Key Takeaways
If you have a fever higher than 100.4°F after an injection, you should get medical help right away. Call 911 if you have trouble breathing or swelling in your face after getting a shot. Rotating injection sites can help prevent lumps and numbness in the skin.
Injection site reactions can vary from mild to severe. Watch for signs like high fever, severe swelling, or trouble breathing, which may indicate a serious problem requiring immediate medical attention.
When to Get Help
Call 911 or head to the ER if you experience symptoms of a severe allergic reaction (anaphylaxis)—such as difficulty breathing, facial swelling, or hives—as these can be life-threatening. Seek prompt medical care if you notice signs of infection, like fever, severe pain, rash, or blisters at the injection site.
When Is a Fever After an Injection a Concern? Tom Merton Getty Images.
Tom Merton/Getty Images
If you develop a fever over 100.4°F after an injection, seek medical help immediately. This may indicate an infection from a contaminated needle. A fever accompanied by chills is particularly serious.
Watch for other signs of infection, including:
Increasing redness, pain, warmth, or swelling at the injection site Muscle aches Feeling very tired Clear fluid or pus coming from the site Red streaks spreading from the area—this could be a sign of a serious infection called cellulitis
Use sterile techniques during injections to prevent infections. Always use a new, sealed syringe and needle for each shot. Many medications are now available in prefilled syringes or single-use vials to reduce infection risks.
Pain After an Injection: When Is It Normal? Fertnig/Getty Images
Most subcutaneous and intramuscular injections cause only mild pain. Some vaccines may leave the area sore for a day or two; this is a normal response from your immune system.
Some swelling after an injection is common and usually subsides within a day or two. However, swelling can occasionally be more serious or last longer.
However, if the pain doesn’t start to get better after 48 hours, or if it’s so severe that you can’t touch the area, it may be a sign of a problem. This is especially true if you also have a fever, muscle aches, or feel unusually tired. In these cases, get medical help right away.
What Does Swelling After an Injection Mean?
PhotoAlto/Michele Constantini/Getty Images
Swelling after an injection is common and typically subsides within a day or two. However, sometimes it can be more serious or persist longer.
Possible reasons include:
Normal vaccine response: Some intramuscular vaccines can cause swelling and firm tissue at the injection site. This usually lasts a day or two and then fades—it’s a normal immune response. Arthus reaction: This intense reaction can occur after a booster shot, especially with diphtheria or tetanus vaccines. It causes severe swelling and skin hardening, and in some cases, a skin ulcer. It may last several days and might require treatment with corticosteroids or antihistamines. Abscess: Rarely, an infection can lead to a pus-filled lump under the skin. This causes severe pain, swelling, fever, and sometimes nearby swollen lymph nodes. It's more likely with self-injections or when needles are reused. This infection requires immediate medical attention. Lipohypertrophy: This is a firm or rubbery lump of fat that can form when injections are given repeatedly in the same spot, like with insulin shots. It may also cause numbness. The lump often goes away over time if you stop using that site. Rotating injection sites helps prevent this.
Consult a healthcare provider for swelling that is severe, unusual, persistent, or unexplained. If you suspect an abscess, seek care immediately.
How to Identify Severe Allergic Reactions Edward McCain/Getty Images
In rare instances, an injection of vaccine or medication can trigger a serious allergic reaction known as anaphylaxis. This whole-body reaction can be life-threatening if not treated immediately.
Anaphylaxis occurs when the immune system overreacts, releasing large amounts of chemicals like histamine and leukotrienes. These chemicals can quickly cause:
Widened blood vessels, which drop your blood pressure and reduce blood and oxygen to your organs (called anaphylactic shock) Narrowed airways (called bronchoconstriction), which make it hard to breathe
Without fast treatment, anaphylaxis can lead to breathing failure, cardiac arrest, coma, or death.
Call 911 immediately if you notice these signs of anaphylaxis after an injection:
A sudden rash or hives Trouble breathing, wheezing Dizziness, fainting, or feeling weak Swelling of the face, lips, or tongue Fast or irregular heartbeat Nausea, vomiting, or diarrhea Trouble swallowing Cold, pale, or sweaty skin A strong sense that something is very wrong
Anaphylaxis usually happens within 30 minutes, but in some cases, symptoms might be delayed for hours. Sometimes there’s a mild reaction first, followed by a severe one later.
A 2017 study in Frontiers in Immunology found that drug-related anaphylaxis affects between 0.04% and 3.1% of people, depending on the drug or vaccine. When treated, the risk of dying is low—around 0.65%. But without treatment, death can happen in as little as 15 minutes.
Common Injection Site Reactions
Injection site reactions are the most common side effect after getting a vaccine. The vast majority of reactions are mild and limited to the area around the injection site.
The most common symptoms of an injection site reaction include:
Pain and soreness Itching Swelling Redness
These reactions are usually mild and last for one to two days.
Less commonly, injection site reactions can be larger and more widespread. More serious injection site reactions can occur with any vaccination. However, they are more common following booster doses of diptheria, tetanus, and pertussis (DTPa/dTpa).
Symptoms of a larger injection site reaction may include:
Difficulty moving nearby limbs More severe pain and swelling Swollen—but not tender—lymph nodes
With a larger reaction, the symptoms tend to develop within 24 to 48 hours after getting the injection, and resolve within a week. Although they are more painful and can be alarming, they rarely become life-threatening.
Treatment Options
Treatment for an injection site reaction depends on what kind of reaction you're having and how severe it is.
Mild injection site reactions, such as pain and swelling, may only require ice, over-the-counter pain relievers, anti-itch medications, or a cold compress to reduce swelling.
Anaphylaxis requires immediate treatment to prevent serious complications. Care typically includes:
Hospital admission A shot of adrenaline (epinephrine) into the thigh to stop the reaction A second dose if symptoms don’t improve within five minutes Oxygen if breathing is affected IV fluids if the person is in shock or has low blood pressure IV adrenaline if symptoms continue after two shots—this is given by specialists Antihistamines and steroids may be used later to help with rash or itching
Localized injection site infections may be treated with prescription topical antibiotics and close monitoring for spreading redness or fever. Moderate to severe infections often require oral or IV antibiotics. If an abscess forms, your provider may drain it and test the fluid to identify the bacteria and guide treatment.
Systemic infections—marked by symptoms like fever, chills, or a rapid heartbeat—need immediate medical care. Treatment may include:
Hospital admission IV antibiotics to control the infection quickly IV fluids to support blood pressure and hydration Oxygen or breathing support, if needed Drainage of abscesses or surgical removal of infected tissue (debridement)
If you think you're having any kind of injection site reaction, contact your healthcare provider for guidance. Don’t assume it’s safe to treat at home—some reactions may need medical attention.
Tips for Prevention
While not all injection site reactions can be avoided, these steps can help reduce the risk of both minor irritation and more serious complications:
Choose the right injection site. Medical providers rotate injection spots to avoid tissue damage, especially for repeated injections (like insulin). Follow your provider’s guidance if self-injecting. Clean the skin properly. Wipe the area with an alcohol swab before an injection to help prevent bacteria from entering the body and causing infection. Use proper injection technique. Whether it’s subcutaneous (into the top layers of skin) or intramuscular (deeper into muscle), using the right needle size, angle, and depth matters. Trained providers know these steps—but people self-injecting should also be educated. Use sterile equipment. Never reuse needles or syringes. Doing so can introduce bacteria and increase the risk of localized or systemic infections. Let your provider know about past reactions. If you've had a severe reaction in the past, your provider may take steps to reduce the risk of a repeat reaction—such as monitoring you after a vaccine or prescribing a different formulation. Stay for observation after vaccines. Most vaccine clinics ask people to wait 15 to 30 minutes in case of a rare allergic reaction. Don’t skip this—anaphylaxis can develop quickly. Keep the area clean and dry afterward. Avoid tight clothing or harsh rubbing on the injection site. If you're prone to swelling or pain, ask if you can apply a cold compress or OTC cream.
If your healthcare provider recommends a vaccine or injected medication, take time to learn about its possible side effects. Don’t hesitate to ask questions and share your medical history, including any past issues with injections or other concerns.
Prognosis
Most localized injection site reactions—like redness, swelling, or itching—clear up on their own within a few days without lasting effects. Localized infections usually respond well to treatment with antibiotics when caught early.
Anaphylaxis is a medical emergency, but with prompt treatment, most people recover fully. Systemic infections can be serious and often require hospitalization, but the outlook is good with early and appropriate care.
Getting medical attention at the first sign of a serious injection site reaction can significantly improve outcomes and lower the risk of complications.
When to See a Healthcare Provider
Contact your healthcare provider right away if you notice:
A fever of 100.4°F or higher Severe pain at the injection site Blisters forming at the injection site Muscle aches or soreness Nausea, headache, or dizziness Skin rash, intense itching, vomiting, or hives (raised, itchy bumps) Symptoms that are getting worse instead of improving
Call 911 or get to the ER immediately if you experience:
Difficulty breathing or shortness of breath Swelling of the face, lips, tongue, or throat Widespread hives or rash Severe dizziness or fainting Rapid heartbeat or chest tightness
Never try to treat a severe allergic reaction at home with over-the-counter antihistamines. They can’t stop anaphylaxis, which can become life-threatening within minutes.