Scientists have found that the active ingredient in the drug cannabis, tetrahydrocannabinol (THC), can alter sperm.
Legal access to cannabis is increasing across the US, prompting scientists to dig deeper into the effects of THC in teens, adults and pregnant women.
Patients in the UK can be prescribed medicinal cannabis as of November, if it is agreed by a specialist doctor.
New research from Duke University has suggested that THC could impact the sperm of men in their child-bearing years and potentially the children they conceive.
Previous research has established that tobacco smoke, pesticides, flame retardants, and even obesity can alter sperm.
In the same way, the new Duke research shows that THC also affects the ways that genes are expressed and causes structural and regulatory changes in the DNA of the drug users' sperm.
Experimenting on rats and in one study with 24 men, the team found that THC targets genes in two major cellular pathways.
Their work has been published in the journal Epigenetics, although they acknowledge they haven't established whether the DNA changes triggered by THC are passed to users' children.
"What we have found is that the effects of cannabis use on males and their reproductive health are not completely null," said Professor Scott Kollins.
"There's something about cannabis use that affects the genetic profile in sperm," added Dr Kollins, a professor in psychiatry and behavioural sciences at Duke and the senior author of the study.
"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," Professor Kollins said.
The study found that the higher the concentration of THC in the men's urine, the more pronounced the genetic changes to the sperm were.
"In terms of what it means for the developing child, we just don't know," said Dr Susan Murphy, the study's lead author.