BACKGROUND: The standardised quality (SQ) treesublingual immunotherapy (SLIT)-tablet has recently been approved for treatment of tree pollen allergy. Health care workers should be providedwith detailed safety data for clinical use. OBJECTIVE: To assess the tolerability and safety of the SQ tree SLIT-tablet (12 SQ-Bet)in adults and adolescents. METHODS: Safety data were pooled fromthree double-blinded, randomized, placebo-controlled trials (2 phase-II/1 phase-III) including adults and adolescents 12-65 years with allergic rhinitis and/orconjunctivitis treated before and during one pollen season once-daily with 12 SQ-Bet (n=471) or placebo (n=458): EudraCTno:2012-000031-59; NCT02481856; EudraCT 2015-004821-15. RESULTS: The most frequently reportedinvestigational medicinal product(IMP)-related AEs with 12 SQ-Bet were oral pruritis (39% of subjects)and throat irritation (29%).IMP-related AEs were mainly mild or moderate in severity,and the majority resolved without treatment and did not lead to treatment interruption/discontinuation. With 12 SQ-Bet, oral pruritus was more frequent among subjects with pollen food syndrome (PFS) (45%) than without PFS (29%).The 12 SQ-Bet did not seem to induce an increased risk of asthma: 7 events were reported in 7 subjects with 12 SQ-Bet and 11 in 10 subjects with placebo. No differences were seen in the risk of moderate to severe IMP-related AEs regardless of age, PFS status and asthma medical history. CONCLUSIONS: The 12 SQ tree SLIT-tablet was well tolerated in tree pollen allergic subjects with no major safety concerns detected.This safety profile supports daily at-home sublingual administration once the first dose is tolerated when administered under medical supervision.