Heavy marijuana users have sperm that move too fast, too soon, and become burnt out, reducing the chance of fertilisation, according to new U.S. research.

Researchers from the University of Buffalo School of Medicine and Biomedical Sciences, led by Dr Lani Burkman, presented their results at the Annual Meeting of the American Society of Reproductive Medicine being held this week in San Antonio, Texas.

In the first study to examine the effect of marijuana smoking on the specific swimming behaviour of sperm, researchers compared samples of seminal fluid from 22 men who were heavy marijuana users with that from 59 fertile men who had produced a pregnancy.

"The bottom line is, the active ingredients in marijuana are doing something to sperm, and the numbers are in the direction towards infertility," says Dr Burkman.

"The sperm from marijuana smokers were moving too fast too early," she says. "The timing was all wrong. These sperm will experience burnout before they reach the egg and would not be capable of fertilisation."

Burkman and collegues found that the sperm from men who used marijuana approximately 14 times a week for an average of five years were less likely achieve fertilisation than non-marijuana smokers. They analysed sperm for a particular, vigorous type of swimming behaviour called 'hyperactivation'. The seminal fluid was also tested for total sperm count, the percent of sperm that were moving, their velocity and shape.

Sperm have two different types of movement, a forward-swimming motion that gets them to the egg and a sideways motion of the sperm head produced by a sudden whiplash of the tail which gives them a strong thrust to help push through the egg's cell wall. If this thrusting motion happens on the way to the egg it impedes progress towards the egg, reducing the chance of fertilisation.

Dr Anne Jequier, President of the Fertility Society of Australia, who has previously studied sperm from five heavy marijuana smokers, says she found Dr Burkman's research very interesting.

"There's been a lot of talk about marijuana as an infertility agent in the past", says Jequier, although her research has showed that this was not the case.

"A year ago we took five heavily addicted marijuana smokers and we looked at their semen. We could find nothing wrong with the routine semen analysis. If Burkman has found there in an alteration in the hyperactivation pattern in this semen - and I haven't seen the abstracts for this meeting yet - that is very interesting."

Like the brain, sperm cells have natural receptors for cannabinoids such as the psychoactive ingredient in marijuana, THC (tetrahydrocannabinol).

"We don't know exactly what is happening to change sperm functioning," says Burkman, "but we think it is one of two things: THC may be causing improper timing of sperm function by direct stimulation, or it may be bypassing natural inhibition mechanisms. Whatever the cause, sperm are swimming too fast too early."

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