In the last year, there has been a wave of legislation legalizing and decriminalizing marijuana for recreational and medical purposes. In response, there has been scientific study after scientific study looking at how cannabis affects smokers' health and brain function, and even crime rates.

Now, researchers have compiled several recent studies for a review published last month in the journal Frontiers in Behavioral Neuroscience, showing how the use and effects of marijuana vary by sex. They argue that it is not just sociocultural factors that differentiate patterns between men and women, but biological aspects too. And it comes down to our hormones.

It is worth pointing out right from the start that these are the conclusions of animal studies, not human studies, and it can be difficult to get lab rats to self-administer cannabinoids in a way that mimics cannabis use in recreational users. However, while they might not be definitive nor can they conclusively prove that the effects observed in female animals are exactly the same as those in human women, they provide convincing evidence to explore further.

So, what did they find? First of all and perhaps unsurprisingly, men are far more likely to smoke pot in the first place – living up to the stoner stereotype as depicted in movies like Pineapple Express and Knocked Up. In fact, men may be four times as likely to try marijuana than women and when they do, they do so more frequently and use higher doses.

"Male sex steroids increase risk-taking behavior and suppress the brain's reward system, which could explain why males are more likely to try drugs, including cannabis," Liana Fattore, co-author and Senior Researcher at the National Research Council of Italy and President of the Mediterranean Society of Neuroscience, said in a statement. 

But while fewer women take up the habit in the first place, the transition from first go to addiction tends to be quicker, the withdrawal symptoms tend to be more severe, and relapse rates tend to be higher. 

"Females seem to be more vulnerable, at a neurochemical level, in developing addiction to cannabis," Fattore added. 

"[F]emale rats have different levels of endocannabinoids and more sensitive receptors than males in key brain areas related to these functions, with significant changes along the menstrual cycle. As a result, the interactions between the endocannabinoid system and the brain level of dopamine – the neurotransmitter of 'pleasure' and 'reward' – are sex-dependent."

Differences in the cannabinoids themselves, the lab animals, and the duration of hormone exposure meant that it was difficult to directly compare the results of all the studies, but the researchers found that human data are consistent with the idea that the hormone estradiol (a type of estrogen) is largely responsible for the female response to cannabinoids. 

"Blood levels of enzymes which break down cannabinoids fluctuate across the human menstrual cycle, and imaging studies show that brain levels of cannabinoid receptors increase with aging in females – mirroring in each case changes in estradiol levels," Fattore explained.

Marijuana is but only the tip of the iceberg – a whole range of drugs could affect men and women differently, morphine and nicotine included.