Werse, B., Benschop, A., Kamphausen, G., van Hout, M.-C., Henriques, S., Silva, J....Korf, D. (2019). Sharing, group-buying, social supply, offline and online dealers: how users in a sample from six European countries procure New Psychoactive Substances (NPS). International Journal of Mental Health and Addiction. 17 (5), 1237-1251