We already know that cannabis lowers sperm count, but new research suggests that the drug actually causes changes to the sperm itself — which might have implications for the health of a potential baby.

For a study published today in the journal Epigenetics, scientists at Duke University compared the sperm of two groups of rats: those who had been given tetrahydrocannabinol (THC), the psychoactive ingredient in cannabis, and those who had not. Then they compared the sperm of 24 human men who smoked marijuana weekly versus a control group who used marijuana no more than 10 times in their life and not at all in the past half-year. In both cases — rats and humans — marijuana changed how genes work in sperm cells.

Think of your DNA as a list of instructions for making proteins, and genes as small subsets of that list. Our body has little chemical tags (called methyl groups) that get added to the DNA at specific regions, explains Susan Kay Murphy, a professor of gynecology at Duke and co-author of the study. These chemicals don’t mutate the genes themselves, but they do affect how they’re used, like deciding which instructions are followed and which aren’t.

In both rats and humans, the cannabis affected many different genes involved in two different pathways. (Think of pathways as another set of instructions, this time for regulating various bodily functions.) One is important for organs to reach full size, and one plays a role in cancer and suppressing tumors. “That just blows my mind,” says Murphy. “How do you even reconcile that, biologically, an entire pathway is going to be affected by these changes?”

This doesn’t mean that smoking cannabis will lead future children to be more vulnerable to cancer. Murphy is very clear that this is a pilot study originally intended to see if cannabis even has any effect of this kind on sperm. The sample size is small, and they didn’t control for the concentration of THC the human recruits smoked. The scientists did, however, measure THC in the urine and noted that more THC in the urine correlated with more changes.

“This is a smaller study, but with concerning implications,” says Bobby Najari, a urologist at NYU Langone who was not involved in the study. Najari already counsels men who use marijuana regularly to cut back because of the effect on sperm count. “I think one of the important positive things about research like this is that it may further motivate men to change their health,” he adds. “It’s one thing to talk about sperm count, another when you’re talking about the potential health of the child.”

Both Najari and Murphy stress that future research needs to be done, and the Duke team is already working on follow-up studies. Are those changes reversible? Will they even end up affecting a potential baby? “I want to be very careful to not have the results turned into something that they’re not,” says Murphy. “It’s not intended to scare people. Our whole objective is to learn more about biology and what effects there might be.”

Update December 20th 8:45PM: Added an updated link to the study and clarified that the changes were not genetic mutations.