There are three crucially important and related facts about the current state of marijuana in the United States: Perceptions of risk associated with cannabis are decreasing, the potency of cannabis is increasing, and it’s more legal to smoke than it’s been in the past 40 years — a trifecta that signals to researchers that we need a better sense of how the drug affects our health. In a study linked to marijuana released Wednesday, scientists zoned in on the wiggly, reproductive cell: sperm.

Tetrahydrocannabinol (THC), an active cannabis compound with psychoactive properties, affects the genetic profile of sperm, researchers from Duke University Medical Center report in the journal Epigenetics. Whether or not THC negatively affects sperm, however, remains to be discovered. The researchers also found that cannabis users have lower sperm counts than non-users.

“Our work and others have shown that cannabis use lowers sperm concentration and this study, for the first time, suggests that it may also alter the genetic profile of sperm cells,” senior author Scott Kollins, Ph.D., tells Inverse. “We do not yet know what the implications of this latter finding are.”

THC affects the genetic profile of sperm. Sarah Calvert / University of Sheffield

Kollins explains that, given the known effects of other kinds of environmental exposure on male sperm epigenetics and the changing legal landscape of marijuana in the United States, the team felt it was important to explore the topic in this preliminary study. Lead author Susan Murphy, Ph.D., says that prior to this study, no one had examined whether cannabis could impact human sperm’s epigenome — the group of chemical compounds that tell the genome what to do — “but there was evidence of intergenerational effects from other animal studies, suggesting that epigenetics could be possible,” she adds.

Epigenetic modifications are heritable alterations that are not due to changes in the DNA sequence. The modifications affect regulating patterns of gene expression by affecting DNA methylation, which is the process of adding methyl groups to the DNA strand, changing the activity of a DNA segment without changing its sequence. The team notes that this effect could change over time, even affecting a marijuana user’s children:

Because sperm maturation is a continual process throughout the adult male’s life, exposures like cannabis could have an impact on the integrity of the sperm methylome, with implications for heritability of such alterations by subsequent generations.

The team doesn't know if these changes are detrimental. Unsplash / Get Budding

To test this idea, they examined a group of 24 men who had smoked marijuana at least weekly for the previous six months. Their sperm was compared the sperm belonging to men who had not used marijuana in the past six months, and not more than 10 times ever. The scientists found what had also been found in previous studies with rats: The higher the concentration of THC in the study subject’s urine, the more pronounced the genetic changes in their sperm.

THC, it appears, alters DNA methylation in sperm and targets genes in two major cellular pathways — one which is linked to the process of body organ growth and another that regulates general growth during a person’s development. As of now, scientists don’t know whether these changes mean that the resulting children linked to this changed sperm are affected at all.

“The methylation changes we have observed seem to be affecting genes involved in early growth and development, but we do not know if the methylation patterns in the sperm are transmitted to the offspring,” Murphy explains.

That huge question is one that the team is currently working on. Murphy and Kollins both emphasize that more studies, with a larger group of study subjects, are needed to say with any confidence exactly what THC-changed sperm means in the long run. But Murphy notes that while they don’t yet know how long the changes to sperm caused by THC last, it’s not a bad idea for fathers-to-be to stop using marijuana for at least six months before trying to conceive.

“Our study needs to be replicated with a larger sample size,” Murphy says. “In the meantime, the most cautious approach, until we know more about potential transmission to the next generation, would be to not use cannabis.”