How Long Should an Injection Site Be Swollen? A Complete Guide

October 9, 2025 •

5 min read

Following COVID-19 vaccination, local injection site reactions are very common, with pain affecting over 72% of recipients and swelling occurring in about 13% [1.7.1]. Understanding how long should an injection site be swollen helps differentiate between a normal response and a potential complication.

Quick Summary

Swelling at an injection site is a common inflammatory response that typically resolves within a few days. Learn to identify normal vs. abnormal reactions, effective home treatments, and crucial signs that indicate a need for medical attention.

Key Points

Normal Duration: Most injection site swelling is mild and resolves within 1 to 3 days [1.2.1].
Cause: Swelling is an inflammatory response to the needle or the medication, indicating the immune system is working [1.4.1].
Home Treatment: Use a cold compress for the first 24-48 hours, followed by a warm compress, and keep the limb moving to reduce swelling [1.5.5, 1.9.4].
Concerning Signs: Worsening pain after 3 days, redness larger than 3 inches, pus, or fever are reasons to see a doctor [1.6.2].
Severe Reactions: An Arthus reaction (severe swelling) or lipohypertrophy (fatty lump from repeated injections) may last longer and require medical advice [1.3.1, 1.9.3].
Emergency Symptoms: Trouble breathing or swelling of the face/tongue requires immediate emergency care (call 911) as it could be anaphylaxis [1.6.3].
Vaccine Variation: The duration of swelling can vary by vaccine; for example, DTaP shot reactions can last up to 7 days [1.2.2].

Understanding Injection Site Swelling

Receiving an injection, whether it's a vaccine or a medication, often results in localized side effects. One of the most common is swelling, medically known as edema, at the site where the needle entered the skin [1.4.5]. This is typically a normal inflammatory response from your body reacting to the needle piercing the skin or to the substance being injected [1.4.1]. This response signifies that your immune system is being activated, particularly in the case of vaccines, as it starts to build protection [1.2.2].

The reaction can happen almost immediately or may develop hours after the injection [1.2.1]. Symptoms that frequently accompany swelling include pain, redness (erythema), itching, and warmth at the site [1.8.2]. For most people, these reactions are mild, not serious, and resolve on their own without intervention [1.2.6].

What Causes the Swelling?

Injection site swelling is primarily caused by two types of reactions:

Irritant Reactions: These occur when the medication itself or another component, like a preservative, irritates the skin tissue. This type of reaction is more common with subcutaneous injections (under the skin) and tends to appear within a couple of hours [1.8.5].
Immune-Mediated Reactions: This happens when the injected substance triggers a more robust immune system response. This is very common with vaccines and booster shots. Immune-mediated reactions can take a few days to fully develop [1.8.5]. In rare cases, swelling can be due to an infection, an abscess, or a more intense reaction like an Arthus reaction, which is a severe local inflammation [1.3.1].

Typical Duration of Swelling

So, how long should an injection site be swollen? For a standard, mild reaction, the swelling, redness, and pain typically begin within the first 24 hours and resolve within 1 to 3 days [1.2.1, 1.2.2].

However, the duration can vary depending on the specific medication or vaccine:

COVID-19 Vaccines: Local symptoms like pain and swelling usually last for 1 to 3 days [1.7.5].
DTaP Vaccine: Swelling and redness can last longer, from 3 to 7 days [1.2.2]. Large swelling (over 4 inches) can occur after the 4th or 5th dose and may take up to 7 days to resolve for 90% of children [1.7.5].
Arthus Reactions: These more intense reactions can last for several days and may require medical treatment [1.3.1]. If a lump forms, it is also usually a normal response that fades within a few days. However, if you receive injections frequently in the same spot (like with insulin), a fatty lump called lipohypertrophy can form, which may take months to resolve after you stop using that site [1.9.1, 1.9.3].

Comparison of Injection Site Reactions

It's crucial to distinguish between a normal, expected reaction and one that requires medical attention.


Feature	Normal Local Reaction	Concerning Reaction (Seek Medical Advice)
Timing	Starts within 24-48 hours after the injection [1.3.2].	Starts more than 48 hours after the shot, or gets progressively worse after 3 days [1.6.2].
Swelling/Redness	Mild to moderate, less than 2-3 inches in diameter [1.2.2, 1.8.4].	Redness is larger than 2-3 inches, or a red streak spreads from the site [1.6.2, 1.6.4].
Pain	Mild to moderate, improves after 48 hours [1.3.1].	Severe pain, or pain that makes it difficult to move the limb [1.3.6].
Duration	Resolves within 3-5 days, up to 7 for some vaccines [1.2.2].	Persists or worsens after 3-5 days; does not improve [1.6.1].
Other Symptoms	Mild warmth at the site. A small, tender lump may be present [1.8.5].	Fever over 100.4°F (38°C), pus or drainage, blisters, or signs of a systemic allergic reaction (hives, trouble breathing, swelling of face/throat) [1.6.3, 1.6.4].

How to Manage and Reduce Swelling

While most injection site swelling is harmless, it can be uncomfortable. Several home care strategies can help provide relief:

Cold Compress: Apply a cold pack or a cloth-wrapped bag of ice to the area for 10-20 minutes at a time during the first 24-48 hours. This helps constrict blood vessels, reducing swelling, pain, and itching [1.5.1, 1.5.5].
Warm Compress: After the first 48 hours, a warm compress can be soothing. The warmth helps dilate blood vessels, improving blood flow to help clear any residual swelling or bruising [1.5.5, 1.9.4].
Keep Moving: Gently moving and stretching the affected limb increases blood flow to the injection site, which can help disperse the fluid and speed up the healing process [1.5.2, 1.9.4].
Over-the-Counter (OTC) Medications: If approved by your doctor, pain relievers like acetaminophen (Tylenol) or nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen (Advil, Motrin) can reduce both pain and inflammation [1.5.4]. For itching, an OTC antihistamine may also help [1.9.4].
Elevation: If the injection was in an arm or leg, elevating the limb above heart level can help reduce fluid buildup [1.5.5].

When to Contact a Healthcare Provider

While most reactions are mild, it's important to know the warning signs of a more serious issue. You should contact your doctor or seek medical attention if you experience any of the following:

The swelling, redness, or pain gets worse after 3 days instead of better [1.2.2].
The reaction does not go away after 7 days [1.2.2].
Redness at the site is larger than 2-3 inches in diameter or you see red streaks spreading away from the site [1.6.2, 1.6.4].
You have severe pain that is not relieved by OTC medications or that limits your ability to use your limb [1.3.6].
The site develops blisters or drains pus [1.6.1, 1.6.4].
You develop a fever of 100.4°F (38°C) or higher, or a fever that lasts more than 3 days [1.6.3, 1.6.2].
A hard lump at the injection site persists for more than two weeks or becomes hot, red, or more painful [1.9.2].

Seek emergency care immediately (call 911) if you have signs of a severe allergic reaction (anaphylaxis), such as difficulty breathing, swelling of the face, lips or tongue, widespread hives, dizziness, or a rapid heartbeat [1.6.3].

Conclusion

Swelling at an injection site is a very common and usually harmless part of your body's natural response. For most people, the discomfort is mild and resolves within a few days with simple home care. By knowing the typical timeline and recognizing the difference between a normal reaction and a potential complication, you can confidently manage the after-effects of an injection. Always consult a healthcare professional if your symptoms are severe, worsening, or persist longer than expected.

For more information on vaccine side effects, you can visit the CDC's page on Possible Side Effects After Getting a COVID-19 Vaccine.

Frequently Asked Questions

Yes, it is a normal and expected reaction to get a hard, swollen, or tender lump under the skin after an injection. This is typically part of the body's inflammatory response and should resolve within a few days [1.8.5, 1.9.4].

A normal lump or knot from an injection usually disappears within a few days to two weeks. If a lump persists longer than two weeks or becomes increasingly painful, hot, or red, you should consult a doctor [1.9.2].

A normal reaction starts within 48 hours and begins improving after 2-3 days. An infection may start later (after 48 hours), gets progressively worse instead of better, and may be accompanied by fever, spreading red streaks, or pus [1.3.5, 1.9.4].

Yes, if your doctor approves, over-the-counter NSAIDs like ibuprofen (Advil, Motrin) can help reduce swelling, pain, and inflammation at the injection site [1.5.4].

This is often an immune-mediated reaction, where your immune system is responding to the injected substance. These types of reactions, common with vaccines and booster shots, can take a few days to develop [1.8.5].

For the first 24-48 hours, use a cold compress to reduce initial swelling and pain. After that, switching to a warm compress can help improve blood flow and resolve any remaining discomfort or bruising [1.5.5, 1.9.4].

You should worry and contact a healthcare provider if the swelling is severe, worsens after 3 days, lasts longer than a week, is accompanied by a fever or pus, or if you have symptoms of an allergic reaction like difficulty breathing [1.6.2, 1.6.3].