A team of US researchers has found that naturally-occurring cannabis-like chemicals may control both sperm motility and their ability to fertilise an egg. The scientists, based at the University at Buffalo, State University of New York, say their study raises the possibility that heavy use of marijuana could interfere with this signalling system.

Dr Herbert Schuel and his colleagues studied the effects on human sperm in a test-tube of THC (the chemical responsible for the 'high' produced by marijuana) and a synthetic 'cannaboid' called AM-356. They found that too much of either chemical prevented 'hyperactivated motility', the vigorous pattern of swimming exhibited by sperm once they become capable of fertilising an egg. The chemicals also prevented the necessary structural changes in the sperm required for fertilisation. 'It really stops them cold' Dr Schuel told New Scientist.

Dr Schuel said the findings suggest that defects in the cannaboid receptor signalling system could account for certain types of infertility. 'A better understanding of these mechanisms might lead to the development of novel drugs useful in reproductive medicine' he added. He also said that men and women who abuse marijuana could 'flood' the natural signalling system, which may cause fertility problems.