Background: Randomized controlled trials have demonstrated the efficacy of allergy immunotherapy (AIT) in allergic rhinitis (AR) and the disease-modifying effects of the SQ grass sublingual immunotherapy (SLIT) tablet. Objective: We sought to assess real-world, long-term effectiveness and safety across AIT subgroups: route of administration, therapeutic allergen, persistence to AIT, and SQ grass SLIT tablet. Methods: The primary outcome of AR prescriptions from a retrospective cohort study (REAl-world effeCtiveness in allergy immunoTherapy; 2007-2017) was assessed across prespecified AIT subgroups in subjects with AR with and without AIT prescriptions (controls). Safety was assessed as anaphylaxis for 2 days or less of the first AIT prescription. Subgroup follow-up continued until samples were fewer than 200 subjects. Results: Subcutaneous immunotherapy (SCIT) and SLIT tablets showed similarly greater reductions in AR prescriptions than controls (SCIT vs SLIT tablets: year 3, P = .15; year 5, P = .43). Comparably greater reductions in AR prescriptions were observed for grass- and house dust mite–specific AIT than for controls, but significantly smaller reductions were observed for tree-specific AIT (tree vs house dust mite, and vs grass: years 3 and 5, P < .0001). Persistence to AIT was associated with greater reductions in AR prescriptions versus nonpersistence (persistence vs nonpersistence: year 3, P = .09; year 5, P = .006). SQ grass SLIT tablet showed sustained reductions versus controls for up to 7 years (year 3, P = .002; year 5, P = .03). Rates of anaphylactic shock were low (0.000%-0.092%), with no events for SQ SLIT tablets. Conclusions: These results demonstrate real-world, long-term effectiveness of AIT, complement disease-modifying effects observed in SQ grass SLIT-tablet randomized controlled trials, and highlight the importance of using newer evidence-based AIT products for tree pollen AR.