The present invention provides a transdermal delivery system comprising (R)-dihydroetorphine, or a salt, hydrate or derivative thereof, wherein said system has a rapid onset of (R)-dihydroetorphine plasma concentration characterised by the mean in vivo plasma concentration of (R)-dihydroetorphine achieving at least 50% of its Cmax in less than 20 hours, preferably in less than 18 hours and more preferably in less than 12 hours, after application of the system to the skin of a human subject, e.g. when based on the mean plasma concentration versus time curve.
