Smoking super strength skunk could make you infertile or stunt your unborn baby, a new study warned.
The chemical that gets you high alters the sperm’s genetic profile.
The new study found the principal psychoactive chemical tetrahydrocannabinol or THC disrupts genes involved in the growth of organs and body of the unborn child.
Scientists do not know if the changes to sperm could be making men infertile, can be reversed or are passed on to children.
So they warn the only solution is to stop using cannabis at least six months before coming a father.
New research by Duke University Medical Centre in Durham, North Carolina found THC affects epigenetics, triggering structural and regulatory changes in the DNA of users’ sperm.
Previous research found tobacco smoke, pesticides, flame retardants and even obesity can alter sperm.
Experiments in rats and a study with 24 men found THC appears to target genes in two major cellular pathways and alters DNA methylation, a process essential to normal development.
One of the pathways is involved in helping bodily organs reach their full size.
The other involves a large number of genes that regulate growth during development.
Both pathways can become dysregulated in some cancers.
But what is unknown is whether DNA changes triggered by THC are passed to users’ children and what effects that could have.
Associate professor Susan Murphy, chief of the Division of Reproductive Sciences, said: “In terms of what it means for the developing child, we just don’t know.”
She explained it was not known whether sperm affected by THC could be healthy enough to even fertilise an egg and continue its development into an embryo.
Senior author Professor Dr Scott Kollins added: “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.
“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.”
The increased availability of skunk and cannabis legalisation made the findings particularly pertinent to teenagers and young men, the professor in psychiatry and behavioural sciences added.
The study defined regular users as those who smoked marijuana at least weekly for the previous six months.
Their sperm were compared to those who had not used marijuana in the past six months and not more than 10 times in their lifetimes.
The higher the concentration of THC in the men’s urine, the more pronounced the genetic changes to their sperm were.
THC appeared to impact hundreds of different genes in rats and humans but many of the genes had something in common – they were associated with two of the same major cellular pathways.
Prof Murphy said the findings was a starting point on the epigenetic effects of THC on sperm and the trial was small and other factors such as nutrition, sleep, drinking and other lifestyle habits could damage sperm.
But she concluded: “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.
“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.”
Further research with a larger number of men is planned to see if stopping using cannabis can reverse the changes.
They also hope to test the umbilical cord blood of babies born to fathers with THC-altered sperm to determine what, if any epigenetic changes, are carried forward to the child.
A recent study found 94% of UK police seizures were high-potency marijuana in 2016 compared to 85% in 2008 and 51% in 2005.
Skunk contains around 14% THC, more than some other types of cannabis, such as hash or herbal cannabis.
The study was published in the journal Epigenetics.