Cannabis can have some pretty strange effects on our bodies, but new research suggests there's another potential benefit of the drug - better night vision.
A study in tadpoles has shown that the drug can make cells in the retina more sensitive to light, and it could not only help people see better in low light, but it could also treat degenerative eye conditions such as retinitis pigmentosa.
It sounds pretty crazy, but this isn't the first time it's been reported that cannabis could improve night vision.
As Mo Costandi reports for The Guardian, 25 years ago, pharmacologist M. E. West from the University of the West Indies in Jamaica reported that local fisherman who smoked weed or drank rum made from the leaves and stems of cannabis plants had "an uncanny ability to see in the dark".
The same thing was rumoured to be happening to Moroccan fisherman and mountain dwellers who smoked hashish - a product of the cannabis plant - and in 2002, a team of researchers from the US and Spain set out to study the phenomenon.
They gave one volunteer a placebo and three others hashish, and found that all three of the test subjects had better night vision after taking the drug. They published their results in 2004 in the Journal of Ethno-Pharmacology.
But, between then and now, no one had been able to figure out on a biological level exactly how weed could have that effect.
The closest researchers had come was to show that there were more CB1 receptors - which the psychoactive ingredient of cannabis is known to bind to - in the eye than the brain, so they were pretty sure that whatever was going on was happening within the retina rather than the visual cortex.
To investigate further, a team led by Lois Miraucourt from the Montreal Neurological Institute in Canada decided to study the transparent tadpoles of the African clawed toad, Xenopus laevis.
In the first part of their experiment, the researchers applied synthetic cannabinoid - made in the lab to replicate the effects of cannabis - to eye tissue of the tadpoles, and used microelectrodes to measure how retinal ganglion cells (which make up the optic nerve) responded to light.
They were able to show that the cannabinoid had made the cells more sensitive, and made them fire more rapidly in response to both bright and dim light.
They were able to show that this happened because of cannabinoid binding with the CB1 receptor, and inhibiting a protein called NKCC1, which shuttled chloride ions in and out of cells to determine their electrical potential.
"Overall, these experiments show that cannabinoids reduce the concentration of chloride ions inside the retinal ganglion cells, making them more excitable and more sensitive to light," writes Costandi.
To investigate whether this was actually affecting the tadpoles' vision at all, the team then placed the tadpoles in a Petri dish under varying light conditions, and monitored how they responded to dark dots when they'd been exposed to cannabinoid or untreated.
Usually, tadpoles will move away from dark dots, as they could signal an incoming predator. And the team showed that, under normal light conditions, there was no difference between the tadpoles that had been treated with cannabinoid and those that hadn't.
But in low light conditions, the cannabinoid tadpoles avoided significantly more dark dots than untreated ones, which only responded to the dots by chance.
This suggests that the tadpoles treated with cannabinoid were more sensitive to light, and were able to see these dots better in the dark.
Of course, it's not year clear yet if the same thing could happens in humans, and we still have a lot to learn about the effect itself - for example, how long it lasts, and whether it can be separated from the 'high' of the drug. It's also not known if there are any side effects to the visual system.
There's a lot more research to be done, but the results suggest that some of the compounds in cannabis could potentially be useful for treating diseases such as retinitis pigmentosa and glaucoma, which kill off cells in the retina and gradually cause blindness.
It's also not terrible to think that maybe cannabis could be giving us better night vision… although we'll need to wait for more verification before we get too excited about that.
The research has been published in eLIFE.