Cancers often spread to distant sites in the body. This process is called metastasis and is regulated by sodium channels located in the membranes of cancer cells. Sodium channels are also found in the nervous system, and drugs used that block these channels are used in the treatment of epilepsy. One such drug, phenytoin, has been found to reduce metastasis in mice, suggesting that anti-epileptic drugs may have a role in treating cancer. However, in previous research we found that patients taking similar drugs were less likely to survive following a diagnosis of cancer, possibly because they also had epilepsy. We now wish to use a richer dataset to study the relationships between epilepsy, cancer, sodium channel blocking drugs and survival in more detail, including examining specific causes of death. If successful, this would facilitate clinical trials of sodium channel blocking drugs for patients with breast cancer.