In men, the best clinical study3 available looked at 1215 patients and showed that smoking marijuana may decrease sperm count by nearly 30%. The effects of marijuana on female fertility are a lot more difficult to study, as we do not have a direct measure similar to semen parameters in men. A study looking at the effect of lifestyle factors on ovulation timing showed that women who smoked marijuana had significantly delayed ovulation by up to 3.5 days.4 Furthermore, there was a disproportionate amount of anovulatory cycles in the group that smoked marijuana. This was a small study of 201 women, of whom only 29 reported smoking marijuana in the preceding 3 months.
Interestingly, population-based studies have not shown an effect of marijuana use in either partner on time to pregnancy or overall pregnancy rates.5 However, based on the underlying mechanisms and the observed effects in men and women, it is plausible that smoking marijuana could make it more difficult to conceive if a couple is already having trouble.
Raul I. Clavijo, MD: The effects of cannabis on fertility are complex, as cannabis may have agonist and antagonist effects on the different influencing physiologic mechanisms that lead to normal fertility. The underlying mechanism that drives these physiologic changes is likely a direct result of cannabinoids, such as THC found in cannabis, binding to receptors that make up the endocannabinoid system. However, no research has been able to fully elucidate the precise mechanism.
Overall, you can split up the effects of cannabis on fertility into the effect cannabis has on reproductive hormones (testosterone likely being the most important hormone) and seminal parameters. Overall, findings are mixed regarding the effect of cannabis on testosterone production, with studies showing an increase, a decrease, and no change in testosterone levels. However, there is overwhelming evidence at this point that cannabis does reduce sperm concentration. As this, along with sperm motility, are the most important seminal parameters, it is safe to say that cannabis likely has a negative effect on fertility potential.
The reason this is not a definitive statement is that cannabis has typically been studied in the smoked form, with several studies using rolled “joints.” This adds the confounder of smoke particles inhaled when cannabis is combusted. It is possible that vaporized or edible cannabis won’t affect hormones and seminal parameters the same way smoked cannabis does.
Sandeep Raha, PhD: There is very little known about the mechanism of how cannabis and its constituents may affect fertility. Mechanistically, some work has been done in animals, and there is evidence that THC may reduce the ability of the fertilized oocyte to properly implant in the uterine wall. However, I am not aware of studies that systemically approached this question. My laboratory hopes to address some of these questions in the future.
Endocrinology Advisor: How should clinicians advise patients regarding this topic?
Dr Ilnitsky: My general advice to any couple trying to conceive is to clean up their lifestyle. Smoking cessation, reducing alcohol and caffeine, healthy eating, and regular exercise are all important for both fertility and pregnancy. To that end, marijuana is a recreational substance with potential effects on reproductive function and pregnancy and should be avoided.
Dr Clavijo: Until we know more, given that cannabis has been consistently shown to diminish sperm concentrations, clinicians should recommend that patients cease cannabis use if they are trying to conceive. This is especially important in patients who have lower sperm concentrations at baseline, and may ultimately affect how they will be able to conceive (ie, natural vs intrauterine insemination vs in vitro fertilization). Clinicians should also warn young men who have conditions that can negatively affect fertility (such as varicocele) that cannabis may worsen their fertility potential, and that the reversibility of these changes is not known.