There’s been evidence for years that heavy marijuana use damages men’s fertility, and researchers may have finally figured out how. It appears that THC, the active ingredient in marijuana, may have pretty much the same effect on sperm that it has on humans.
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NEW SCIENTIST reports that when sperm in test tubes were treated with high levels of THC, rather than swimming briskly like gametes with places to go and people to see, they meandered lazily, not getting very far. If they did happen to reach an egg, they seemed unable to bind to it and penetrate. “It really stops them cold,” said one researcher.