In the present study, we firstly assessed Cetraria islandica and Pseudevernia furfuracae to avoid detrimental effects on multiple tissues of rats. Diabetes mellitus (DM) with the subsequent generation of oxidative stress represents a major risk factor for organs. The second aim of this study is to investigate whether administration of both lichens could prevent type 1 diabetes (T1D)-induced organ dysfunctions. During two weeks, both control and diabetic rats were treated with aqueous lichen extracts. The metabolic changes were determined. On day 14, after animals were decapitated, required samples for biochemical and genetic analysis were collected. Oxidative damage of DNA was estimated by measuring the increase in 8-hydroxy-2'-deoxyguanosine formation. Biochemical parameters were used to observe and evaluate the functional changes in tissues. Experimental data showed that the increasing doses of lichens alone have not any detrimental effect on above parameters. Moreover, C. islandica decreased the diabetes-induced glucose and malondialdehyde (MDA) levels. Thus, it seemed that the antioxidant treatment has an important effect on the organ failure in ill rats. However, the protective effect of C. islandica was inadequate on diabetes-induced disorders and DNA damages. Lichens are a safe in the studied dose range but the power of C. islandica is limited because of intensive oxidative stress in essential organs of T1D rats.