Background: Many drug users are not receiving treatment for their drug use. Little is known about drug users not receiving treatment, as they are difficult to identify and recruit for research. Methods: We identified 479 autopsied decedents with illegal/unmarketed drug or opioid agonist treatment positive toxicological screenings from 2015 to 2016 in Denmark. Toxicological screenings from autopsy, and information on treatment status at time of death, health care utilization, educational attainment, employment status and prescription drug use from Danish national health registers were used for comparison between groups. Results: Drug users not in treatment constituted 63.3% of the study population and died at a younger age than those in treatment (41 vs. 44 years). Fatal overdose was the most common cause of death in both groups. Nearly thrice as many drug users not in treatment died from somatic causes compared with drug users in treatment (18.2% vs. 6.8%). On average, drug users not in treatment received fewer prescriptions prior to their deaths than those in treatment, but non-prescribed medications were equally prevalent among both groups (74.3% vs. 81.3%) except for non-prescribed methadone which was significantly less prevalent among drug users not in treatment (33.3% vs. 42.6%). Conclusion: Two-thirds of decedents were not in treatment at time of death. Drug users not in treatment died more often from somatic causes compared to those in treatment. Decedents had equal amounts of non-prescribed psychotropic medication in the blood, but non-prescribed methadone was more common among those in treatment at the time of death.