In allergic patients coexistence of other diseases such as cardiovascular, metabolic or neoplastic is frequently encountered. This creates a potential risk of interaction between allergen immunotherapy (AIT) and therapies used for therapy of coexistent diseases. The risk implies potential effects of medications used for coexistent diseases on efficacy and/or safety of AIT as well as potential effects of AIT on concomitant pharmacotherapy. The decision making regarding introduction of AIT in such patients should rely on profound analysis of risks and benefits of therapeutic options under consideration performed by an experienced allergist.