Aims: In October 2019, a citizens’ initiative to decriminalise cannabis use started a large debate about drug policy in Finland. This study examines online discussions about the initiative to supplement the current knowledge about citizens’ drug opinions. The focus is especially on argumentation techniques that are used to support or object to the decriminalisation. Design: Methodologically, the study is based on discourse studies, new rhetoric, and argumentation analysis. The data of 1,092 messages were collected from a popular Finnish anonymous discussion forum Ylilauta. Results: Online discussions about the legal status of cannabis are highly polarised. Decriminalisation is often both supported and resisted in a strong and affective manner, and even hate speech is not rare in the data. Statements made by both discussion parties often lack any argumentation or are based on fallacies, especially ad hominem arguments. Some discussants refer to scientific studies and expert statements, even though such references are usually inaccurate. Cannabis is compared to alcohol more often than to other illegal drugs. Conclusions: The emotional responses and inadequate argumentation might be partially explained by the general nature of online discussions and the culture of the investigated website, but also by the powerful stigma related to illegal drugs and insufficient knowledge on the subject. A future objective is to create a societal atmosphere where the complex question of the legal status of cannabis could be discussed more neutrally and rationally.