To estimate the prevalence of self-reported adverse reactions (AdR) to subcutaneous airborne allergen immunotherapy (SCIT) and to describe factors associated with its occurrence. Real-life, observational, descriptive study of all patients treated with SCIT at a Portuguese allergy unit between 03/2017 and 06/2019, and who answered ≥1 time to a pre-SCIT evaluation questionnaire assessing the occurrence of local and/or systemic AdR in the previous administration. 939 questionnaires from 231 patients (42% female, 35% with asthma) were included. Most (60%) SCIT preparations had multiple allergens with concentration adjusted to prevent dilution (MA-NoDil), 26% were single allergen with standard concentration (SA-SC), 10% single allergen with higher than standard concentration (SA-HC), and 4% mixtures without concentration adjustment (MA-Dil). SCIT-related AdR were self-reported in 313 (33%) administrations, 97% at the injection site and 11% grade 1 systemic symptoms. In a multivariable model, being a female and having asthma were associated with higher risk of AdR. MA-NoDil SCIT presented a lower risk of AdR compared to SA-SC SCIT. SCIT-related AdR were self-reported in 1/3 of the administrations, most at the injection site. The risk of AdR was higher in females and in patients with asthma. The lower risk of adverse reactions observed in SCIT preparations with multiple allergens with no dilutional effect should be further explored in future, targeted studies.