Cancer and diabetes share a bidirectional relationship, with cancer increasingly recognised as an associated factor of diabetes, both from the disease itself and its treatments. In this Review, we aim to: summarise the distinct mechanisms of dysglycaemia arising from cancer and neuroendocrine tumour treatments, including targeted therapies such as PI3K–AKT–mTOR inhibitors, antibody–drug conjugates, immune checkpoint inhibitors, corticosteroids, and somatostatin receptor ligands; examine the often overlooked aspect of the secondary cause of diabetes as an early manifestation of cancer; and highlight research gaps and encourage a collaborative care approach to manage the rising rate of dysglycaemia as a result of cancer and its evolving treatments.