The Most Common Side Effect of an Allergy Shot: What to Expect

The most common side effect of an allergy shot is a local reaction at the injection site — redness, swelling, and itching at the spot where the needle was inserted, typically the posterior upper arm. This occurs in 26-86% of allergy shot patients depending on the study and definition used, making it by far the most frequently experienced adverse event of SCIT. The reaction consists of an immediate IgE-mediated wheal forming within 20-30 minutes, and sometimes a late-phase swelling that develops 6-12 hours later. Most local reactions resolve within 24 hours. They are expected, normal, and do not require clinical intervention unless they exceed palm-size or persist beyond 48 hours. By comparison, systemic reactions — the more serious category — occur in only 0.1-0.2% of injection visits, roughly 500-1000 times less frequently.

The immediate wheal — the small bump appearing within 20-30 minutes of the injection — typically resolves within 1-3 hours. A late-phase local reaction may develop 6-12 hours after the injection as eosinophils migrate to the site; this can persist for 24-48 hours and sometimes longer for larger reactions. Most injection-site reactions from allergy shots are fully resolved within 24-48 hours. Large local reactions (LLRs — swelling larger than 2.5 cm or palm-sized) can take 24 hours to 10 days to fully resolve in the most significant presentations. If swelling has not begun to improve at all after 48 hours, or if it is worsening instead of improving, contact your allergist. Persistent firm nodules lasting weeks to months are a separate entity (associated with aluminum-adsorbed depot extracts) and should be evaluated by the allergist.

Yes, it is normal for an allergy shot to cause a bump at the injection site, though the size matters for how to interpret it. A small bump or wheal — under about 2.5 cm in diameter, roughly the size of a U.S. quarter — is an expected local allergic reaction occurring in the majority of allergy shot patients. A larger bump exceeding 2.5 cm (a large local reaction, or LLR) occurs in 0.4-5.3% of injections and should be reported to your allergist before the next injection, but is not itself an emergency. Large local reactions can range from quarter-sized to palm-sized and may peak at 24-48 hours. A single large local reaction does not predict that you will have a serious systemic reaction from your next shot. However, a recurrent pattern of large local reactions across multiple shots should be evaluated by your allergist for possible premedication or dose assessment.

The late-phase reaction to an allergy shot develops 6-12 hours after the injection, often after the patient has already left the clinic and noticed the immediate swelling subsiding. It feels like a return and sometimes amplification of the initial injection-site reaction: swelling and induration (firmness) at the shot location, with warmth, itching, and aching. The area may feel larger and more uncomfortable than the immediate reaction. The late-phase is driven by the recruitment of eosinophils and T-cells to the injection site by cytokines from the initial mast cell response — an inflammatory cellular infiltrate that takes hours to develop. It peaks at 24-48 hours and then gradually resolves. Ice applied to the site, oral antihistamines, and over-the-counter ibuprofen or acetaminophen for discomfort are all reasonable symptomatic measures.

Fatigue or malaise after allergy shots is one of the most commonly reported patient experiences but one of the least studied. It does not appear as a discrete, quantified adverse event in AAAAI/ACAAI national surveillance data. The mechanism is plausible: allergen injection triggers late-phase immune activation including release of cytokines such as IL-1, IL-6, and TNF-alpha — the same mediators responsible for sickness behavior and central-nervous-system fatigue signaling in other inflammatory contexts. This is similar to the fatigue mechanism seen in cancer immunotherapy. Post-injection fatigue, when it occurs, typically lasts hours to about 24 hours and is more common during build-up or after dose escalations. Fatigue lasting more than 48 hours, or accompanied by hives, throat tightness, wheezing, or fever, should be reported to the allergist as it may represent a delayed systemic reaction rather than routine post-injection fatigue.

Most — but not all — allergy shot patients experience at least some degree of local injection-site reaction. Rates in published studies range from 26-86% of patients experiencing any local reaction, depending on how the reaction is defined and measured. The wide range reflects genuine variability: sensitization level, allergen type, dose, and individual immune responsiveness all influence how much local reaction a given patient experiences. Some patients notice virtually no injection-site response; others have consistent large local reactions at every visit. Over the course of treatment, local reaction frequency and intensity typically decrease for most patients as immunologic tolerance develops — one of the signs that the treatment is working. Serious systemic reactions — the more dangerous category — occur in only 0.1-0.2% of injection visits across AAAAI/ACAAI surveillance of 54 million injection visits.

For a local injection-site reaction — redness, swelling, or itching at the shot site — apply ice for 10-20 minutes, take an oral antihistamine (cetirizine 10 mg or loratadine 10 mg) if the itching is bothersome, and do a brief self-observation after the dose. You do not need to contact anyone for a typical small local reaction. For any symptom extending beyond the injection site — hives elsewhere on your body, throat tightness, wheezing, or dizziness — message your care team right away. Do not wait to see if symptoms resolve on their own; systemic reactions require prompt evaluation. For signs of anaphylaxis — severe breathing difficulty, throat swelling with stridor, loss of consciousness, or cardiovascular symptoms — use your prescribed epinephrine auto-injector and call 911. Confirming a prescribed epinephrine auto-injector is on hand before the first injection, and having your first dose and every dose change supervised live over Zoom, are core safeguards of at-home SCIT.