Background: Symptoms and treatment of benign prostatic hyperplasia (BPH) or erectile dysfunction (ED) may lead to prostate cancer workup, and patterns of prescriptions before diagnosis may affect findings of pharmacoepidemiological studies. Usage of BPH and ED drugs after diagnosis may be related to prostate cancer treatment. We investigated differences in prescription rates of BPH and ED drugs among prostate cancer patients and cancer-free comparisons and between patients with localized and non-localized disease. Material and methods: A nationwide register-based study, including all Danish men aged 50–85 years diagnosed with prostate cancer during 1998–2015 and an age-matched comparison cohort without cancer. We calculated rates of new and total prescriptions in 1-month intervals from 3 years before to 3 years after cancer diagnosis for drugs used to treat BPH and ED, overall and stratified by clinical stage. Results: We identified 54,286 men with prostate cancer and a comparison cohort of 249,645 age-matched men. The new prescription rate for BPH drugs increased for men with prostate cancer in the year before diagnosis and peaked 1 month before diagnosis with an 18-fold higher rate. Men with prostate cancer had a higher total prescription rate of BPH drugs 3 years before diagnosis, notably among men with localized disease. Before diagnosis, the new prescription rates for ED drugs were similar among men with prostate cancer and comparisons. After diagnosis, men with prostate cancer had a 7-fold higher rate of new prescriptions for ED drugs. Among men with localized disease, the total prescription rate of ED drugs increased in the months following diagnosis. Conclusion: Differences in prescription rates suggest increased prostate cancer surveillance among men receiving BPH drugs, whereas the post-diagnostic increase in ED drugs among men with localized disease is compatible with the increased risk of ED following prostate cancer treatment.