The prevalence of seasonal allergic rhinitis (hereinafter “hay fever”) is increasing annually. While self-medication with over-the-counter (OTC) drugs is growing, some hay fever patients also use OTC drugs alongside medical visits. Issues arising from co-administration, such as excessive dosing and reduced quality of life (QOL) due to drowsiness, have been highlighted. However, no research has investigated the factors contributing to the concomitant use of prescribed and OTC drugs. Therefore, this study examined the following three patient groups: those using only prescribed drugs, those using only OTC drugs, and those using a combination of both. Initial analyses compared adherence, overuse extent, QOL (as measured by the Japanese Rhino-conjunctivitis Quality of Life Questionnaire [JRQLQ]), treatment satisfaction, lifestyle habits, and health literacy (as measured by the Communicative and Critical Health Literacy Scale [CCHL]) among the groups. Subsequently, a multinomial logistic regression analysis focusing on combination users identified factors associated with co-administration. The results revealed that combination users exhibited lifestyle issues and lower QOL compared to those using prescribed or OTC drugs only, although differences in CCHL were not significant. Furthermore, co-administration was linked to social life-related JRQLQ and excessive usage during symptom onset. In conclusion, patients with hay fever should avoid the concurrent use of prescribed and OTC drugs. Co-administration may not necessarily improve symptoms and might even lead to overuse risks. Thus, patients should be guided to consult medical professionals before purchasing OTC drugs, and lifestyle improvements should be emphasized as well.