The addition of a drug to a blood product intravenous line raises the question of physical and chemical compatibility of the drug with the blood and any preservatives or additives in the blood product. This clinical scenario is encountered frequently, however the practice has not been well studied. Protocols at most institutions prohibit the addition of drugs to blood products. We investigated the addition of analgesic drugs used for patient-controlled analgesia (morphine 1 mg/ml, pethidine 10 mg/ml and ketamine 1 mg/ml) to a standard red cell concentrate, resuspended red blood cells. The red cells were analysed by a Blood Transfusion Service Haematologist and subjected to standard quality control tests. The morphology of the red cells in resuspended red blood cell preparations was unchanged by the addition of these drugs at any stage during storage. The drug concentration in the resuspended red blood cell serum was measured at 0 and at 20 minutes and there was no decrease in concentration which showed that there was no loss of free drug in the resuspended red blood cell serum. This study demonstrates that the concern regarding injury to red cells in standard red cell concentrates by addition of these drugs is unjustified.