When to Call Your Healthcare Provider About an Injection Side Effect

Injection Site Reactions, Fever, Swelling, and More

Injection site reactions are common after vaccines or treatments delivered through a shot. These may include a little swelling, tenderness, pain, itching, or redness around the area where the injection was given. Typically, any minor discomfort goes away in one or two days.

However, as with all medications, there can be serious adverse reactions or side effects that require prompt medical care.

This article explains some serious injection reactions that you should be aware of and what to do if you experience them.

Injection site reactions can vary depending on if the shot was subcutaneous (under the skin), intravenous (in a vein), or intramuscular (in a muscle). 

What Is an Injection Site Reaction?

An injection site reaction is your body's response to an immunization or other injection at the site where it was administered. It typically manifests as swelling, pain, rash, redness, or bleeding. 

Injection site reactions are usually caused by an immune response to the needle, vaccine, or other medicine. These types of reactions are typically mild and resolve in a day or two without treatment.

Less commonly, injection reactions are due to an infection or allergy. Some may be easily treated, while others may be far more serious and lead to a potentially deadly, all-body reaction (such as anaphylaxis or sepsis).

Certain reactions to an injection are potentially serious and should be evaluated by a healthcare provider. If you have any of the following symptoms after an injection, seek medical attention right away.

High Fever

Sick man taking temperature with digital thermometer

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Fevers higher than 101 F following an injection warrant a call to your healthcare provider or visit the nearest emergency room. That’s because the fever may indicate an infection caused by needle contamination or an allergic reaction to the medication itself. Both are serious.

By and large, allergies tend to happen quickly, while symptoms of an infection may take one to 10 days to appear.

While many infections occur due to a self-administered injection, such as with diabetes or autoimmune disorders. However, they can also happen at the healthcare provider’s office or hospital if the person administering the injection does not adhere to aseptic techniques. 

Extreme Pain at the Injection Site

Woman with shoulder pain
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While most people dislike the idea of a shot, the good news is it is usually over quickly and causes little pain. However, if the pain persists or worsens, you should call a healthcare provider and have it checked out.

While it is not uncommon to have localized swelling or redness for a day or two following an injection (or even longer for certain types of intramuscular shots), some symptoms warrant further investigation. These include:

  • Injection site that is tender to the touch
  • Fever
  • Body aches
  • Creeping discoloration

In some cases, the pain may be extreme but not particularly dangerous (such as when an intramuscular injection accidentally hits the sciatic nerve). But, at other times, it may be due to an infection that might only get worse if it's not untreated.

Swelling or Hardness Under the Skin

Doctor comforting woman in the waiting room

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While swelling and minor bruising can happen after a shot, they usually get better within a day or so. However, if swelling and discoloration persist, it may signify an infection.

A lump under the skin that feels soft, mushy, and painful may indicate a developing abscess. An abscess is a walled-off collection of pus. It is often warm to the touch and accompanied by enlarged lymph nodes, small bean-shaped glands that are part of the immune system.

You should never squeeze abscesses. That's because to prevent infection from spreading throughout the body, a healthcare provider must properly drain it. If you try yourself, it could burst under the skin and spread the infection through the bloodstream, causing a potentially life-threatening blood infection known as sepsis.

Signs a lump may be an abscess include:

  • The lump oozes: While a bit of drainage following an injection may be expected (caused by medication leaking out of the needle track), a doctor should look at any discolored or abnormal discharge immediately.
  • The swelling grows: If the lump is small and you're not sure if it's an abscess, take a pen and draw a circle along the border. If it starts to expand beyond the edge or fails to go away in several hours, call a healthcare provider to look at it as soon as possible.

A Sudden, All-Body Reaction

emergency room ER patient hospital anaphylaxis
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Following an injection, the most severe reaction is an all-body allergic response known as anaphylaxis. This type of response can occur if the body reacts adversely to the injected medication, causing a cascade of severe and potentially life-threatening symptoms.

Anaphylaxis develops very quickly and needs to be treated immediately with a shot of epinephrine (adrenaline).

The first signs of anaphylaxis may be similar to those for an allergy, including a runny nose and congestion (rhinitis), and an itchy skin rash. However, within 30 minutes or so, more severe symptoms can develop, including:

  • Coughing, wheezing, and shortness of breath
  • Chest tightness
  • Hives 
  • Dizziness or fainting
  • Rapid or irregular heartbeat
  • Weak pulse
  • Facial swelling
  • Swollen or itchy lips or tongue
  • Difficulty swallowing 
  • A blueish tinge to the lips, fingers, or toes (cyanosis)
  • Pale, clammy skin
  • Nausea, vomiting, or diarrhea

People who have anaphylaxis often report having a feeling of impending doom and panic. Anaphylaxis can lead to shock, coma, or even death if left untreated.


Injection site reactions are usually mild when they do occur. However, sometimes they can indicate something more serious, like an infection or allergic reaction. High fever, swelling, drainage from the injection site, severe pain, or a whole-body response are all reasons to see a healthcare provider for an evaluation.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Jung Kim H, Hyun Park S. Sciatic nerve injection injury. J Int Med Res. 2014;42(4):887-897. doi:10.1177/0300060514531924

  2. McNeil MM, DeStefano F. Vaccine-associated hypersensitivity. J Allergy Clin Immunol. 2018;141(2):463-472. doi:10.1016/j.jaci.2017.12.971

  3. Long B, Gottlieb M. Emergency medicine updates: anaphylaxis. Am J Emerg Med. 2021;49:35-39. doi:10.1016/j.ajem.2021.05.006

By Nicole Galan, RN
Nicole Galan, RN, is a registered nurse and the author of "The Everything Fertility Book."