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. https://doi.org/10.1007/s11469-018-0043-1