| Sumario: | Objectives This study aimed to retrospectively assess whether community pharmacy customers accepted the pharmacist's recommendations for the selection of an antihistamine based on medicine optimization guidelines. Methods A retrospective study was conducted on the implementation of an antihistamine use optimization guide for patients who were seeking first-generation antihistamines between July and December 2019 across forty-five community pharmacies in Singapore. The primary outcome measure was the acceptance rate of ceasing or substituting first-generation antihistamine with a second-generation antihistamine. Secondary measures included the reduction in types of first-generation antihistamines used, adverse drug-related events reported, intended use of antihistamines and the types of recommendations. Key findings During the study period, 2328 patients fulfilled the inclusion criteria, out of which 523 patients agreed to optimize their use of sedating first-generation antihistamines. Chlorpheniramine (95.2%) was the most widely accepted first-generation antihistamine for optimization, with 59.6% of the users experiencing adverse events, the most common being drowsiness (53.2%). The main indication of use was allergic conditions (allergic rhinitis or atopic dermatitis) (70.3%). After implementation of the guide, most interventions were direct substitution (72.8%) with a less-sedating antihistamine, followed by gradual tapering (22.6%). Loratadine, a second-generation antihistamine, was most frequently (51.4%) used to substitute first-generation antihistamines. The optimization guide can potentially reduce adverse effects in 59.6% (297 patients) of chlorpheniramine users, which were mainly drowsiness (265 patients; 53.2%) and dry mouth (14 patients; 2.8%). Conclusions This study highlighted the importance of assessing and reducing potentially inappropriate first-generation antihistamine self-use and that a guided approach and substitution with less-sedating antihistamines can be employed in the community pharmacy setting.
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