Seasonal purchase of antihistamines and ovarian cancer risk in the Cancer Loyalty Card Study (CLOCS): results from an observational case-control study

Objective: Antihistamine use has previously been associated with a reduction in incidence of ovarian cancer, particularly in premenopausal women. Herein, we investigate antihistamine exposure in relation to ovarian cancer risk using a novel data resource by examining purchase histories from retailer loyalty card data. Study Design: A subset of participants from the Cancer Loyalty Card Study (CLOCS) for which purchase histories were available were analysed in this study. Cases (n=153) were women in the UK with a first diagnosis of ovarian cancer between Jan 2018 to Jan 2022. Controls (n=120) were women in the UK without a diagnosis of ovarian cancer. Up to 6 years of purchase history was retrieved from two participating high street retailers from 2014 to 2022. Main outcome measures: Logistic regression was used to estimate the odds ratio (OR) and 95% confidence intervals (CIs) for ovarian cancer associated with antihistamine purchases, ever versus never, adjusting for age and oral contraceptive use. The association was stratified by season of purchase, age over and under 50 years, ovarian cancer histology, and family history. Results: Ever purchasing antihistamines was not significantly associated with ovarian cancer overall in this small study (OR:0.68, 95% CI: 0.39,1.19). However, antihistamine purchases were significantly associated with reduced ovarian cancer risk when purchased only in spring and/or summer (OR: 0.37, 95% CI: 0.17,0.82) compared with purchasing all year (OR: 0.99, 95% CI: 0.51,1.92). In the stratified analysis, the association was strongest in non-serous ovarian cancer (OR: 0.41, 95% CI:0.18,0.93). Conclusions: Antihistamine purchase is associated with reduced ovarian cancer risk when purchased seasonally in spring and summer. However, larger studies and more research is required to understand the mechanisms of reduced ovarian cancer risk related to seasonal purchases of antihistamines and allergies.