Mixtures of different analgesics given parenterally by PCA (patient controlled analgesia) devices are an adequate, safe and comfortable analgesic therapy for patients with severe chronic pain even at home. The mixture of different chemical entities often brings the risk of instabilities or incompatibilities. Three successfully used binary analgesic mixtures contain the opioid analgesics morphine, hydromorphone or oxycodone with the non-opioid analgesic dipyrone (metamizole). This paper presents the first studies of the physical and chemical stability of such mixtures over 3 weeks at temperatures between 4C and 37C.