To evaluate trends in the clinical development of new pain and reformulated pain medications given the ongoing opioid crisis and the public health burden of inadequately controlled pain. We conducted a retrospective cohort study of new drugs starting clinical testing between January 1, 2000, and December 31, 2015. We searched two comprehensive commercial databases of global research and development activity. The primary outcomes were trends in new and reformulated pain drugs starting clinical testing, proportion of new pain drugs targeting a novel biological pathway, and rates and reasons for discontinuation of development. The proportion of new pain drugs entering phase 1 testing (relative to all new drug trials) declined from 2.5% between 2000 and 2002 to 1.7% between 2013 and 2015. No significant changes in the proportion of new pain drugs entering phase 2 or phase 3 trials were observed. Most new pain drugs failed to reach late-stage clinical development, with 52% of pain drugs successfully advancing from phase 1 to phase 2 and 11% advancing from phase 2 to phase 3 trials. The number of reformulated products starting clinical testing increased over the study period and was greater than that for new analgesics in 2012 and every year thereafter. Pain drug development activity has largely shifted from new therapeutics to reformulated ones. New policies, such as increased funding for basic pain research, may help address the urgent need for new therapies for pain.