The present invention refers to a medical line set (10) having a breakable coupling device (90), and to such a coupling device (90). More particularly the present invention refers to an medical dialysis line set (10), such as a peritoneal dialysis (PD) line set, including a liquid introducing portion (20), a liquid distribution portion or PD portion (40) and a drain portion (80), for introducing and extracting dialysis liquid from a patient, the line set particularly including said coupling device (90) for connecting and disconnecting the liquid introducing portion (20) in order to introduce and extract spent medical solution.