A 2013 study  on rats found that the administration of cannabis significantly decreased sperm count, sperm motility, and the diameter of seminiferous tubules (the cell system where sperm is gestated, matured, and transported in the testes). A 2007 study  seeking a safe method of contraception found that cannabis sativa reduced sperm count in male Wistar rats. A third study  , from 2011, found that bhang, an edible cannabis preparation common in India, reduced fertility in male rats, possibly due to alterations in the testicular endocannabinoid system.-
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.
We’ll start by examining the results, taking a closer look at male fertility, reviewing past research, and finally, seeing what conclusions we can draw.
PAST RESEARCH ON CANNABIS AND MALE FERTILITY
As one meta-analysis  acknowledges, the research is contradictory, but the preponderance of evidence suggests that cannabis could hurt sperm count. There could be a few reasons for these messy results: some might site researchers biased against cannabis, while others might point to confounds in studies on human populations. It’s equally possible that cannabis use affects fertility differently at different use profiles: cannabis is known to have a “biphasic effects distribution”, where low levels of use often have opposite results to high levels. It’s entirely possible that consuming small amounts of cannabis increases sperm count, while consuming large amounts decreases it.
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.
Furthermore, when we go back and examine previous studies on the topic of cannabis and male fertility, the plot thickens.
It’s important to note that sperm count isn’t the only factor determining male fertility, though it’s an important one. Another factor is sperm motility, the ability of sperm to “swim” properly through the female reproductive tract. Reduced seminal volume can also cause fertility problems, even if sperm count is normal. Others factors causing infertility can include white blood cells in sperm, abnormally formed sperm, or a high volume of dead sperm.
“In terms of what it means for the developing child, we just don’t know,” Murphy said. It’s unknown whether sperm affected by THC could be healthy enough to even fertilize an egg and continue its development into an embryo, she said.
“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.
The study was a starting point on the epigenetic effects of THC on sperm and is limited by the relatively small number of men involved in the trial, Murphy said. The findings in men also could be confounded by other factors affecting their health, such as their nutrition, sleep, alcohol use and other lifestyle habits.
Whether genetic changes can be reversed or are passed on to children is still unknown
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.
The higher the concentration of THC in the men’s urine, the more pronounced the genetic changes to their sperm were, the authors found.
One of the pathways is involved in helping bodily organs reach their full size; the other involves a large number of genes that regulate growth during development. Both pathways can become dysregulated in some cancers.
“What we have found is that the effects of cannabis use on males and their reproductive health are not completely null, in that there’s something about cannabis use that affects the genetic profile in sperm,” said Scott Kollins, Ph.D., professor in psychiatry and behavioral sciences at Duke and senior author of the study.