The increasing global consumption of cannabis, particularly within unregulated markets, raises concerns about potential health risks associated with contaminants and adulterants. This study investigates the presence of active compounds and adulterants in illicit cannabis products within the UK. Data was identified from 1,635 samples of cannabis products submitted to the Welsh Emerging Drugs & Identification of Novel Substances (WEDINOS). Of these, 695 (42.51%) did not contain any naturally occurring cannabinoids and in 162 (9.91%) samples no active compound was identified. Over one-third (n = 624; 38.17%) of tested samples contained a psychoactive adulterant. The most common were synthetic cannabinoid receptor agonists (n = 435; 26.61%). These findings highlight the potential dangers of unregulated cannabis consumption and emphasize the need for greater consumer awareness and harm reduction strategies. The study also underscores the urgency for policy reform and stricter enforcement of regulations to mitigate the risks associated with adulterated cannabis within the illicit market.