What causes a knot after getting a shot? Understanding post-injection lumps

September 28, 2025 •

5 min read

Approximately 1 in 10 million people experience a rare, more severe reaction known as an Arthus reaction, but for most, a simple knot is a normal part of the body's healing process. So, what causes a knot after getting a shot? It's typically a mild and temporary inflammatory response by your immune system to the needle or the substance injected.

Quick Summary

A knot after an injection is caused by a localized inflammatory reaction, where immune cells and fluid gather at the site to process the injected substance, often accompanied by minor tissue trauma from the needle. This is a normal and harmless sign that the immune system is doing its job, but there are rarer, more serious causes to be aware of.

Key Points

Immune Response: A knot is typically a sign of a normal, localized inflammatory response by your immune system to a foreign substance, such as a vaccine or medication.
Tissue Trauma: The needle itself can cause minor trauma to muscle and tissue, which contributes to swelling and lump formation.
Arthus Reaction: In rare cases, a very painful, hard swelling known as an Arthus reaction can occur after a booster shot, representing an intense immune system overreaction.
Infection: Signs of a serious infection include increasing pain, swelling, redness, pus, fever, or red streaks radiating from the injection site.
At-Home Care: Most knots can be managed with cold compresses initially, followed by warm compresses, gentle movement, and over-the-counter pain relievers.
When to See a Doctor: Seek medical attention if the lump worsens, persists for more than a week or two, or is accompanied by fever or severe pain.

Common Causes of Post-Injection Lumps

For the vast majority of people, a small, tender lump at the injection site is a normal and expected side effect. The body views the injection—and its contents—as foreign, prompting a localized immune response. This causes inflammation, which leads to the characteristic swelling, redness, and pain.

Localized Immune Response

When a substance is injected into the muscle or under the skin, the immune system sends a rush of blood, fluid, and immune cells to the area. This influx of fluid and cells is the body's natural defense mechanism, which results in the small bump you can feel. The knot is essentially the collection of these immune agents doing their job and processing the vaccine or medication.

Minor Tissue Trauma

Another reason for the formation of a knot is the minor trauma caused by the needle itself. The needle physically displaces muscle and tissue fibers as it enters the skin and muscle. In some cases, it can also cause a small hematoma (a localized collection of blood outside of blood vessels) by nicking a tiny blood vessel. This tissue disruption, combined with the immune response, contributes to the feeling of a hard knot.

Injection Technique Factors

Certain aspects of the injection technique can also influence whether a knot forms and how pronounced it is.

Speed of Injection: A fast injection can sometimes irritate the tissue more, especially with thicker medications.
Needle Size: An inappropriate needle size for the type of injection can cause more tissue damage.
Injection Site: Not rotating injection sites, especially for those receiving frequent injections like insulin, can lead to lipohypertrophy, a buildup of fatty tissue.
Volume of Medication: A larger volume of medication or vaccine injected into one spot can take longer for the body to absorb, creating a more noticeable and longer-lasting lump.

Less Common and More Serious Causes

While most post-shot lumps are harmless, there are less common occurrences that may warrant closer attention. It's important to understand the difference between a normal reaction and one that indicates a potential problem.

Arthus Reaction

An Arthus reaction is a rare, severe inflammatory reaction that occurs after a booster vaccination, typically for diphtheria and tetanus. It is a type III hypersensitivity response where immune complexes build up at the injection site, causing more intense swelling, pain, and hardening of the area. This reaction is much more pronounced than a typical injection site knot and often takes longer to resolve. In very rare cases, it can cause tissue death (necrosis).

Sterile Abscess

If a knot becomes large, warm, or painful, it could be a sterile abscess—a pus-filled lump that is not caused by bacteria. This can sometimes happen in response to certain components of the vaccine, such as adjuvants. Unlike a normal post-injection reaction, a sterile abscess may need medical evaluation to determine the best course of action.

Infection

In rare cases, bacteria can enter the injection site during or after the shot, leading to a local infection known as cellulitis or an abscess. This is more likely with self-injections or unsanitary conditions. Signs of infection include increasing redness, warmth, swelling, and severe pain, often accompanied by fever and chills.

Granulomas

Some vaccines containing aluminum salts, used as adjuvants, can cause persistent, itchy nodules called granulomas. These are essentially a collection of immune cells that have walled off the foreign substance. They can sometimes persist for months but are typically harmless and resolve on their own.

Comparison of Common vs. Serious Lumps


Feature	Common Injection Site Reaction	Arthus Reaction	Abscess/Infection
Onset	Within a few hours to a day or two	4–12 hours after a booster shot	Can develop over several days
Size	Small, localized lump	Large, hard, and painful swelling	Expanding, painful mass
Tenderness	Mild to moderate soreness	Very painful	Severe pain
Warmth	Mild warmth, if any	May feel warm or hot	Warm or hot to the touch
Duration	Resolves within a few days	Lasts for several days to weeks	Persists and worsens without treatment
Associated Symptoms	Mild redness, swelling	Redness, swelling, possibly an ulcer	Fever, chills, pus, red streaks

How to Manage a Post-Shot Knot

For a normal, non-serious lump, simple at-home care is usually all that is needed to manage discomfort and speed up resolution.

Cold and Warm Compresses

Immediately after the shot, a cold compress or ice pack can help to reduce initial swelling and pain. After the first 24 hours, switching to a warm compress can increase blood flow to the area, which helps the body absorb the injected substance and resolve the knot more quickly.

Gentle Movement and Massage

Keeping the muscle moving, for example by gently exercising your arm, can improve circulation and promote lymphatic drainage. Gentle massage around the injection site, but not directly on it, can also be beneficial.

Over-the-Counter Pain Relievers

For pain and discomfort, over-the-counter pain medications like acetaminophen can be effective. It is sometimes recommended to avoid NSAIDs like ibuprofen, which can suppress the inflammatory response that is crucial for the vaccine's effectiveness. Always consult a healthcare provider for personalized advice.

When to Seek Medical Attention

While most knots are not a cause for concern, there are clear signs that you should consult a healthcare provider. Seek immediate medical care if you experience:

Severe, increasing pain, swelling, or redness
Fever, chills, or muscle aches
Pus or fluid draining from the injection site
Red streaks spreading from the lump
Signs of a systemic allergic reaction, such as difficulty breathing, swelling of the face or tongue, or widespread hives

Conclusion

A knot after a shot is a common, often temporary, sign that your body's immune system is actively responding to the injected substance. While most lumps are harmless and resolve with simple at-home care, understanding the underlying reasons and recognizing the signs of a more serious reaction is important for your health and peace of mind. Always consult a healthcare professional with any concerns about an injection site reaction. Staying informed ensures you can properly care for your body and identify when a lump is more than just a minor inconvenience.

For more detailed information on injection site reactions and other vaccine-related topics, consider reviewing guidelines from a trusted medical source, such as the Centers for Disease Control and Prevention.

Frequently Asked Questions

Yes, it is very common and normal to feel a small, hard knot or lump at the injection site. This is a sign of your immune system's normal, localized inflammatory response to the injected substance.

Most knots are temporary and should resolve on their own within a few days to a week or two. A warm compress can often help speed up the process.

An Arthus reaction is a very rare but severe inflammatory response that causes a hard, painful lump and significant swelling, typically after a booster shot. It's more intense than a normal injection site reaction and may require medical evaluation.

Yes, but it is rare. Signs of an infection include worsening redness, warmth, pain, swelling, pus drainage, or the development of a fever. If these symptoms occur, you should see a doctor.

A normal reaction involves mild soreness, redness, and swelling that improves over a few days. A serious reaction may include severe pain, expanding redness, fever, pus, or signs of a systemic allergic response.

You can apply a cold pack to the site immediately after the shot to reduce swelling. After 24 hours, a warm compress can increase circulation. Gentle movement and over-the-counter pain relievers can also provide relief.

Gentle movement and light massage around the injection site can help promote circulation and absorption. However, you should avoid vigorous massage directly on the lump, as it could cause more irritation.

Lipohypertrophy is a condition where repeated injections in the same area lead to a buildup of fatty tissue, forming a firm, rubbery lump. This is common with frequent self-injections, like for insulin, and can be avoided by rotating injection sites.