Marijuana is the most commonly used drug worldwide, but even as access to legal weed improves, researchers are still teasing out the long-term health outcomes.

The trouble is, not all of the results match up, and this makes it difficult for users to make informed decisions. For men who smoke marijuana and are planning on having children, the advice keeps getting more confusing.

A 2015 study found that Danish men who smoked marijuana more than once a week had lower sperm counts. In addition, a study from last year found that marijuana's active ingredient, THC, can change the structure and development of a sperm's DNA.

Whether those changes can be passed on to a child remains unknown, but for now, some researchers are warning that men in their childbearing years should consider how weed might impact their fertility and possibly their offspring.

Nevertheless, because much of this research is based on animal models and those with histories of drug abuse, the average pot smoker still can't be sure there's anything sinister to worry about.

A long-term study from Harvard University has now placed this past research on even shakier ground, tipping the equation in the exact opposite direction. 

Examining more than a thousand semen samples from 662 men between 2000 and 2017, the researchers found something unexpected.

The men who admitted to smoking marijuana at some point in their life seemed to have higher concentrations of sperm than those who had never had a puff.

"These unexpected findings highlight how little we know about the reproductive health effects of marijuana, and in fact of the health effects of marijuana in general," says co-author Jorge Chavarro, an expert in nutrition and epidemiology at Harvard University.

"Our results need to be interpreted with caution and they highlight the need to further study the health effects of marijuana use."

The men involved in this study reported their own marijuana use, answering questions about whether they currently smoked weed, how much they smoked weed now, or if they had ever done so in the past.

Of all those who participated, 55 percent had smoked marijuana before - 44 percent of those men had tried it in the past, while 11 percent were current marijuana smokers.

Comparing these results to their semen samples, the researchers found that men who had smoked marijuana had average sperm concentrations of 62.7 million sperm per millilitre. Those who had never smoked marijuana only had 45.4 million sperm per millilitre.

It also didn't seem to matter whether the men had smoked in the past, or if they were current smokers, the difference remained the same.

Additionally, the hormone levels of these men were also different. Among marijuana smokers, the researchers found greater use was associated with higher serum testosterone levels.

What's more, those who had never smoked pot were more likely to fall into the extremes. In fact, this group was more than twice as likely to drop below the World Health Organisation's threshold for "normal" sperm levels.

This is not what the authors were anticipating, but they are taking their results with a grain of salt. It's true that marijuana acts on the endocannabinoid system, which is known to play a role in fertility, so the authors say there could be some benefit to sperm production from low levels of marijuana use.

But that's not the only explanation for what's going on - there could be other variables at play which have nothing to do with the presence of weed itself.

"An equally plausible interpretation is that our findings could reflect the fact that men with higher testosterone levels are more likely to engage in risk-seeking behaviours, including smoking marijuana," says Feiby Nassan, a researcher in environmental health and nutrition at Harvard.

And this difference in physiology or other associated behaviours might then also be responsible for the difference in sperm counts, rather than the marijuana.

For now, there's just not enough evidence to make any conclusions about the use of marijuana on male sperm. Watch this space.

This study has been published in Human Reproduction