Background
Because long-term effectiveness of pollen allergen immune therapy (AIT) for allergic rhinitis (AR) is not well-described, we studied effectiveness over 18 years in Denmark.

Methods
A register-based cohort study using data on filled prescriptions, 1995–2016, Denmark. In a cohort of 1.1 million intranasal corticosteroid inhaler users (proxy for AR), we matched users treated with grass, birch or mugwort AIT 1:2 with non-treated users on baseline year and 24 characteristics in the 3 years prior to baseline. The primary outcome was the odds ratio (OR) of using anti-allergic nasal inhaler during the pollen season in the treated versus non-treated group by years since baseline.

Results
Among 7760 AR patients treated with pollen AIT, the OR of using nasal inhaler 0–5 years after baseline was reduced when compared with 15,520 non-treated AR individuals (0–2 years, OR 0.84 (0.81–0.88); 3–5 years, OR 0.88 (0.84–0.92)), but was close to unity or higher thereafter (6–9 years, OR 1.03 (0.97–1.08); 10–18 years, OR 1.18 (1.11–1.26)). In post hoc analyses, results were more consistent for those who already had 3 of 3 baseline years of use, and in patients using nasal inhaler in the latest pollen season (0–2 years, OR 0.76 (0.72–0.79); 3–5 years OR 0.86 (0.81–0.93); 6–9 years, OR 0.94 (0.87–1.02); 10–18 years, OR 0.94 (0.86–1.04)) as opposed to no such use.

Conclusions
Patients treated with pollen AIT in routine care to a higher degree stopped using anti-allergic nasal inhaler 0–5 years after starting the standard 3 years of therapy, and not beyond 5 years. Post hoc analyses suggested effectiveness was more consistent among patients with persistent AR.