Drug repurposing is potentially cost-effective, low risk, and necessary in psychiatric drug development. The availability of large, routine data sets provides the opportunity to evaluate the potential for currently used medication to benefit people with serious mental illness. Using linked Swedish register data we set out to determine whether hydroxylmethyl glutaryl coenzyme A reductase inhibitors, L-type calcium channel antagonists, and biguanides are associated with reduced psychiatric hospitalisation and self-harm in individuals with severe mental illness.