Bulgarian researchers found that frequent use of mobile phone has reduced the concentration of sperm by 30%. However, the authors did not take into account other factors that affect fertility. Researchers at the University of Belfast have determined that Viagra reduces the fertilizing capacity of sperm and causes intrauterine growth retardation in mice. It is hardly possible to extrapolate the results to human, especially because the drug has been used for a long time with no apparent adverse effects on fertility.