Men who smoke too much marijuana could face fertility problems because the drug makes sperm 'mellow' causing it to 'swim in circles'.

Cannabis - which is the most widely-used illegal drug in Britain - tends to leave users feeling chilled out and relaxed.

But now researchers have revealed that it has the same effect on sperm and regular weed smoking can cut counts of the cells by as much as a third.

Dr Victor Chow, of the University of British Columbia in Canada, said: 'The weight of the ­evidence is that marijuana ­probably has a negative impact not only for sperm counts but sperm function.'

Researchers have revealed that cannabis has a relaxing effect on sperm leaving it 'mellow' and potentially cutting counts of the cells by a third

Previous studies have shown that marijuana can affect fertility by cutting sperm count and suppressing ovulation in women.

The exact way in which that happens has not yet been made clear but the new research appears to shed more light.

But fertility problems are not the only danger of smoking marijuana.

The drug can increase the heart rate and affect blood pressure - while increasing the risk of a pregnant woman's baby being born smaller than expected.

It was also recently revealed that pot can affect the way a smoker's knees, elbows and shoulders move when they are walking.

Cannabis - which is the most widely-used illegal drug in Britain - tends to leave users feeling chilled out and relaxed (stock photo)

The study, from the University of South Australia, found that there are differences between the way marijuana users and non-users walk.

Those who smoke the drug tend to have stiffer shoulders, more flexible elbows and quicker knees - which move faster than those of non-users.

While differences in movements were detected, there were no significant differences between the balancing abilities and neurological functions.

The study's authors are calling for more research that can determine exactly how marijuana affects movements, as the drug continues to be legalized in the US.