What Happens If You Give an IM Injection Wrong?

A misplaced intramuscular (IM) injection can cause problems ranging from a sore arm that lingers for weeks to permanent nerve damage, tissue death, or a medication that simply doesn’t work because it never reached the muscle. The specific consequences depend on what went wrong: hitting the wrong structure, using the wrong needle length, injecting at the wrong depth, or choosing a risky site. Most IM injections go fine, but when they don’t, the complications can be surprisingly varied and sometimes severe.

Hitting a Nerve

The most feared complication of a badly placed IM injection is nerve injury, and the sciatic nerve is the most commonly damaged nerve from injections because of its large size and its proximity to the buttock, which remains a popular injection site worldwide.¹ The damage can happen in two ways: the needle physically pierces the nerve, or the injected substance leaks into the tissue surrounding the nerve sheath and causes chemical injury to the area.²

What does sciatic nerve injury from an injection feel like? It typically begins with sudden, severe pain shooting down the leg during or immediately after the injection. This can progress to weakness in the foot, numbness along the back of the leg, and chronic neuropathic pain that persists long after the injection site itself has healed. In one documented case, a 68-year-old man developed intractable nerve pain after a gluteal injection of a painkiller. The pain did not improve even as his foot weakness gradually recovered, and he ultimately required a surgical procedure called external neurolysis three months later, which provided only partial relief.¹

The prognosis depends on how badly the nerve is damaged. When imaging shows no neuroma formation or nerve discontinuity, the injury is typically classified in the less severe range and generally carries a more favorable outlook. But when the nerve shows focal neuroma or actual discontinuity on MR neurography, the injury is more severe and may require surgery.³ Recovery can take months to years, and some patients never fully regain function.

Shoulder Injury from Vaccines Injected Too High

If you’ve ever received a vaccine in your upper arm and had shoulder pain that lasted far longer than a day or two, you may have experienced something called shoulder injury related to vaccine administration, or SIRVA. This happens when an injection intended for the deltoid muscle lands too high, hitting structures adjacent to the muscle like the bursa, rotator cuff tendons, or the joint capsule itself. The result is a combination of mechanical and chemical trauma, worsened by the inflammatory immune response to the vaccine and any adjuvants it contains.⁴

SIRVA gained public attention during the COVID-19 vaccination campaigns, when billions of deltoid injections were administered worldwide, some by personnel with limited injection training. Symptoms include persistent shoulder pain, reduced range of motion, and weakness that begins within hours to days of the injection and does not resolve with typical post-vaccination soreness timelines. The condition can require physical therapy, steroid injections, or even surgery in severe cases. The key error is almost always injecting too high on the arm, above the bulk of the deltoid muscle where subcutaneous tissue is thinner and underlying structures are more exposed.

When the Medication Never Reaches the Muscle

Not every injection error causes dramatic harm. One of the most common mistakes is simply failing to get the medication into the muscle at all, depositing it into the subcutaneous fat layer instead. This matters because drugs and vaccines given intramuscularly are designed to be absorbed at a specific rate through the muscle’s rich blood supply. When they pool in fat tissue, absorption slows, and the medication may be less effective or essentially wasted.

Current injection guidelines may not account for this well enough. Research on deltoid injections has found that standard needle lengths can result in subcutaneous rather than intramuscular delivery in women and people with obesity, potentially reducing medication effectiveness. For people with higher body mass, clinicians using too-short needles may inadvertently create a disparity in how well treatments work.⁵ The same problem arises if the skin is bunched or pinched during injection, which effectively adds tissue between the needle tip and the muscle.

A systematic review of needle length for deltoid injections found that standard 25-millimeter needles may miss the muscle entirely in a meaningful proportion of recipients with normal or elevated BMI. The concern is greatest for women with obesity, who tend to have a thicker layer of subcutaneous tissue over the deltoid.⁶ For the ventrogluteal site (the hip), BMI reliably predicts fat thickness, and standard needles would fail to deliver medication into the muscle in a considerable proportion of obese adults.⁷

This is a genuinely underappreciated problem. A vaccine that lands in fat instead of muscle may still generate some immune response, but you won’t necessarily know it was suboptimal. For medications like certain antibiotics, antipsychotics, or hormone injections that depend on a predictable absorption curve, a subcutaneous deposit can alter both the timing and intensity of the drug’s effect.

Tissue Death and Nicolau Syndrome

One of the rarest but most alarming complications is Nicolau syndrome, a condition where tissue around the injection site dies. It begins with intense pain at the injection site, followed by a purplish discoloration of the skin that progresses to full-thickness necrosis of the skin, underlying fat, and sometimes the muscle itself.⁸ The pattern is distinctive: the skin develops a mottled, net-like appearance before turning black as the tissue dies.

Documented cases show the damage can extend well beyond the injection site. In one report, a 25-year-old man developed purplish skin discoloration at his right hip after an intramuscular diclofenac injection. In another case, a 60-year-old man who received an intramuscular antihistamine injection developed discoloration not only at the injection site but also at his left shoulder blade and left elbow.⁹ The exact mechanism is debated, but it likely involves accidental injection into or near a blood vessel, causing local vascular spasm or blockage that cuts off blood supply to the surrounding tissue.

Nicolau syndrome is rare enough that many clinicians will never see a case. But when it happens, the consequences can be severe, sometimes requiring surgical removal of dead tissue and skin grafting. The non-steroidal anti-inflammatory drugs commonly given by injection (like diclofenac) appear in a disproportionate number of case reports, possibly because of their chemical irritancy.

Infection at the Injection Site

Any time a needle breaks the skin, there is a risk of introducing bacteria. With proper skin cleaning and sterile technique, injection-site infections are uncommon. But when technique fails, the consequences can be extreme. Necrotizing soft tissue fasciitis, the aggressive “flesh-eating” infection, has been documented after gluteal intramuscular injections. A review of published cases between 1997 and 2017 identified 19 such cases, making it rare but real.¹⁰

More commonly, poor technique leads to localized abscesses or cellulitis. These are typically treatable with antibiotics and drainage, but they turn a routine injection into a multi-week medical problem. People who inject medications at home, whether insulin, testosterone, or other drugs, face particular risk if they reuse needles, skip alcohol swabs, or inject through unclean skin.

Why Injection Site Matters So Much

The classic buttock injection, given in the upper outer quadrant of the gluteal muscle (the dorsogluteal site), has been falling out of favor for good reason. This area puts the sciatic nerve and the superior gluteal artery at risk, and the thickness of subcutaneous fat over the dorsogluteal region makes it harder to reach the muscle in many patients. A systematic review and meta-analysis comparing the dorsogluteal site to the ventrogluteal site (the hip, slightly forward) found that the dorsogluteal site carries greater risk of adverse effects.²

The ventrogluteal site is considered less risky for nerve injury and local or systemic side effects across all age groups, including children.¹¹ Despite this evidence, the dorsogluteal site remains deeply entrenched in clinical practice and self-injection habits. Many healthcare workers were trained on the traditional buttock site and continue using it out of familiarity, even though guidelines increasingly recommend alternatives.

For vaccines specifically, the deltoid muscle of the upper arm is standard in adults, and the vastus lateralis (the outer thigh) is preferred in infants and young children. Each site has its own anatomy to respect. The deltoid is small, which means injecting too high risks SIRVA, while injecting too low or too far to the side can hit the radial nerve or the brachial artery. The vastus lateralis is generally considered one of the safest sites because the area used for injection has relatively few major nerves and blood vessels running through it.

The Aspiration Debate

If you’ve ever watched someone give an injection and pull back slightly on the plunger before pushing the medication in, that’s aspiration, a technique historically used to check whether the needle tip has accidentally entered a blood vessel. The idea was that if blood appeared in the syringe, you’d know to reposition before injecting. For decades, aspiration was considered standard practice for all IM injections.

Current evidence and international guidelines have moved away from routine aspiration for most injection sites. A systematic review found no clinical reason to aspirate when using the deltoid, ventrogluteal, or vastus lateralis sites. The one exception is the dorsogluteal site, where aspiration is still recommended because of its proximity to the gluteal artery.¹² A more recent meta-analysis reinforced this position, finding that skipping aspiration at recommended sites does not worsen clinical outcomes and actually reduces pain and procedure time.¹³

This is worth knowing if you give yourself injections or if you’re a patient watching a nurse skip the aspiration step. At most sites, they’re following current best evidence, not cutting corners. But if someone is injecting into the traditional buttock site without aspirating, that’s a gap in technique that does carry risk.

Angle, Technique, and Leakage

Textbooks have long taught that IM injections should go in at a strict 90-degree angle to the skin. In practice, that standard is more flexible than most people realize. A mathematical analysis showed that an injection given at 72 degrees reaches 95 percent of the depth achieved at 90 degrees. Combined with research on how hands naturally move during injection, this supports a relaxed standard: any comfortable angle between 72 and 90 degrees is fine.¹⁴ Obsessing over a perfect right angle is unnecessary and may actually make the injector tenser, leading to other technique problems.

A separate concern is medication leaking back out of the muscle after injection. The Z-track technique addresses this by pulling the skin and subcutaneous tissue to one side before inserting the needle, then releasing it after the medication is deposited. This creates a zigzag path through the tissue layers that seals off the injection track and prevents the drug from escaping back to the surface.¹⁵ The technique is especially important for medications that are irritating to subcutaneous tissue, like iron dextran or certain antipsychotics, where leakage can cause pain, staining, or local tissue damage.

Who Is Most Vulnerable to Injection Errors

Body composition plays an outsized role in whether an IM injection actually reaches the muscle. Women generally have more subcutaneous fat over common injection sites than men, meaning a standard needle that works reliably for one group may fail for another. People with obesity face the highest risk of receiving an accidental subcutaneous injection, and current guidelines may not adequately account for this.⁵

Infants and very young children present the opposite challenge. Their muscles are small, so there’s less room for error before the needle overshoots into bone or hits structures it shouldn’t. The vastus lateralis is the go-to site for this age group precisely because it offers the most muscle mass relative to the child’s size, but even here, using a needle that’s too long can cause problems.

Older adults with muscle wasting (sarcopenia) are another group at risk. Even at a normal BMI, someone who has lost significant muscle mass may have a thinner-than-expected muscle layer, making it easier for an injection to pass through the muscle entirely and into the underlying tissue or periosteum. Conversely, very muscular individuals may tolerate standard needles fine in terms of depth but could experience more bleeding or discomfort if a superficial vein is nicked on the way in.

People who self-inject chronically, such as those on testosterone replacement, certain psychiatric medications, or allergy immunotherapy, face cumulative risks. Repeated injections in the same spot can lead to scar tissue buildup, which makes the area harder to inject into and may alter drug absorption over time. Rotating injection sites is standard advice, but in practice many people develop a preferred spot and stick with it.

What to Do If Something Goes Wrong

The signs that an IM injection has gone badly vary by the type of error. If you feel electric, shooting pain down a limb during the injection, that’s a red flag for nerve contact. The injection should be stopped immediately and the needle withdrawn. Subsequent numbness, tingling, or weakness warrants medical evaluation, ideally with imaging to assess nerve integrity.

Skin that turns dusky, purple, or mottled around the injection site in the hours after an injection is concerning for vascular compromise, as seen in Nicolau syndrome. This is a medical emergency. Early intervention can sometimes limit the extent of tissue death, but there is no specific antidote. Treatment is supportive: wound care, possible surgical debridement, and management of pain.

Increasing redness, warmth, swelling, and pain over the days following an injection suggest infection. A mild local reaction is normal and expected, particularly with vaccines. But if the redness is expanding, you develop a fever, or the area becomes fluctuant (feeling like there’s fluid underneath), you need medical attention. Catching an abscess early usually means antibiotics alone can handle it; waiting too long may mean drainage.

For SIRVA, the hallmark is shoulder pain and stiffness that persists or worsens beyond the first 48 hours after a deltoid injection, accompanied by difficulty raising the arm. If this describes your situation after a vaccination, documenting it early matters both for treatment and, in some countries, for injury compensation programs.

Common Misconceptions About IM Injections

One widespread belief is that all injections should be given in the buttock. As the evidence above makes clear, the traditional dorsogluteal site is actually one of the riskier options for IM injection. The ventrogluteal site, the deltoid, and the vastus lateralis all carry fewer complications in most circumstances.

Another misconception is that a longer needle is always more dangerous. In reality, using a needle that’s too short is often the bigger problem, because it results in a subcutaneous rather than intramuscular deposit. For people with more subcutaneous tissue, a longer needle is the correct and safer choice. The discomfort of the injection itself is more related to needle gauge (thickness) than length. A longer, thinner needle may actually hurt less than a shorter, thicker one.

Many people also believe that if an injection didn’t cause immediate, intense pain, it went fine. Nerve injuries, SIRVA, and Nicolau syndrome can all have delayed presentations. Sciatic nerve injury from an injection can take hours to fully manifest, and SIRVA often doesn’t become obvious until the expected post-vaccine soreness should have cleared but hasn’t. The absence of immediate pain is reassuring but not a guarantee that everything landed where it should.

Finally, there’s a persistent idea that aspiration should always be done before pushing medication in. While this was once universal practice, current evidence supports skipping aspiration at the deltoid, ventrogluteal, and vastus lateralis sites. Routine aspiration at these locations adds pain and procedure time without improving safety.¹³ The dorsogluteal site remains the exception where aspiration still has a role, which is yet another reason to prefer other sites when possible.¹²

1
PubMed Central. Iatrogenic Injury to the Sciatic Nerve due to Intramuscular Injection: A Case Report
2
PubMed Central. Adverse effects of dorsogluteal intramuscular injection versus ventrogluteal intramuscular injection: A systematic review and meta‐analysis
3
PubMed Central. Post-Injection Sciatic Neuropathy: MR Neurography Findings in a Case of Iatrogenic Nerve Injury
4
PubMed Central. Shoulder injury related to vaccine administration (SIRVA) after COVID-19 vaccination
5
Medical Research Archives. Intramuscular Injection Guideline Revisions are Needed Based on Body Mass Index, Needle Length, Sex, and Skin to Muscle Depth
6
PubMed Central. What variables should inform needle length choice for deltoid intramuscular injection? A systematic review
7
PubMed. Body Mass Index: A Reliable Predictor of Subcutaneous Fat Thickness and Needle Length for Ventral Gluteal Intramuscular Injections
8
PubMed Central. Nicolau syndrome after intramuscular injection: 3 cases
9
PubMed Central. Nicolau syndrome: an iatrogenic cutaneous necrosis
10
PubMed Central. Necrotizing Soft Tissue Fasciitis after Intramuscular Injection
11
Türk Hemşireler Derneği Dergisi. Preference Of the Ventrogluteal Region in Intramuscular Injection in Children: Systematic Review
12
PubMed. Aspirating during the intramuscular injection procedure: a systematic literature review
13
PubMed. Aspiration in intramuscular injection: a meta-analysis of pain, duration, and clinical outcomes
14
PubMed Central. The myth of the 90 degrees-angle intramuscular injection
15
PubMed Central. Production and validation of an educational video on the use of the Z-Track Technique