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Many people have had to take a urine test for cannabis, perhaps as a job requirement. Using the popular procedure, marijuana metabolites can in some cases be detected for weeks after a person’s last use. But here’s a question few may have thought to ask: Can THC be detected in semen?

According to a new study by a team of Harvard Medical School researchers, the answer is yes—at least sometimes. In a study of 12 participants who regularly consumed marijuana by inhalation, the researchers were able to detect delta-9 THC, the main psychoactive ingredient in cannabis, in two subjects’ semen samples. And at least one metabolite of THC—what’s left over after the body processes the compound—could be detected in all samples capable of being analyzed. “Two semen samples,” the report says, “had insufficient volume to be analyzed.”

Why the focus on THC in semen? In a word, pregnancy. Men of reproductive age, the study’s authors note, “are the most prevalent consumers of marijuana, with 19.4% of men in the USA reporting use.” A 2018 study cited by the authors found that 16.5 percent of men and 11.5 percent of women reported using marijuana while attempting to conceive.

How exactly THC affects reproductive systems and childhood development are questions the Harvard authors don’t attempt to answer in the study. The primary goal of the proof-of-concept research, they explain, “was to determine whether THC can cross the blood-testis barrier.” On that front, they appear to have succeeded.

“In the setting of a growing repository of data surrounding the effects of the endocannabinoid system in the regulation and maintenance of fertility and early pregnancy,” the study says, “ours is the first report that the exogenous cannabinoid THC can be detected in any human reproductive matrix.”

Because of the interest in whether THC could be detected at all, researchers focused on regular, long-term marijuana consumers. All participants indicated they had used the drug between 25 and 30 days of the last month, and most said they had been regular consumers for at least five years. “Consequently,” the team said, “our study findings cannot be generalized to include ever users, light, or moderate users of marijuana.”

Of the two participants whose semen contained detectable levels of THC itself, samples contained 0.97 nanograms per milliliter and 0.87 ng/mL.

But it wasn’t clear what set those two participants apart. There was no correlation between semen THC and concentration of the metabolite THC carboxylic acid in urine, nor with time since last cannabis consumption, participant age or participant body mass index.

“It is puzzling that some, but not all, semen samples tested positive for THC,” the study says. “There were no obvious factors that were strongly associated with detectable semen THC; thus, we can propose few predictors of the presence of THC in human semen. Future directions (of research) include identifying characteristics that may affect semen detectable THC levels.”

How precisely THC affects semen—or the sperm within it, not to mention conception, pregnancy or childhood development—is still hard to say with certainty. As the Harvard researchers note in the study, “Evidence linking marijuana to reproductive outcomes is scarce and to date, often conflicting.”

One study of 1,200 young Danish men, for example, found that those who smoked marijuana regularly had lower sperm counts than those who did not. Another study, of 662 older, subfertile men in Massachusetts, found that men who had ever smoked marijuana had significantly higher sperm counts than those who’d abstained.

As for the effects of THC on sperm, or conception itself, those also remain unclear. “The effect of marijuana on human gametes and fertilization is relatively unknown,” the new paper says. Endocannabinoid receptors have been reported on sperm themselves, but “studies examining the direct effect of THC on human sperm are limited.”

Most research so far has either been observational, by measuring THC through self-reporting or blood testing, or studied the behavior of sperm that had been washed in a laboratory with a THC solution. “Our findings, that THC can be directly quantified in human seminal fluid, lay the groundwork to allow for future studies,” the new study says. “Since THC can be detected in the seminal fluid of some individuals, this might provide a direct method of measurement (rather than relying on self-reporting marijuana use, which is subjective and potentially unreliable, or serum levels which only reflect recent exposure) to bridge real-world clinical studies with the prior staged studies in which THC was directly incubated with washed sperm.”

While the THC-washed sperm showed some concerning effects, including decreased motility and mitochondrial oxygen consumption, the Harvard team acknowledged the concentrations of THC used in those studies were significantly stronger than anything observed in their semen study: “It should be noted that even the lowest concentration of THC with which former studies incubated sperm was over tenfold higher than the concentration of THC detected in the semen of our study subjects.”

In other words, the study is a stepping-stone to further research. And while the top-line findings might elicit some giggles, authors say the study is serious business.

“The ability to quantify cannabinoids in human reproductive tissues and fluids,” they conclude, “gives us the capability to directly study the effects of cannabis on early human reproduction.”

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