Side Effects of an Allergy Shot: What to Expect From Your Next Injection

Three pre-visit checks matter for every allergy shot appointment. First, confirm your asthma is well-controlled — uncontrolled asthma is present in 88% of confirmed immunotherapy fatalities (Bernstein 2004, JACI), and injections should be delayed if your asthma is actively flaring. Second, avoid intense exercise for at least 2 hours before your shot, since vigorous activity increases circulation and can amplify the systemic distribution of the injected allergen. Third, inform your allergist if you are in your peak pollen season, if you recently started or stopped any medications (especially beta-blockers or ACE inhibitors), or if you had a significant local or systemic reaction at your last visit. These three pieces of information help your allergist make the dose decision for the day.

A normal reaction at the injection site is a small red wheal — smaller than a US quarter, roughly 2.5 cm or less — with mild itching and warmth that develops within minutes of the shot and resolves within a few hours. Apply ice for 10-20 minutes and take an oral antihistamine if the itching is bothersome. Mild arm soreness is also normal, similar to a flu vaccine. If the swelling is still present the next morning and has grown larger than expected, this is a large local reaction (LLR) — peaks at 24-48 hours and resolves over several days. An LLR looks more alarming than it is. It warrants a call to your allergist before your next injection, but it is not a medical emergency on its own (James & Bernstein 2017, Curr Opin Allergy Clin Immunol).

You should avoid intense exercise for at least 2 hours after each allergy shot. Physical activity increases your heart rate and circulation, which can accelerate the systemic absorption of the allergen extract and potentially increase the risk of a systemic reaction. Light walking or gentle activity is generally acceptable, but vigorous exercise, hot showers, saunas, and hot tubs should all be avoided during the 2-hour post-injection window. Some allergists extend this recommendation to 4 hours for patients who are starting a new vial, entering peak pollen season, or have a history of previous systemic reactions. If you exercised immediately before your last injection and had a reaction, mention this to your allergist — the timing may be relevant.

Leaving before the 30-minute observation window is complete significantly increases the risk that a systemic reaction will occur without trained medical staff present. Approximately 85% of systemic reactions from allergy shots begin within 30 minutes (Epstein 2011, Ann Allergy), which means the observation period is specifically designed to catch the great majority of reactions while epinephrine and resuscitation equipment are immediately available. If you leave early and a reaction develops, you may be alone, without epinephrine, and too far from emergency care for timely treatment. Three of 17 confirmed fatalities in the Bernstein 2004 survey occurred after patients had already left the clinic — underscoring how the 30-minute window is a safety infrastructure element, not a bureaucratic formality.

Contact your allergist's office before your next injection to describe what happened in detail — the size of any local swelling, whether symptoms appeared outside the injection arm, and how long they lasted. Your allergist will use this information to adjust your protocol. After a Grade 1 systemic reaction (mild hives or sneezing only), the standard approach is to reduce to approximately 50% of the reaction-causing dose. After a Grade 2 reaction (multiple organ systems), a more significant reduction of about 10% of the reaction-causing dose is typical. Your allergist may also recommend antihistamine premedication for future visits, a slower escalation schedule, or extended observation time. A single large local reaction at the injection site without any systemic symptoms typically does not require dose adjustment, though recurrent large local reactions do warrant discussion (Calabria 2011, REPEAT Study, Ann Allergy).

Getting an allergy shot when you are acutely ill, especially with a respiratory infection, is generally not recommended and may be postponed by your allergist. Active illness — particularly respiratory infections — can cause temporary changes in airway reactivity that are similar to uncontrolled asthma, potentially increasing your reaction risk during the injection. The 2011 AAAAI/ACAAI Practice Parameter recommends pre-injection assessment of symptoms, and most allergists will delay an injection if you have fever, active wheezing, or significant upper respiratory symptoms. Call your allergist's office before your appointment if you feel unwell — most practices would rather reschedule than risk an injection on a day when your baseline is altered. Missing one or two injections due to illness is manageable; your allergist will adjust the dose when you return depending on how long the gap was.