In 2013, Marin Cilic returned a positive drug test for the substance nikethamide. Other drugs of this category are prethcamide, pentylenetetrazole, and nikethamide. Walilko tested positive for Nikethamide and was initially suspended for two years. Nikethamide is now withdrawn due to risk of convulsions. Nikethamide is a stimulant which mainly affects the respiratory cycle. In some sports, nikethamide is listed by the World Anti-Doping Agency as a banned substance.