Reasoning: Concept of the usual block drug dose (UBDD) adopted in Penal Code (PC) has crucial importance in the court sentence for the severity of the penalty. Not only police, and lawyers, but also legal experts ­- psychiatrists have difficulties with UDD assessment when they are asked to answer this question. Our effort was to demonstrate which possibilities has psychiatrist to provide a correct answer based on the latest findings of the medical sciences. Methods:The theoretical work is based on the analysis of the professional legal publications, judicatives written by legal experts in comparison with clinical experience and the results of the clinical research in the field of adictology. Results: Great differences and difficulties exist in the assessment of UDD. While judges, prosecutors and barristers indicate that the problem is that accused person had no opportunity to find out about concentration and the amount of effective psychoactive substance in the drug bought by him, and also that amount of the drug bought for his own consumption may vary significantly, on the other hand medical science, due to variability of tolerance is solving the problem of the assessment of UDD, which could be correctly done only by clinical experiment with accused person. It would be illegal. Conclusions: Medical science does not set up maximal lethal dose, especially for the persons with dependency on psychoactive substances and does not recognize the concept of the usual block drug dose. UBDD is legal concept, which should help the courts in their decision-making process. Because of the ethical and legal reasons, the psychiatrist must not perform a clinicalexperiment with accused person to assess UBDD. So, frequently the only correct answer about UDD expressed by the psychiatrist should be: „I do not know“. In such cases police, prosecution and judges must rely on the other evidence.

Key words: usual block dose of drug – drug – Penal Code