Injection Site Reactions: Why Your Arm Hurts After a Shot and What to Do About It

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Vaccines and injectable medications keep people healthy. But getting a shot isn't very pleasant. Beyond the injection pain or discomfort of the shot, injection site reactions can look worrisome. Just when the memory of the pain has faded - your arm swells and turns red.

If you've had injection site reactions, you may be wondering if it's an allergy or sign of something more serious. Here's what you should know about injection site reactions and how to take care of them.

What is an injection site reaction?

Injection site reactions are skin changes that develop after receiving injectable medications, including vaccines. These reactions are very common for both children and adults. Most of the time they aren't caused by any sort of allergy.

Injection site reactions can happen if something in the injection:

• Irritates your skin or tissues (irritant reactions)

• Causes your immune system to kick into high gear (immune-mediated reactions)

Irritant reactions are more common after subcutaneous injections. They tend to develop quickly - usually within a couple of hours. Immune-mediated reactions can happen fast too, but more often they take a few days to develop.

Injection site reactions can form whether you receive a medication through an intramuscular injection or a subcutaneous injection. And they can form on any part of the body where you inject medications - like your arm, thigh, stomach, or buttocks.

Some medications are more likely to cause injection site reactions. For example, about 4 in 10 people who receive Stelara (ustekinumab) - a medication that treats different autoimmune conditions and ulcerative colitis - develop injection site reactions.

What are the symptoms of an injection site reaction?

Symptoms of an injection site reaction include the following skin changes in or around the spot where you received the injection:

• Redness

• Warmth

• Swelling

• Itching

• Mild pain or discomfort

• A lump under the skin

You may experience some or all of these symptoms. Keep in mind that while injection site reactions can be bothersome, they aren't dangerous.

How long do injection site reactions last?

Most irritant injection site reactions only last 1 to 2 days. Immune-mediated reactions can last longer.

Injection site reactions may get worse over the first 24 hours. But things should get better after that. If your symptoms just keep getting worse, something else could be going on. Talk to a healthcare professional right away if the redness is spreading or your pain is getting worse after 24 hours.

When should I worry about an injection site reaction?

More serious symptoms could be a sign that what you're experiencing isn't related to an injection site reaction. The following symptoms could be due to an allergic reaction or a skin infection like cellulitis:

• Fever

• Swelling of your face, lips, or tongue

• Hives

• Trouble breathing

• Vomiting or nausea

• Excessive fatigue

• Trouble moving because of pain

• Blisters at the injection site

• Severe pain over the area

Get medical attention if you experience any of these symptoms after receiving an injection.

How do you treat an injection site reaction?

Injection site reactions will get better and go away on their own. So you don't have to do anything if you have an injection site reaction.

But since they can be uncomfortable, some things that can bring relief while you wait include:

• Warm compress:Warm compresses can be soothing if your muscle is irritated from the injection. You can apply a warm compress for 10 to 20 minutes as needed.

• Ice packs: Ice packs can help bring down redness, warmth, and swelling. They may also help calm itching. Make sure to wrap the ice pack in a towel before placing it on your skin. You can use an ice pack for 10 to 20 minutes as needed for symptom relief.

• Over-the-counter (OTC) pain relievers:Tylenol or Advil will help with pain and bring down swelling and redness.

• Anti-itch medications: If you're having a lot of itching from your injection site reaction, you can try an OTC antihistamine like diphenhydramine (Benadryl).

• Movement: Moving the part of your body where you got an injection will increase blood flow and help the reaction clear up faster. This is especially true for intramuscular injections (into the muscle).

Can you prevent injection site reactions?

You can't completely prevent injection site reactions. But if you take injections frequently, there are things you can do to lower your chance of getting them:

• Lower the dose. Irritative injection site reactions are more likely to develop when you take a higher dose of a medication. Talk to your healthcare team about whether you can try a lower dose. If you can't, you can also ask if you can split your dose.

• Try a new brand. Different brands of medication use different carriers and preservatives. Many injection site reactions are due to these things, not the actual medication itself. So switching brands might stop injection site reactions.

• Get feedback on your technique. Your injection technique could be leading to injection site reactions. Ask a healthcare professional to watch you give yourself an injection. They may have tips on how to prepare your skin and avoid hitting blood vessels.

• Rotate injection sites. If you've been giving yourself injections, you know some sites are much less painful than others. Even though it's tempting to stick to these sites, rotating injection sites will decrease your risk of developing injection site reactions.

• Get medications to room temperature. Room temperature medications are less painful to inject and are less likely to cause injection site reactions.

• Avoid irritated skin and blood vessels. Avoid any sites where your skin is irritated. Whether it's a mild rash, a cut, or an eczema flare, irritated skin is inflamed and more likely to have a reaction.

• Apply a cold compress. Applying a cold compress for 5 to 10 minutes after your injection can limit swelling and redness.

If you get an injection site reaction with one vaccine, will you have a reaction with all vaccines?

Maybe. All vaccines are different - they don't all have the same ingredients. So, you're unlikely to get exposed to the same irritant with every vaccination.

An exception is if you get another injection with the same brand of the same vaccine. If you're in the middle of a vaccine series, like the COVID vaccine, you could get an injection site reaction after follow-up doses if you had a reaction with the first injection. You might also get injection site reactions with yearly vaccines (like the flu shot) or booster shots (like the tetanus shot).

If you developed an injection site reaction to an injectable medication, like a biologic, you'll likely develop an injection site reaction with additional doses. But studies show that your body will get used to the medication. Injection site reactions tend to become smaller and less severe after you've been using the medication for 4 to 8 weeks.

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