Quick Answer
The most common allergy shot side effects are local reactions at the injection site — redness, swelling, and itching that typically peak within 30 minutes and resolve within 24 hours. Systemic reactions (hives, asthma, anaphylaxis) are rare, occurring in approximately 0.1% of injections. All reactions occur during or within 30 minutes of injection, which is why you must wait at the office after each shot.
Local Reactions: Normal and Expected
Local reactions at the injection site are the most common and expected side effect of allergy shots. Redness, swelling, itching, and a raised bump (wheal) at the injection site are normal immune responses indicating that allergen is engaging mast cells. Local reactions that remain smaller than a quarter (approximately 25mm) are considered acceptable and do not require dose adjustment.
Large local reactions — swelling greater than 25mm lasting more than 24 hours — should be reported to your allergist, as they may prompt dose reduction at the next visit to minimize the risk of a larger systemic reaction. Applying a cold pack to the injection site immediately after the shot and taking an antihistamine beforehand (if your protocol allows) can reduce local reaction size.
Systemic Reactions: Rare but Important
Systemic reactions involve symptoms beyond the injection site and occur in approximately 0.1% of all allergy shot injections. They range from mild (generalized hives, nasal symptoms, asthma) to moderate (angioedema, significant asthma) to severe (anaphylaxis with hypotension, bronchospasm). The vast majority of systemic reactions occur within 20–30 minutes of injection — explaining why all patients must wait in the office for 20–30 minutes after each shot.
Risk factors for systemic reactions include poorly controlled asthma on the day of the injection, dosing errors (receiving too high a dose), high-pollen-day appointments (when baseline allergen load is already elevated), and beta-blocker use (which impairs epinephrine response). Your allergist's office assesses these risk factors before each dose.
What Happens If a Systemic Reaction Occurs
If a systemic reaction occurs in the allergy office during the mandatory observation period, trained medical staff administer epinephrine immediately, place the patient supine, establish IV access if needed, and monitor vital signs. This is why all immunotherapy must be administered in physician offices equipped for anaphylaxis treatment — home self-injection of allergy shots is absolutely contraindicated.
After a systemic reaction, the dose is reduced by 50% or more for the next visit, and the buildup phase is slowed. Most patients who have mild systemic reactions can safely continue immunotherapy at adjusted doses. Patients who have severe anaphylactic reactions may need evaluation for whether to continue or modify the treatment protocol.
Minimizing Side Effects
Pre-treatment antihistamine administration 30–60 minutes before allergy shots significantly reduces local reaction size and lowers the risk of systemic reactions. Several randomized trials support pre-medication with cetirizine or loratadine before shots during the buildup phase. Discuss pre-medication with your allergist.
Avoid exercising vigorously within 2–4 hours after an allergy shot — exercise increases blood flow and allergen absorption, increasing systemic reaction risk. Avoid allergy shots on high-pollen days if your protocol allows flexibility, and always disclose any respiratory symptoms or asthma flare before each injection. Never skip the post-injection observation period — even if prior visits have been uneventful.
Key Takeaways
- Local reactions (redness, swelling, itch) are normal and occur in 80% of patients — wheal under 25mm is acceptable.
- Systemic reactions occur in ~0.1% of injections — always occur within 30 minutes, requiring mandatory waiting period.
- Pre-medication with cetirizine 30–60 minutes before shots reduces local reaction size significantly.
- Avoid vigorous exercise within 2–4 hours after allergy shots to reduce absorption and reaction risk.
- Never self-administer allergy shots — emergency equipment must be immediately available at all times.
Related Guide
Allergy Treatment Hub →