The ongoing opioid crisis in the United States raises concern about a possible shift of the issue towards Europe. Concomitantly, severe intoxications and deaths in Canada and Australia, among others, have been atributed to fentanyl-laced heroin. As heroin remains the most frequently consumed opioid in Europe, information on composition of retail heroin is of high value. Despite being valuable, the statistics coming from customs and police do not provide a comprehensive view of retail heroin. A questionnaire was conducted through grassroots organisations among people who use drugs (PWUD). It included questions on current heroin use and poly-drug use and also details of purchase and experienced unforeseen effects after consuming heroin. In addition, participants were required to hand in a sample of 100mg heroin for quantitative and qualitative analysis by GC-MS and UPLC. Data (n =128) shows that the average purity of heroin has an average of 13.0%. Few adulterants were found, including acetaminophen, diacetamate, caffeine, cocaine, bromazepam, pseudococaine and ketamine. None of the samples showed the presence of fentanyl. Furthermore, 8% of participants pointed out having experienced mild unexpected effects, including nausea, enhanced effect, itchiness and local irritation. The study confirms the absence of fentanyl-laced heroin on the Belgian drug market. Knowing that the production of heroin is evolving and this type of PWUD is extra vulnerable, the continuous monitoring of retail heroin quality and involvement of PWUD, as done in RADAR-heroin-23, remains of high priority.