Booze could be more damaging to your brain than weed - according to a recent study which linked alcohol consumption to long-term changes to parts of the brain, but didn't find any long-term changes in relation to marijuana.

The study found that the negative effects of drinking were worse for adults over the age of 18, but that even in younger people there was a noticeable effect on reducing the levels of grey matter, the stuff that processes information in the brain.

But the researchers are keen to point out that it's a complex picture – more work is going to be needed to figure out exactly what these drugs do to the connections inside our neural networks.

"With alcohol, we've known it's bad for the brain for decades," says one of the team, Kent Hutchison from the University of Colorado Boulder. "But for cannabis, we know so little."

The research looked at brain scans collected in previous studies, covering 853 people aged between 18 and 55, and 439 young adults aged between 14 and 18. A range of different levels of cannabis and alcohol use were reported by the participants.

As well as reducing the volume of grey matter in the brain, alcohol use was associated with a loss of integrity in the white matter – the parts of the brain that handle communications between grey matter clusters, as well as links to the rest of the body.

In other words, any kind of damage to grey or white matter can impair the brain's normal function. The negative effects were particularly pronounced for people with a long history of drinking, the researchers said.

One of the main aims behind the study was to gather more information about the impact of cannabinoids, the active ingredient in cannabis, on our health – especially as the drug is now legal in some parts of the United States.

In this particular study at least, no link was found between cannabis use and damage to the grey or white matter inside the brain. That backs up other research that weed isn't as bad for your health as drink is.

So is marijuana in the clear? Not quite.

"Particularly with marijuana use, there is still so much that we don't know about how it impacts the brain," says one of the researchers, Rachel Thayer from the University of Colorado Boulder.

"Research is still very limited in terms of whether marijuana use is harmful, or beneficial, to the brain."

At least now we have another set of figures to go off when it comes to assessing the health risks of these two drugs. The researchers are calling for more consistency in future studies in terms of how they assess damage to the structure of the brain.

And while cannabis may not be as damaging as previously thought, the jury is still out on whether it provides any tangible benefits to health, something that's important when the drug is prescribed for medical use.

Particularly in the US, there's a lot of debate about just how good or bad for our bodies weed actually is, with claims and counter-claims not always backed up by solid science. This latest study hopes to go some way to correcting that.

"Considering how much is happening in the real world with the legalisation movement, we still have a lot of work to do," says Hutchison.

The research has been published in Addiction.