Could cannabis make men more fertile? Researchers at Harvard recently published a study[1] linking cannabis use to increased sperm count. They collected 1,143 sperm samples from 662 men who sought treatment at a fertility clinic between 2000 and 2017, and found that those who reported smoking weed showed sperm concentrations of 62.7 million sperm per millilitre (million/ml), while those who had never smoked showed 45.4 million/ml. Additionally, while 5% of the men who smoked weed showed sperm counts below the WHO’s “normal” threshold of 15 million/ml, 12% of men who didn’t smoke fell into this territory. Cannabis smokers were half as likely to score below WHO minimum thresholds for sperm motility and sperm concentration.

The average age of the men was 36, and the majority were white and college educated. The study was conducted at the Fertility Clinic at Massachusetts General Hospital.

These results fly in the face of a slew of previous research linking cannabis use to reduced male fertility. As lead study author Feiby Nassan stated, “Our findings were contrary to what we hypothesized at the start of the study”.

We’ll start by examining the results, taking a closer look at male fertility, reviewing past research, and finally, seeing what conclusions we can draw.


This study seems to suggest that weed can increase male fertility. This may well be true; cannabis has been associated with a slew of positive effects, and it’s entirely possible that there’s some mechanism in cannabis consumption that increases sperm count. Furthermore, research[2] has shown that endocannabinoids play a large role spermatogenesis, suggesting that phytocannabinoids may have a large impact on this biological function.

However, the study should be interpreted with caution. It shows a correlation between weed smoking and sperm count, but correlation does not imply causation. There could be a third factor causing both the higher sperm count and the weed smoking; testosterone, for example, can cause both high sperm count and greater risk-taking behaviour, so it’s possible that men who happen to have higher testosterone are more likely to both smoke weed and have more sperm.

It’s also possible that the subjects of this study were non-representative of the general population, given they were men seeking treatment for infertility. The men might also have been dishonest about their cannabis use, given that cannabis was illegal at the time and location of the experiment. Further, the study didn’t control for the magnitude of lifetime cannabis use; it only looked at whether the subjects had previously smoked weed or not.

As the study’s authors stated, “Whether these findings are reflective of the previously described role of the endocannabinoid system in spermatogenesis or a spurious association requires confirmation in further studies”.

Furthermore, when we go back and examine previous studies on the topic of cannabis and male fertility, the plot thickens.

Male Fertility


It’s important to note that sperm count isn’t the only factor determining male fertility, though it’s an important one. Another factor is sperm motility, the ability of sperm to “swim” properly through the female reproductive tract. Reduced seminal volume can also cause fertility problems, even if sperm count is normal. Others factors causing infertility can include white blood cells in sperm, abnormally formed sperm, or a high volume of dead sperm.

Further, 15% of infertile men have normal sperm production in their testes, and normal semen. Their problem is often getting the sperm into the semen. This can happen due to blockages in the vas deferens, the tubes transporting sperm from the testes to the ejaculatory ducts. Some men are born without a vas deferens, for genetic reasons. Other men produce antibodies that attack sperm cells on their way out of the testes. Others still experience retrograde ejaculation, where the sperm ejaculates backwards into the bladder.


Past research on this topic further complicates the story. A 2015 study[3] on 1,215 Danish military recruits found that men with a history of weekly pot use had 28% lowered sperm concentration and 29% lower sperm count, though the weed smokers had higher testosterone.

Animal research adds a whole new dimension to the picture. The main advantage of animal research is that scientists can control for extraneous variables, thereby eliminating the confounds we’ve been grappling with. The disadvantage is that humans are different from animals, and the results might not always carry over the species boundary.

A 2013 study[4] on rats found that the administration of cannabis significantly decreased sperm count, sperm motility, and the diameter of seminiferous tubules (the cell system where sperm is gestated, matured, and transported in the testes). A 2007 study[5] seeking a safe method of contraception found that cannabis sativa reduced sperm count in male Wistar rats. A third study[6], from 2011, found that bhang, an edible cannabis preparation common in India, reduced fertility in male rats, possibly due to alterations in the testicular endocannabinoid system.- 

Narrowing down to a possible mechanism of action, one 2009 study[7] found that THC inhibited the cellular respiration of sperm cells. This effect was much less in “neat” sperm, still contained in seminal fluid, suggesting a protective effect from the plasma. Further, the action of isolated mitochondria was also affected by cannabis, supporting the idea that cannabis might make it hard for sperm cells to produce energy. Over time, this could cause lower sperm count.

Sperm Structure


As one meta-analysis[8] acknowledges, the research is contradictory, but the preponderance of evidence suggests that cannabis could hurt sperm count. There could be a few reasons for these messy results: some might site researchers biased against cannabis, while others might point to confounds in studies on human populations. It’s equally possible that cannabis use affects fertility differently at different use profiles: cannabis is known to have a “biphasic effects distribution”, where low levels of use often have opposite results to high levels. It’s entirely possible that consuming small amounts of cannabis increases sperm count, while consuming large amounts decreases it.

Overall, it’s yet unclear how cannabis affects sperm count and male fertility, though this recent study is promising. That being said, men trying to conceive may want to consider keeping their pot use on the lighter side of the spectrum.

External Resources:
  1. Marijuana smoking and markers of testicular function among men from a fertility centre | Human Reproduction | Oxford Academic
  2. The Endocannabinoid System and Spermatogenesis
  3. Association Between Use of Marijuana and Male Reproductive Hormones and Semen Quality: A Study Among 1,215 Healthy Young Men. - PubMed - NCBI
  4. Efecto del Extracto Hidroalcohólico de Cannabis sativa sobre los Parámetros Espermáticos e Histología Testicular en Ratas
  5. The Effect of Cannabis sativa Hydroalcoholic Extract on Sperm Parameters and Testis Histology in Rats
  6. Effects of chronic bhang (cannabis) administration on the reproductive system of male mice
  7. Cannabinoids inhibit the respiration of human sperm - ScienceDirect
  8. Adverse Effects of Cannabis on Male Reproduction. - PubMed - NCBI
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