Marijuana-smoking college men volunteered for the study led by Lani J. Burkman, PhD, director of andrology at the University of Buffalo School of Medicine and Biomedical Sciences.
The smokers weren't the only ones who got high. The drug affected their sperm, too. These stoned sperm party hard. And then? They burn out, researchers say.
"Marijuana-smoking men's sperm are hyper. They are way out there," Burkman tells WebMD. "They already have begun the vigorous swimming called hyperactivation. Sperm should be quiet at first. They should be waiting to be washed into cervix and approach the egg before they start hyperactivation."
So the little guys are fast out of the gate, right? What's wrong with a little head start?
"It is not a head start. They are going to blow it," Burkman says. "They're too fast, too early. Each individual sperm can maintain this swimming only so long, only several hours. Then it poops out. If it has run out of hyperactivation before it gets close to the egg, it will not fertilize. These sperm are going to burn out."
Burkman announced the findings at this week's meeting of the American Society of Reproductive Medicine.
Marijuana and Fertility Timing
"The reason men have millions of sperm is because the fertility process is more difficult than people think," Dominguez tells WebMD. "The whole process of ascending up the tract to the fallopian tubes and then finding the egg is delicately balanced."
As the sperm approaches the egg, it receives a signal to start swimming -- hard. This hyperactivation lets it push through the egg cover. Pooped out sperm don't have a chance.
Of course, men who smoke marijuana do get women pregnant. But some men are more fertile than others, or are more fertile at different times of their lives. Smoking marijuana, Burkman warns, will make a borderline-infertile man frankly infertile.
"The marijuana-smoking men had significantly lower semen volume," Burkman says. "Many had pretty low volume, about half the male norm. If they came to our clinic as patients, we'd tell them they are abnormal. ... They are delivering significantly fewer sperm to the female when they have sexual intercourse."
Burkman's team studied only men. But she says that when women smoke marijuana, the active ingredient -- THC -- appears in their reproductive organs and vaginal fluids. Sperm exposed to this THC are likely to act just as sperm exposed to THC in the testes.
"When women smoke marijuana, nicotine, or other drugs, their reproductive fluids contain these drugs," Burkman says. "The woman smoking marijuana is putting THC into her oviduct, into her cervix. If the man is not smoking but the woman is, his sperm go into her body and hit THC in the vagina, oviduct, and uterus. Her THC is changing his sperm."
Dominguez says that Burkman's study is more important than merely warning men and women to avoid marijuana if they want to get pregnant. She says that by learning how the reproductive tracts of men and women respond to different chemical signals, researchers will learn more about how to help people get pregnant -- or even to avoid it.