Illicit or 'designer' benzodiazepines are increasingly contributing to overdose deaths. This study utilized natural language processing (NLP) to analyze benzodiazepine mentions over a decade on 270 online drug forums (subreddits) on Reddit. NLP was used to automatically detect mentions of both illicit and prescription benzodiazepines, including their misspellings and non-standard names, and to group these mentions by quarter. The analysis covered 17,861,755 posts from 2012 to 2021, focusing on 26 benzodiazepines (8 prescription; 18 illicit) and detecting 173,275 mentions. The findings showed a consistent increase in posts about both prescription and illicit benzodiazepines, correlating with the rise in overdose deaths involving these drugs. This demonstrates the potential of Reddit for drug surveillance.