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<b><i>Background:</i></b> As the number of allergic sensitizations increases the severity of allergic respiratory diseases worsens. Multiple monoallergen immunotherapy can be accompanied by poor treatment adherence and high costs, single multiallergen immunotherapy needs to prove efficacy whilst maintaining a good safety profile. <b><i>Methods:</i></b> Observational, retrospective, multicenter study using a 2-pollen single undiluted multiallergen subcutaneous immunotherapy (SCIT) in routine clinical practice in Spain. Patients with rhinoconjunctivitis, with/without controlled asthma, sensitized to grass, olive, Parietaria, Cupressus, plane tree and/or Salsola pollen were included. Primary and secondary clinical efficacy endpoints were quality of life (mini Rhinitis Quality of Life Questionnaire (miniRQLQ)) and visual analogue scale (VAS) respectively. All adverse events were documented. <b><i>Results:</i></b> Ten centers included 97 patients, median age 32 years. SCIT treatment included combinations of grass mix with olive, Parietaria, Cupressus, plane tree or Salsola or olive with Parietaria, Cupressus or Salsola. The mean duration of SCIT was 1.8 years with a high treatment adherence (73%). Significant improvement in quality of life, nasal and ocular symptoms, activity limitations and practical problems (<i>p</i>&#x3c; 0.0001) and other symptoms (<i>p</i>= 0.024) was observed. Most patients did not develop asthma-like symptoms and a significant improvement of all allergic symptom severity was perceived. VAS showed a significant improvement in rhinoconjunctivitis and asthma by patients and physicians. Twenty-nine patients experienced adverse reactions, 25 had local and 6 had systemic reactions. <b><i>Conclusions:</i></b> Single undiluted multiallergen SCIT treatment of two different pollens is efficacious and safe in both children and adults, showing that it is a suitable option for the treatment of polyallergic patients.

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