The evidence is mixed, and the honest answer is that frequent use is the pattern to worry about. In a study of 1,215 young Danish men from the general population, smoking cannabis more than once a week was linked to 28% lower sperm concentration and 29% lower total count, and the gap roughly doubled in men who also used other drugs [1]. But a study of 662 men at a US fertility clinic found the opposite, a 2021 meta-analysis of 9 studies found no clear link with abnormal semen results, and a study of 1,125 couples found no clear link with how quickly they conceived [2, 4, 5]. All of it is observational. If you use cannabis most weeks and you are trying for a baby, cutting down is a low-cost change, and a semen analysis about three months later will show where you are.
What the studies found
The Danish population study. Gundersen and colleagues (American Journal of Epidemiology, 2015) examined 1,215 men aged 18 to 28 at their military fitness assessment, with a semen sample, a blood test and a questionnaire about the previous three months [1]. 45% had smoked cannabis in that time. Smoking it more than once a week was linked to 28% lower sperm concentration and 29% lower total sperm count after adjusting for other factors, and men who combined that with other recreational drugs had 52% lower concentration and 55% lower total count. The confidence intervals are wide (the 28% could plausibly be anywhere from 1% to 48%), and the association the paper reports is for more-than-weekly use.
A later Danish study of 2,008 young men from the general population (Human Reproduction, 2020), a third of whom used cannabis, was designed mainly to look at e-cigarettes. For cannabis, its summary reports 8.3% higher testosterone in daily users, and notes that cannabis users were often cigarette smokers too [10].
The fertility clinic study that found the reverse. Nassan and colleagues (Human Reproduction, 2019) followed 662 men at a Boston fertility center between 2000 and 2017 [2]. Men who had ever smoked cannabis had higher average sperm concentration than men who never had, 62.7 against 45.4 million per mL after adjustment, and current and past users did not differ. The authors were careful: the men were already attending a fertility clinic, use was self-reported, and they said the result could be a chance finding that needs confirming.
Pooling the studies. A 2021 meta-analysis in Andrology combined 9 studies with semen data on 4,014 men [4]. Abnormal results were more common in users (44.9%) than non-users (24.5%) in the raw numbers, but after pooling, the risk of an abnormal semen result was not clearly higher (relative risk 1.16, with a range from 0.84 to 1.60 that includes no difference). The authors concluded that the current evidence does not suggest a clinically significant association, while saying an effect cannot be ruled out because the studies are few and different from each other.
A broader review. A 2019 systematic review in the Journal of Urology read the same literature more cautiously in the other direction: it found the strongest evidence for an effect was on semen measures (count, concentration, motility, morphology), with hormone findings inconclusive for testosterone, and concluded that cannabis "may negatively impact male fertility" [3].
Does it change the chance of pregnancy?
That is the question that matters, and few studies have looked at it for men directly. One is PRESTO, a North American study of couples trying to conceive (Journal of Epidemiology and Community Health, 2018), 1,125 couples had information on the male partner's use [5]. Couples where the man used cannabis once a week or more did not conceive at a clearly different rate from couples where he did not (fecundability ratio 1.24, with a range from 0.90 to 1.70 that includes no difference). The authors found "little overall association" for either partner, and pointed out how hard it is to measure the amount people use.
What about the DNA in sperm?
A small study at Duke (Epigenetics, 2018) compared the sperm of 24 men, regular users and non-users, and found DNA methylation, a set of chemical tags that helps control which genes are switched on, differed by at least 10% at 3,979 sites [6, 7]. Similar pathways were affected in rats given THC, and men with more THC in their urine showed bigger changes. Nobody yet knows whether these changes last, or whether they pass to a child; the authors flag both as open questions and the small sample as a limit [7]. It is a reason for caution, not a measured harm.
Why the studies disagree
- Who was studied. Healthy young men from the general population in Denmark, versus men already at a fertility clinic in the US. A finding in one group may not hold in the other.
- How use was measured. Every study relies on men saying how much they use, often while it was illegal, and "use" covers anything from a joint a month to daily heavy use.
- Everything that travels with cannabis. Tobacco, alcohol and other drugs often go with it, and the Danish study's biggest differences were in men who used other drugs as well.
- Frequency. Where an association shows up, it is with frequent use. Occasional use is not consistently linked to anything.
CBD, edibles and vaping
The human studies above are almost all about smoked cannabis, and none of them separates CBD from THC or edibles from smoking. There is not enough human evidence on CBD products, edibles or cannabis vapes on their own to say whether they are safer for sperm, so treat them as unknown rather than as a safe swap.
If you stop, how long until a test shows it?
No study has followed men after they stopped and measured their sperm, so the timing is an inference from how sperm are made. One cycle of production takes about 74 days in the testis, and sperm then spend one to two weeks in the epididymis, so a semen analysis about three months after stopping reflects sperm made without it [8]. The author of the Duke study goes further, advising men to stop at least six months before trying as a precaution, given what is not yet known about the DNA changes [7].
What the NHS says
The NHS lists illegal drugs, naming marijuana and cocaine, among the things that "can also affect semen quality" [9]. The 2019 review suggests clinicians ask about cannabis use as part of a male fertility work-up [3], so it is worth being straight with your GP or clinic about how much you use.
What to do
- If you use cannabis more than once a week, cut down or stop while you are trying to conceive. Frequent use is where the evidence of lower counts is.
- Give it three months before a semen analysis, so the test reflects sperm made after the change. See when to retest.
- Don't swap to edibles, CBD or vapes on the assumption they are safe for sperm. Nobody has shown that either way.
- If you smoke tobacco with it, tobacco smoking is one of the most consistently confirmed factors for lower sperm quality; the smoking guide covers it.
- Tell your GP or clinic how often you use. It is a standard question in a fertility work-up, and the honest answer helps them read your results.
- If cutting down is hard, the NHS has drug support services, and talking to a GP is a good first step.
Questions
Does cannabis lower sperm count?
In a study of 1,215 young Danish men, smoking it more than once a week was linked to 28% lower concentration and 29% lower total count [1]. A fertility clinic study found the opposite and a meta-analysis found no clear link, so the evidence is mixed, but the signal that does exist is for frequent use.
Is occasional use a problem?
The studies have not found a consistent link with occasional use. Where a link with lower counts has been reported, as in the Danish study, it was for smoking more than once a week [1].
Does cannabis affect testosterone?
Not in the direction people expect. The Danish study found higher testosterone in users, in the same range as cigarette smokers [1], a second Danish study found 8.3% higher testosterone in daily users [10], and the 2021 meta-analysis found no clear difference in testosterone [4].
Does Motily track cannabis?
The daily check-in asks whether you used any recreational drugs, as part of the substances habit. If you never do, you can tell it to stop asking.
Related
Sources
- Gundersen TD, et al. Association between use of marijuana and male reproductive hormones and semen quality: a study among 1,215 healthy young men. American Journal of Epidemiology, 2015.
- Nassan FL, et al. Marijuana smoking and markers of testicular function among men from a fertility centre. Human Reproduction, 2019.
- Payne KS, et al. Cannabis and male fertility: a systematic review. Journal of Urology, 2019.
- Belladelli F, et al. The association between cannabis use and testicular function in men: a systematic review and meta-analysis. Andrology, 2021.
- Wise LA, et al. Marijuana use and fecundability in a North American preconception cohort study. Journal of Epidemiology and Community Health, 2018.
- Murphy SK, et al. Cannabinoid exposure and altered DNA methylation in rat and human sperm. Epigenetics, 2018.
- Duke Health. Exposure to cannabis alters the genetic profile of sperm. December 2018.
- Heller CG, Clermont Y. Kinetics of the germinal epithelium in man. Recent Progress in Hormone Research, 1964.
- NHS. Infertility: causes.
- Holmboe SA, et al. Use of e-cigarettes associated with lower sperm counts in a cross-sectional study of young men from the general population. Human Reproduction, 2020.
