Pot-smoking men can end up with sperm which are as chilled out as themselves.
A Canadian study has found that even just a couple of joints a week can lower sperm counts by almost a third.
And those sperm that are around can be more laid-back than usual "mellow and swimming in circles", researchers say.
The research was carried out by the University of British Columbia Medical School's department of urological sciences.
Spokesman Dr Victor Chow told The Edmonton Journal the occasional puff won't cause too much harm, but heavy users would "definitely affect the quality of the sperm".
While a number of substances can lower sperm counts, Chow said it was interesting how marijuana also appeared to affect how sperm moved.
He said marijuana had a strong effect on male fertility because sperm was a "very rapidly turning over cell". With new sperm made every three months, it meant heavy marijuana use had an immediate effect on production.