“…Of the twenty-two papers discussed in this section, three are focussed on opioid abuse [35,41,42], eight on tobacco and marijuana use [6,12,13,40,43,45,46,49], one on alcohol abuse [36], and one on the street drug, mephedrone [44]. Twitter is the most popular source of data (18 papers) [6,11,12,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49], with Reddit [11][12][13], and online health communities [12,13], both represented. Supervised machine learning (8 papers -all utilising Twitter data) and unsupervised machine learning (11 papers) were both evident in the reviewed papers, with classical machine learning approaches more common than modern neural-network-based approaches (17 and 2 papers, respectively).…”