What do tobacco smoke, pesticides, flame retardants, obesity, and marijuana have in common?

The ability to alter the DNA of men’s sperm.

As legal access to cannabis expands across North America, more scientists are turning their attention to the drug’s active ingredient, tetrahydrocannabinol (THC).

New research from Duke Health suggests men in their childbearing years (aged up to about 45) should think twice before lighting up while trying to conceive.

Experiments in rats and a study with 24 men found that THC targets genes in two major cellular pathways, and alters DNA methylation, a process essential to normal development.

Whether genetic changes can be reversed or may be passed on to children remains unknown.

“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 study author Scott Kollins, a professor of psychiatry and behavioral sciences at Duke University, said in a statement.

“We don’t yet know what that means,” he continued. “But the fact that more and more young males of childbearing age have legal access to cannabis is something we should be thinking about.”

Canada in October became the second country to legalize recreational marijuana use.

Colorado and Washington, meanwhile, were first in the US to legalize medicinal and recreational cannabis in 2012; their policies have led to an increase in weed tourism—i.e. people travel there for the express purpose of obtaining or using drugs.

For its study, Duke recruited a sample of “regular users,” defined as those who smoked marijuana at least weekly for the previous six months. Their sperm were compared to a selection of men who did not use the drug in the last six months, and not more than 10 times in their life.

The higher the concentration of THC in the men’s urine, the more pronounced the genetic changes to their sperm were, according to the results, published online this week in the journal Epigenetics.

“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 Susan Murphy, associate professor and chief of the Division of Reproductive Sciences in obstetrics and gynecology at Duke, said. “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,” she added.

This content was originally published here.

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