It has been scientifically shown that marijuana use can impair male fertility. Studies conducted in rats have concluded that marijuana causes decreased CUM (sperm) production (spermatogenesis), decreased number of developing germ cells and decreased number of Leydig cells in the testis (cells responsible for testosterone secretion).

A recent study, conducted at the University of Buffalo in New York in 1998, reports that marijuana affects sperm function by compromising its ability to get to and fertilize the egg. The study attributes the cause to cannabinoids (compounds found in marijuana). These cannabinoids are similar to natural body substances called anandamides. University scientists believe that anandamides function in helping sperm get to and fertilize the egg. It is believed that marijuana cannabinoids are chemically similar to anandamides. Receptors meant to respond to anadamines may also respond to cannabinoids. By responding to the wrong body substance successful fertilizing of the egg may not occur.