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Background: The safety profile of venom immunotherapy (VIT) is a relevant issue, and considerable differences have been reported in the safety and efficacy of this treatment modality. The primary aim of this study was to evaluate the safety of angiotensin-converting enzyme inhibitors and ß-blockers during VIT. In a second analysis, we evaluated data on premedication and venom preparations and their association with systemic adverse events (AEs) during the up-dosing phase and the first year of the maintenance phase, as well as the outcome of field stings and sting challenges., Methods: Ours was an open, prospective, observational, multicenter study that recruited 1425 patients, of whom 1342 underwent VIT., Results: Premedication with oral antihistamines was taken by 52.1% of patients during up-dosing and 19.7% of patients during the maintenance phase. Antihistamines had no effect on the frequency of systemic AEs (P=.11), although large local reactions (LLRs) were less frequent (OR, 0.74; 95%CI, 0.58-0.96; P=.02). Aqueous preparations were preferred for up-dosing (73.0%), and depot preparations were used for the maintenance phase (64.5%). The type of venom preparation had no influence on the frequency of systemic AEs or on the effectiveness of VIT (P=.26 and P=.80, respectively), while LLRs were less frequent with depot preparations (P<.001)., Conclusions: Pretreatment with oral antihistamines during VIT significantly reduces the frequency of LLRs but not systemic AEs. All venom preparations were equally effective and did not differ in terms of the frequency of systemic AEs.