However, the study should be interpreted with caution. It shows a correlation between weed smoking and sperm count, but correlation does not imply causation. There could be a third factor causing both the higher sperm count and the weed smoking; testosterone, for example, can cause both high sperm count and greater risk-taking behaviour, so it’s possible that men who happen to have higher testosterone are more likely to both smoke weed and have more sperm.
Past research on this topic further complicates the story. A 2015 study  on 1,215 Danish military recruits found that men with a history of weekly pot use had 28% lowered sperm concentration and 29% lower sperm count, though the weed smokers had higher testosterone.
LINK BETWEEN SPERM COUNT AND MALE FERTILITY
As the study’s authors stated, “Whether these findings are reflective of the previously described role of the endocannabinoid system in spermatogenesis or a spurious association requires confirmation in further studies”.
Animal research adds a whole new dimension to the picture. The main advantage of animal research is that scientists can control for extraneous variables, thereby eliminating the confounds we’ve been grappling with. The disadvantage is that humans are different from animals, and the results might not always carry over the species boundary.
It’s also possible that the subjects of this study were non-representative of the general population, given they were men seeking treatment for infertility. The men might also have been dishonest about their cannabis use, given that cannabis was illegal at the time and location of the experiment. Further, the study didn’t control for the magnitude of lifetime cannabis use; it only looked at whether the subjects had previously smoked weed or not.
Much like previous research that has shown tobacco smoke, pesticides, flame retardants and even obesity can alter sperm, the Duke research shows THC also affects epigenetics, triggering structural and regulatory changes in the DNA of users’ sperm.
Experiments in rats and a study with 24 men found that THC appears to target genes in two major cellular pathways and alters DNA methylation, a process essential to normal development.
Whether genetic changes can be reversed or are passed on to children is still unknown
“We know that there are effects of cannabis use on the regulatory mechanisms in sperm DNA, but we don’t know whether they can be transmitted to the next generation,” Murphy said.
THC appeared to impact hundreds of different genes in rats and humans, but many of the genes did have something in common — they were associated with two of the same major cellular pathways, said lead author Susan K. Murphy, Ph.D., associate professor and chief of the Division of Reproductive Sciences in obstetrics and gynecology at Duke.
In addition to Kollins and Murphy, study authors include Nilda Itchon-Ramos, Zachary Visco, Zhiqing Huang, Carole Grenier, Rose Schrott, Kelly Acharya, Marie-Helene Boudreau, Thomas M. Price, Douglas J. Raburn, David L. Corcoran, Joseph E. Lucas, John T. Mitchell, F. Joseph McClernon, Marty Cauley, Brandon J. Hall, and Edward D. Levin.