The potent opioid and veterinary drug, carfentanil has recently entered the illicit drug market, especially in relation to heroin and cocaine. Recent publications have reported carfentanil concentrations found in fatalities occurring in the USA. This article presents the toxicological findings in seven heroin/cocaine cases occurring in the UK within a short period of time where carfentanil was detected and measured. Carfentanil was detected along with other drugs in all cases with no alcohol detected in the post-mortem blood in any of the cases. Of the other drugs detected, of particular note, cannabinoids were detected in three, cocaine in four, other opioids (methadone, dihydrocodeine and tramadol) in four and benzodiazepines were detected in four of the seven fatalities. A high concentration of ketamine and norketamine was found in one case. Morphine and its glucuronide metabolites were also measured where detected in six of the seven cases. The carfentanil concentrations were found to be between 0.22 and 3.3 ng/mL (mean 0.93, median 0.66 ng/mL) in post-mortem femoral blood. In one case where aortic and ventricular post-mortem blood was submitted for analysis in addition to femoral blood, comparative concentrations of 1.05 (aortic), 0.57 (ventricular) and 0.50 (femoral) ng/mL were found. The concentrations support the necessity to ensure laboratory detection methods for carfentanil and subsequent measurement are appropriate as concentrations below 0.3 ng/mL may be present in post-mortem blood. The concentrations also support the notion that there is no particular "toxic" or "fatal" post-mortem blood carfentanil concentration associated with its use.