Anaphylaxis (a severe allergic reaction affecting two or more body systems) is the most serious potential side effect of allergy shots.  The condition can quickly progress to life-threatening anaphylactic shock, which is characterized by difficulty breathing and a dangerous drop in blood pressure. Symptoms of anaphylaxis include:

• Hives on areas of the body other than the injection area
  
  
• Shortness of breath
  
  
• Chest pain or tightness
  
  
• Swelling of the throat or tongue (angioedema)
  
  

  
  

• Choking
  
  
• Loss of consciousness

A medical professional can administer epinephrine (and, possibly, an antihistamine), which usually provides immediate relief from the dangerous reaction. Those with medical conditions such as asthma are at increased risk for anaphylaxis, so the condition must be well-controlled before any asthmatic undertakes immunotherapy. Though rare, the risk of anaphylaxis must be weighed by anyone considering immunotherapy.

More common – and less serious – potential side effects include:

• The triggering of allergic rhinitis or asthma symptoms
  
  
• Itching or hives
  
  
• Swelling of the skin near the injection site or of the entire arm
  
  
• Burning, stinging or redness at the injection site

These symptoms usually pass within a few hours.