Animal studies have shown that exposure to the active ingredient in marijuana can result in a decrease in testosterone levels, a reduction of testicular size, and abnormalities in the form and function of sperm.
In humans, the effects of marijuana on testosterone and estrogen levels aren’t as clear. Lower testosterone levels have been reported in chronic marijuana users compared to nonusers, but not all studies support this.
Few studies have examined a direct causative effect between smoking marijuana and gynecomastia. A report in 1972 made the initial connection between cannabis and gynecomastia. This study is contrasted with a 1977 survey of U.S. Army soldiers which showed no association between smoking marijuana and gynecomastia. This study was limited, however, due to its very small sample size.
http://www.cnn.com/2013/12/05/health/youn-pot-moobs/
