The most successful transdermal patch is for the delivery of fentanyl. It was developed by ALZA and by the mid 2000swas selling over $1 billion per year. The design of the patch was based on interesting physicochemical properties which had to address safety, efficacy, high cost of active and potential abuse of goods. It has also been the subject of litigation in which patch failure was thought to be responsible for a number of deaths. The scientific rationale of one of these cases will be discussed and the reasons behind the favourable outcome described.