Research area, country of origin, and funding source
The proportion of neurosciences & neurology and pharmacology & pharmacy articles decreased over time. Substance abuse articles were uncommon in the first two periods before spiking in the third period. The other research areas did not show a clear trend over the three time periods. In all three periods, more papers were published in the US and Canada than in the rest of the world. The proportion of papers published in the US and Canada stayed relatively constant in the first two periods before spiking in the third period. Most citing papers (79%) had no disclosed funding source. The largest funder of research was the National Institute on Drug Abuse, which funded 35% of citing articles with known funding sources.