The role of oestrogens and antibiotics on the development of cancer

Despite advances in cancer treatment and surgery, cancer-related mortality remains the leading cause of death globally, harvesting over ten million lives annually. While the risk factors are both hormonal and non-hormonal, common prescription drugs’ long-term effects are understudied and likely even underestimated. Particularly drugs including oestrogens or those altering the metabolism of oestrogens may promote carcinogenesis of oestrogen receptor sensitive cancers and possibly even lower the risk of sex hormone-associated cancers. Two common examples of such prescription drugs are systemic menopausal hormone therapy (MHT) and oral antibiotic treatment. Historically, preventive measures have been the key factor in our unending battle against cancer’s burden. This doctoral thesis within clinical epidemiology was conducted to evaluate the association between exposure to contemporary MHT use and cancer risk (Studies I and II); and colorectal cancer mortality (Study IV). These associations were investigated on a population level, longitudinally, based on nationwide and population-based cohort studies, taking advantage of the unique Swedish prescription feature, including various MHT treatment options. Notably, these large-scale cohort studies are feasible to conduct only in a few other countries than Sweden, underscoring the importance of valid evidence from methodologically well-grounded studies with complete and reliable data sources. Furthermore, Study III aimed to clarify the posed association between exposure to oral antibiotic treatment and colorectal cancer risk based on a systematic review and dose-response meta-analysis. For Studies I, II, and IV, data were retrieved from the Swedish Prescribed Drug Registry, Swedish Cancer Registry, Causes of Death Registry, Patient Registry, and the Total Population Registry (Study I only). Study I evaluated the net effect of systemic MHT use on cancer risk; and 16 different cancer types. The findings suggested an associatio