteen to twenty minutes. A negative skin test makes a true allergic reaction extremely unlikely, and the patient can proceed with the injection. This testing has been increasingly adopted in hospital inpatient settings to clear patients for penicillin-based treatments they would otherwise miss out on. Given that penicillin remains the only recommended treatment for syphilis in pregnancy, and that alternatives are inferior for neurosyphilis, confirming or ruling out allergy before resorting to a substitute drug can make a real difference in outcomes.
If you have a reported penicillin allergy and need a penicillin injection, ask about skin testing rather than simply accepting an alternative drug. The odds are good that you are not truly allergic, and confirming that opens the door to the most effective treatment.