Cannabis is changing sperm, but scientists are still not exactly clear how it is being altered and the extent to which this will affect users' children, according to a new study. 

Scientists at Duke Health are cautioning male users to take a six month from the drug if their partner is actively trying to get pregnant.

They issued the warning after studying the effects of the active ingredient in cannabis, tetrahydrocannabinol (THC), on sperm. 

They found that THC is altering the structure of DNA in the sperm of men who take cannabis in ways that may affect the development of the child. 

After studying the impact of THC in rats and 24 men, they discovered that THC targets certain genes and affects how DNA structures are formed. 

The study, published in the Epigenetics journal, compared the sperm of men who smoked marijuana at least weekly for the previous six months with those who had not taken the drug at all during the same period and who had used it no more than 10 times in their lifetimes.

The researchers found that the higher the concentration of THC found in the men's urine, the greater the genetic changes to their sperm. 

However, the scientists were unable to determine whether the DNA changes are passed onto the children and what possible effects this could have if they are. 

'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,' said Scott Kollins, PhD, professor in psychiatry and behavioral sciences at Duke and 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.'

The warning is timely as more and more states in the US legalise marijuana use.  As regulation of the drug has loosened over the year, research has found a correlating decrease in perceived risk. 

One study by the National Institute on Drug Abuse found that the prevalence of perceptions of great risk in using marijuana once or twice a week decreased by 17 per cent between 2002 and 2014, after several US states legalised the drug.