Allergen immunotherapy, commonly known as allergy shots, is a long-term treatment designed to reduce allergy symptoms by gradually exposing the body to increasing amounts of specific allergens. This process, also called subcutaneous immunotherapy (SCIT), helps the immune system build tolerance over three to five years. While the goal is to shift the immune response away from allergic reactions, the introduction of the allergen extract can temporarily cause side effects. Patients often wonder if this treatment can lead to gastrointestinal issues, such as nausea.
The Likelihood of Gastrointestinal Side Effects
The most frequent side effects experienced with allergy shots are confined to the injection site, appearing as local redness, swelling, or itching. Gastrointestinal symptoms like nausea or stomach cramps are far less common, as they are considered systemic reactions that affect the entire body. Systemic reactions can range from mild symptoms, such as sneezing or hives, to a serious event like anaphylaxis.
Mild nausea is a recognized, though infrequent, manifestation of a mild systemic reaction. This response usually occurs rapidly, typically within the 30-minute observation period following the shot. Although generally brief and self-limiting, nausea should be reported to clinic staff, as it signals a systemic reaction to the injected allergen dose.
Why Immunotherapy Can Affect the Stomach
Nausea following an allergy shot is a direct consequence of the immune system’s response to the allergen extract. When the body encounters an allergen, immune cells release chemical mediators, primarily histamine. Histamine is responsible for classic allergy symptoms, such as itching and swelling.
Histamine also significantly impacts the digestive system, not just the skin and airways. Receptors for histamine (H1 and H2 types) are present within the lining of the gastrointestinal tract. When released systemically, histamine stimulates smooth muscle contractions in the intestines, which can cause cramping or discomfort. This systemic release of inflammatory mediators can also affect the nervous system, potentially triggering nausea.
Strategies for Managing Nausea After Treatment
If mild nausea occurs after an injection, immediate communication with the allergist is necessary. The healthcare provider may need to adjust the dose of the allergen extract, often by moving back to a lower, better-tolerated concentration. Adjusting the dose helps prevent repeated systemic reactions while allowing the immune system to build tolerance.
Patients can also employ behavioral strategies. It is helpful to avoid receiving the shot on an empty stomach, as eating a small amount of food beforehand can lessen the likelihood of digestive upset. Patients should report any signs of nausea during the mandatory 30-minute observation period following the injection, which is in place to monitor for systemic reactions.
If nausea persists despite dose adjustments, the allergist may suggest pre-medicating with an oral antihistamine before future injections. Severe nausea, especially if accompanied by vomiting or other systemic symptoms like difficulty breathing, widespread hives, or dizziness, signals a severe allergic reaction. Immediate medical attention is required in such instances.