Abstract: The invention features devices and methods for treatment of pain. The drug delivery device is a drug delivery system adapted for whole implantation in a subject and to provide pain relief by delivery of fentanyl or a fentanyl congener (e.g., sufentanil) over a protracted period of time (e.g., at least 3 days or more than 3 days). The device comprises a housing defining a reservoir that contains a drug formulation, a pump operatively connected to the housing so as to facilitate movement of drug out of the reservoir and out of the device, and a thermal expansion element which defines a flow pathway comprising a thermal expansion channel to accommodate thermal expansion of formulation in the reservoir. The device can further comprise a valve positioned within the flow pathway so as to prevent movement of drug out of the reservoir prior to use.