Heroin is among the most widely used and dangerous addictive opiate. The World Health Organization (WHO) estimated that more than 15 million people are under the influence of opiate addiction. The aim of this study was to investigate copper zinc-superoxide dismutase (Cu, Zn-SOD), catalase (CAT) and selenium-dependent glutathione peroxidase (Se-GPx) antioxidant enzyme activities, malondialdehyde (MDA) levels and the frequency of micronuclei (MN) in addicts using heroin, the most commonly abused opiate in Turkey. Addicts were defined as individuals diagnosed according to "Diagnostic and Statistical Manual of Mental Disorders, fourth edition (DSM-IV)" criteria by the "Alcohol and Substance Abuse Treatment and Education Centre-Ankara (AMATEM)". The control group had no addiction. In comparisons between the groups, a significant decrease in Cu, Zn-SOD activity and increases in MDA levels and MN frequency were observed in addicts. It can be concluded that opiates may cause oxidative stress and that antioxidant supplementation, in addition to pharmacological and psychiatric approaches, can reduce the toxicological effects of these opiates.