Lifetime prevalence of illicit drug use is high in the Czech Republic (CR) - it reached 31.2 % of the population in the age group 15-64 years in 2017. Diabetes mellitus (DM) applies to 8.8 % of the Czech population. Risks of illicit drug use in diabetic patients are related both to their effect on glucose metabolism and to the lifestyle associated with illicit drug use, which might lead to worsened glycemic control and increased morbidity and mortality of the patients. Cannabis use, being the most common illicit drug use in the CR, presumably does not have a direct effect on glucose metabolism. However, it increases appetite and decreases self-control. Opioids/opiates disrupt insulin secretion, which consequently leads to hyperglycaemia. On the other hand, hypoglycaemia might be an adverse effect of opioid treatment in diabetic patients. Cocaine and other stimulants such as MDMA increase a blood glucose level and increase the risk of diabetic ketoacidosis in Type 1 DM. Patients with DM who use illicit drugs should therefore be sufficiently informed about health risks related to illicit drug use in combination with DM.