New research from Duke University Medical Center has discovered that one of the active ingredients in cannabis, tetrahydrocannabinol (THC), can trigger structural and regulatory changes in the DNA of the user’s sperm.

As reported in the journal Epigenetics, a team of researchers conducted experiments in rats as well as 24 men. They discovered that THC appears to affect two important cellular pathways and cause DNA methylation, where the activity of a DNA segment can change without alterations to the sequence itself.

This has potentially important consequences for reproduction and for this reason, the researchers suggest that fathers-to-be are cautious when it comes to using cannabis and call for more in-depth studies. It is unclear at this stage if the effects seen on the sperm’s genetic profile are reversible or if they can be passed on to the next generation.

"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," senior author Professor Scott Kollins said in a statement. "We don't yet know what that means, but the fact that more and more young males of child-bearing age have legal access to cannabis is something we should be thinking about."

Epigenetics studies changes in organisms that affect gene expression without altering the DNA. Many factors can lead to epigenetic changes in sperm, such as obesity, tobacco smoke, and exposure to pesticides and flame-retardant materials. This is a very small study but it is a start in understanding the effects that using cannabis high in THC might have on the body.

The team plans to continue this work in larger studies to see if the changes are reversible when cannabis use is stopped. They are also interested in testing children whose fathers have THC-altered sperm to see if the changes are transmitted to the next generation.

"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," lead author Dr Susan K. Murphy added. "In the absence of a larger, definitive study, the best advice would be to assume these changes are going to be there. We don't know whether they are going to be permanent. I would say, as a precaution, stop using cannabis for at least six months before trying to conceive."