Smoking is the most consistently confirmed lifestyle factor in male fertility research. A 2016 meta-analysis of 20 studies and 5,865 men found smokers had lower sperm count, lower motility and poorer morphology than non-smokers, with bigger effects in moderate and heavy smokers. A 2019 meta-analysis of 10,823 men attending fertility clinics found smokers were 1.29 times as likely to have a low sperm count. Vaping has one large study: of 1,221 young Danish men asked about e-cigarettes, the 25 who vaped daily had an adjusted total sperm count of 91 million against 147 million in non-users. Because sperm take about 74 days to develop, a test two to three months after quitting is the first that can reflect it.
What smoking does to sperm
Sharma and colleagues (European Urology, 2016) pooled 20 studies covering 5,865 men and compared smokers with non-smokers on the three measures every semen analysis reports. Smokers came out lower on sperm count, lower on motility and lower on the share of normally shaped sperm, and the differences were larger in men who smoked moderately or heavily than in light smokers, and larger in men attending fertility clinics than in the general population. Bundhun and colleagues (BMC Public Health, 2019) looked only at infertile men, 16 studies and 10,823 participants, and found smokers were 1.29 times as likely to have oligozoospermia (a count below the reference limit) and had more defects of the sperm head, neck and tail, though in that group motility and pH didn't differ. Two meta-analyses, one dose effect, no threshold below which nothing happened: that is why the app scores even light smoking steeply.
Why
Cigarette smoke delivers nicotine, cadmium, lead and a long list of reactive compounds that raise oxidative stress throughout the body. The developing sperm cell is thought to be especially exposed during the middle of its cycle, when its genetic material is being reorganized and repacked, which is where the morphology and DNA-related findings in the smoking literature are usually placed. The meta-analyses are built on observational studies, so strictly they show association rather than cause, but the consistency across countries, decades and study designs is about as strong as observational evidence gets.
Vaping
Holmboe and colleagues (Human Reproduction, 2020) studied 2,008 young Danish men examined for military service and asked about cigarettes and cannabis; 1,221 of them were also asked about e-cigarettes. The 25 daily e-cigarette users had an adjusted total sperm count of 91 million, against 147 million in men who didn't vape, with cigarette smoking taken into account. That is one cross-sectional study, the first of any size, and it needs replicating before anyone can say vaping is as harmful as smoking, less harmful, or harmful in a different way. What can be said is that the only decent evidence points in the same direction as cigarettes. If you switched from cigarettes to vaping to try to conceive, the switch is not the finish line.
Cannabis
The same Danish study looked at cannabis and found a murkier picture: in the main analysis, daily and occasional users didn't differ from non-users on total sperm count, while a secondary analysis found weekly users had lower counts, and daily users had about 8% higher testosterone. Other studies have gone both ways; the cannabis guide goes through them. If you use it and are trying to conceive, the evidence is unsettled rather than reassuring, which is a reason to pause it rather than to worry.
What happens after you quit
Here the evidence is thinner than you'd hope. The meta-analyses compare smokers with non-smokers at one point in time rather than following men after they stop, so recovery time is an inference from the production cycle rather than a measured result. The inference is straightforward: sperm take about 74 days to develop plus one to two weeks in the epididymis, and cells that develop entirely after you quit are the ones a test two to three months later measures. The sauna study in the heat guide shows that kind of recovery for a different exposure, which makes it a reasonable expectation here too, but nobody should promise you a number.
What to do
- Stop, rather than cut down. The dose effect in the meta-analyses runs all the way down; there is no light-smoking band the evidence calls safe.
- Treat vaping as part of the same problem. The one large study points the same way as cigarettes.
- Use the free support. In the UK, the NHS Better Health quit-smoking service offers a plan, an app and local stop-smoking services; nicotine replacement and prescription medicines make a quit attempt more likely to stick than willpower alone.
- Pause cannabis too, if that applies.
- Put the retest three months after your quit date, so the first result after stopping is one that can show it.
Related guides
Sources
- Sharma R, et al. Cigarette smoking and semen quality: a new meta-analysis examining the effect of the 2010 World Health Organization laboratory methods for the examination of human semen. European Urology, 2016.
- Bundhun PK, et al. Tobacco smoking and semen quality in infertile males: a systematic review and meta-analysis. BMC Public Health, 2019.
- 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.
- NHS. Quit smoking (Better Health).
- Heller CG, Clermont Y. Kinetics of the germinal epithelium in man. Recent Progress in Hormone Research, 1964.
Correction, 8 October 2026. The vaping figures come from the 1,221 men asked about e-cigarettes, of whom 25 vaped daily, and 91 and 147 million are adjusted estimates, not medians.
