According to our favourite magazine:
Sperm motion, sperm concentration and the levels of hormones were all the same for mild, moderate and heavy coffee drinkers. “However, sperm motility was higher in patients who drink coffee compared those who do not,” says the team, which presented their work at the American Society for Reproductive Medicine meeting in San Antonio, Texas, US.
But a second study presented at the same meeting on another widely used drug – marijuana – revealed a detrimental effect. Smoking cannabis not only reduces the number of sperm and sperm volume produced by a man, it also causes sperm to speed up temporarily and then “burn-out”.
This premature “burn-out” may mean a sperm is not active enough to fertilise an egg when it reaches it.