The current study was done to investigate the preclinical safety and toxicology of Althaea officinalis extract as naturopathic medicine in common carp (Cyprinus carpio). Specimens were treated with 0 (control), 2.5, 5 and 10 g of marshmallow extract for 45 days. Plasma biochemical parameters were measured after 15 and 45 days. Total protein, albumin and globulin levels (p<0.05)  were significantly higher in the fish fed with 10 g A. officinalis extract than that in control groups on day 45. Although, alanine aminotransferase (ALT), aspartate aminotransferase (AST) and alkaline phosphatase (ALP) activities significantly decreased (p<0.05) in fish fed with A. officinalis extract on day 15. The use of the A. officinalis extract (10 g) led to a significant increase in AST, lactate dehydrogenase (LDH) and ALP activities and cholesterol levels on day 45 (p<0.05) and a significant decrease in plasma glucose and cholesterol levels on day 15 (p<0.05). There was no significant difference in glucose levels and creatine kinase (CK) activity between all treatments and the control group on day 45 (p>0.05). During the experimental period, triglyceride levels noticeably decreased in fish fed with 2.5 g of A. officinalis extract (p<0.05). Although, administration of marshmallow extract up to 5 g per kg of feed did not show any side effect on fishes, the use of the A. officinalis extract (10 g) led to cytotoxicity and modifications in blood biochemical parameters of fish. Therefore, we recommend the use of the lower concentrations than 10 g A. officinalis extract in prospective clinical studies.