Men who smoke marijuana frequently damage their fertility in several different ways, research suggests.
Drugs can damage sperm
Scientists at Buffalo University found regular smokers had significantly less seminal fluid, and a lower sperm count.
Their sperm were also more likely to swim too fast too early, leading to burn-out before they reach the egg.
Lead researcher Dr Lani Burkman said: "The bottom line is, the active ingredients in marijuana are doing something to sperm."
Marijuana contains several chemicals known to have an impact on human physiology, including THC (tetrahydrocannabinol).
The researchers believe that THC may disrupt the way sperm swim, or it may block mechanisms designed to weed out malfunctioning sperm.
It is known that human sperm contains receptors which are stimulated by substances like THC and other cannabis-related chemicals.
Tests have shown that sperm exposed to high levels of THC began to swim in an abnormal fashion, and were less able to attach to an egg so that fertilisation could take place.
In the current study, Dr Burkman's team carried out a battery of tests on sperm samples taken from 22 men who had smoked marijuana on average 14 times a week for at least five years.
In particular, the scientists assessed the sperm to see when they began to swim in a very vigorous fashion.
It is vital that a sperm begins to swimming vigorously as it approaches the egg to improve its changes of successful fertilisation. However, if it begins to swim too fast too soon its changes of reaching the egg are significantly reduced.
Dr Burkman said: "The sperm from marijuana smokers were moving too fast too early.
"The timing was all wrong. These sperm will experience burnout before they reach the egg and would not be capable of fertilization."
Dr Burkman conceded that many men who smoke marijuana have fathered children.
However, she said that smoking the drug may tip the balance against men whose fertility was borderline to start with.
"THC from marijuana may push them over the edge into infertility."
Dr Burkman said it was unclear whether fertility would recover after men stopped smoking marijuana.
But she warned that THC remained stored in fat for a long period.
"We can't say that everything will go back to normal.
"I definitely would advise anyone trying to conceive not to smoke marijuana, and that would include women as well as men."
Professor Alison Murdoch, chair of the British Fertility Society, told BBC News Online the results were not surprising as marijuana was a potent drug which was likely to have an effect on many systems in the body.
However, she said many other factors, such as diet, alcohol consumption, cigarette smoking and exercise also had an impact on fertility.
"Male fertility is quite complicated. The partners of men with low sperm counts can sometimes achieve pregnancy, and it is only when men produce very, very small amounts of sperm that they can be considered infertile."
Details of the study were presented to a meeting of the American Society of Reproductive Medicine in San Antonio.