The reviewed emergency room studies (n = 11) show overall prevalence rates of illicit drug use of 35-40% in studies using blood and urine toxicology and 1-5% in self-report studies. Cannabis and cocaine are the substances most prevalent in these studies, with a higher prevalence of cocaine in emergency rooms in the USA than in other countries where cannabis is the most common substance. Illicit drugs and alcohol are often used in combination. No relationship was found between injury severity and illicit drug use. Less clear associations emerged concerning patient and injury characteristics and illicit drug use. However, illicit drug use seems to be more common in men aged 20-40 years and is strongly associated with violence-related injuries. Variations in the prevalence rates and patient characteristics can partly be explained by locale and/or country of research. Moreover, because methodological differences influence the study outcomes, methodological aspects of emergency room studies should be taken into consideration when interpreting the results.