Sublingual Immunotherapy vs Allergy Shots: A Clinical Comparison

Both deliver allergen to drive immune tolerance, but the entry route differs. Sublingual immunotherapy places allergen under the tongue, where the oral mucosa contains a high density of tolerance-promoting Langerhans-type dendritic cells that capture antigen and prime regulatory T-cell responses with limited systemic exposure. Allergy shots inject allergen into subcutaneous tissue, producing more direct systemic uptake and a more robust antibody shift but also a higher chance of systemic reactions. Both ultimately raise blocking IgG4 antibodies and shift the immune response away from the allergic Th2 pattern. The shared endpoint — durable tolerance after years of dosing — is reached by different routes, which is why monitoring needs differ even when efficacy is similar.

For grass pollen, the strongest head-to-head evidence suggests comparable efficacy. Nelson et al. 2015, a network meta-analysis in the Journal of Allergy and Clinical Immunology: In Practice, found a symptom-score difference of just 0.01 between the routes. Placebo-controlled Cochrane reviews show meaningful symptom reduction for both. However, indirect comparisons such as Di Bona et al. (2012) favored injections with larger effect sizes, and direct trials have been small. The injection route has a broader and stronger evidence base for cat, mold, and tree pollen, where sublingual data are thinner. Overall, the routes appear broadly comparable for common pollens while shots retain an evidence edge for certain allergens.

The difference comes down to systemic-reaction risk tied to the route. Because the oral mucosa promotes tolerance with limited systemic allergen exposure, sublingual immunotherapy has a substantially lower rate of systemic reactions and no fatalities reported worldwide, which supports at-home dosing after an initial supervised dose. Subcutaneous injection delivers allergen more directly to the circulation, carrying a small but real risk of systemic reactions, so allergy shots are given in a clinic with a 30-minute observation window to manage any reaction promptly. The route, not the allergen itself, is what changes the safety logistics. Even so, sublingual users are typically prescribed epinephrine and should follow their provider's guidance.

Allergy shots are well suited to multi-allergen treatment: a clinician can formulate customized vials covering grasses, trees, weeds, dust mites, animal dander, and mold matched to a patient's sensitization profile. On the sublingual side, FDA-approved tablets each treat only a single allergen, which is limiting for the majority of patients who react to several triggers. Compounded multi-allergen sublingual drops can address multiple allergens at once and are used off-label, but they lack the standardized phase-3 evidence base of the approved tablets. So while both routes can in principle cover multiple allergens, the regulatory and evidentiary picture differs, and the choice depends on how many allergens you need to treat.

Both routes require a sustained course to achieve lasting benefit. The GRASS trial (Scadding et al., JAMA 2017) showed that two years of either sublingual or subcutaneous immunotherapy was not enough to produce benefit that persisted after stopping. Three to five years is the generally accepted course for durable disease modification by either route. Allergy shots typically involve a weekly build-up phase followed by maintenance dosing on an individualized schedule, while sublingual immunotherapy is taken daily at home throughout. Stopping early shortens the durability of benefit. Patients should plan for a multi-year commitment regardless of route and discuss the expected timeline with their allergist before starting.