Cannabis consumption is legally prohibited in most countries in the world. Several countries are legalising cannabis for adult consumption. It is important to monitor the public health effects of these policy changes. In this paper, we summarise the evidence to date on the legalisation of adult non-medical cannabis use in Canada and the USA. We describe regulatory models for cannabis legalisation, changes in cannabis products and pricing, effects on the illicit cannabis market, changes in cannabis use, and changes in cannabis-related physical and mental health harms. We discuss the challenges in assessing the effects of cannabis legalisation on public health outcomes and emphasise the importance of continuous and rigorous monitoring of adverse health effects to inform the design of public health policies and regulations.