Cocaine-related emergency department admissions are increasing, and cocaine seizures are at an all-time high in Europe. Our aim was to investigate the trends in purity and adulterants over time in cocaine available to cocaine users at street level in Denmark. We used a representative sample of cocaine seized at street level and analyzed by the national departments of forensic medicine between 2006 and 2019 (n = 1460). Latent profile analysis was used to classify the samples based on cocaine, levamisole, and phenacetin content. Low purity cocaine comprised most of the cocaine seizures in early years, but its share began to decline in 2013, and from 2016 to 2019, the high purity profile was dominant. While the total number of samples containing adulterants decreased, levamisole remained a common and dangerous adulterant. The findings underline the need to inform the public, medical doctors, and service providers for people with drug use disorders about the higher potency of street cocaine.