In many Western jurisdictions cannabis, unlike most other psychoactive drugs, cannot be prescribed to patients even in cases where medical professionals believe that it would ease the patient’s pain or anxiety. The reasons for this prohibitionare mostly ideological, although medical and moral arguments have been formulated to support it. In this paper, it is argued that freedom, properly understood, provides a sound ethical reason to allow the use of cannabis in medicine. Scientific facts, appeals to harm and autonomy, and considerations of symbolic value cannot consistently justify prohibitions.