Reactions to research published in Human Reproduction that states men who had ever smoked marijuana had high sperm concentration and count than men who had never smoked marijuana. In subjects, each taking the same moderate dose of marijuana daily over only several weeks, their sperm quality plummeted. Sperm motility decreased, acrosome reactions failed to occur and worst of all, sperm counts dropped and the nurse Sertoli cells that help to make sperm disappeared irreversibly. Despite its large numbers and authorship from a highly esteemed university, it has several major design flaws: we do not know the baseline of sperm counts before marijuana smoking; making the reported increase less reliable. Secondly, the levels of marijuana taken were just self-reported so are open to high variation.
Credit: Getty Images. As legal access to marijuana continues expanding across the US, more scientists are studying the effects of its active ingredient, tetrahydrocannabinol THC , in teens, adults, and pregnant women. Experiments in rats and a study with 24 men found that THC appears to target genes in two major cellular pathways and alters DNA methylation, a process essential to normal development. The researchers do not yet know whether users pass the DNA changes THC triggers to their children and what effects that could have. National research has shown a steady decline in the perceived risk of regular marijuana use. This, combined with the demand and wide availability of marijuana bred specifically to yield higher THC content, make this research especially timely, Kollins says. The study defined regular users as those who smoked marijuana at least weekly for the previous six months.
Past research has suggested that marijuana is bad for men's fertility, but — to researchers' surprise — a new Harvard study finds no such evidence of harmful effects. The researchers collected semen and blood samples from hundreds of men at the Massachusetts General Hospital fertility clinic between and , and asked them about their marijuana use. The study cannot be taken to mean that marijuana is good for male fertility, Chavarro emphasizes; more research on that is needed.