One of the more recently investigated adverse long-term side effects of gonadotropin-releasing hormone (GnRH) agonists for prostate cancer (PCa) is cardiovascular disease (CVD). Studies suggest lower risk of CVD following GnRH antagonists (degarelix) than GnRH agonists. This protocol describes precise codes used to extract variables from five European databases for a study that compares risk of CVD following GnRH agonists and antagonists for PCa. PCa men on primary GnRH agonists or antagonists were identified from the UK THIN (The Health Improvement Network) database, National Health Service (NHS) Scotland, Belgian Cancer Registry (BCR), Dutch PHARMO Database Network and French National Database (SNIIRAM). Cohort entry was defined as date of treatment initiation. CVD event was defined as any first incident or fatal CVD after cohort entry. Readcodes in THIN and ICD codes in NHS Scotland, BCR, PHARMO and SNIIRAM were used to extract variables. Risk of Bias in Non-randomised studies of Interventions (ROBINS-I) tool was used to assess the potential risk of biases in this study. 51 572 men with a median follow-up time of 2 years started on GnRH agonists and 2 417 men with a median follow-up time of 1 year started on GnRH antagonists between 2010 and 2017 in the UK, Scotland, Belgium, the Netherlands and France. Data from five countries improved the study power and internal validity required to compare risk of CVD between GnRH agonists and antagonists, the latter being a fairly new drug with limited data in individual countries.