Dependence on psychoactive substances is a major public health, security and development problem today, both in developed countries and in developing countries. Addictions are a health problem with environmental, biological and psychosocial features that require a comprehensive and multidisciplinary response from different institutions. Over the past few decades, researchers have increasingly recognized the link between the use of psychoactive substances and psychological disorders. The World Health Organization (WHO) defines "dual diagnosis" as the simultaneous existence of addiction to psychoactive substances and some other psychiatric disorder. At the beginning of 2020, the world was hit by the COVID-19 pandemic, which affected all aspects of the population's life. Fear and panic are the consequences of the pandemic that the population felt, which could result in changes in the habits of people treated for addiction and the consumption of psychoactive substances. This research paper describes psychoactive substances and their impact on the body, restrictions introduced due to the spread of the SARS-CoV-2 virus infection, and examines the connection between these facts. Data on the number of treated addicts of opiates, cocaine, amphetamines and cannabis before the COVID-19 pandemic (in 2017) and data from 2020, i.e. during the pandemic, were taken. Data for the Republic of Croatia were taken from the Croatian Institute of Public Health: Report on persons treated for the abuse of psychoactive drugs in Croatia in 2017 and 2020, and for the European Union, data were taken from the European Center for Monitoring Drugs and Drug Addiction: European drug report, Trends and Developments 2017 and 2020. The aim of this paper is to show the connection between the consumption of psychoactive substances and the impact of the COVID-19 pandemic on the population of the European Union and especially the Republic of Croatia. The area of the Republic of Croatia and the European Union in the mentioned time period was compared. The data of treated addicts on opiates, cocaine, amphetamines and cannabis were observed. The results of the research show that the number of treated addicts of opiates, amphetamines and cannabis during the COVID-19 pandemic in the Republic of Croatia decreased, while the number of treated cocaine addicts increased. In the European Union, the number of treated opiate and amphetamine addicts also decreased, while the number of treated cocaine and cannabis addicts increased. We associate the changes in numbers with changes in the habits of the population during the declared pandemic.