Our ideas about the "munchies" have come a long way: We're no longer just giggling about a magically empty bag of Doritos in a smoky basement. Now experts are really considering cannabis as a potential treatment for chemotherapy-induced nausea and HIV/AIDS-related lack of appetite. But we still don't quite understand how pot makes us suddenly crave Taco Bell's latest abomination.
Beyond that, though, researchers are still developing a concrete explanation for why the munchies happen. And, of course, most of this research has been done in non-human animals. So we don't know for sure how much of it is applicable to humans — but researchers are on the case.
For instance, one study published in 2014 in Nature Neuroscience found that your brain's CB1 receptors are a key part of the equation. These are activated both by chemicals your body makes naturally and compounds found in marijuana (cannabinoids). The researchers in this study found that CB1 receptors are surprisingly crucial for your ability to smell. Therefore, the researchers suggest that any effect pot has on our hunger could be a direct results of CB1 receptors heightening our sense of smell. After all, that pizza's a lot harder to resist once those cheesy, carb-filled fumes start wafting their way through your apartment.
According to a 2015 study published in Nature, though, it's probably a little (or a lot) more complicated than that. Here, researchers were able to pinpoint changes in a part of the mouse brain called the hypothalamus, which, in humans, is responsible for a bunch of our internal hunger chemistry. Other research has found that cannabinoids also manipulate the messages sent by ghrelin, a hunger-related compound hanging around your hypothalamus. Essentially, this suggests that those cannabinoids in weed switch on a fair amount of our brain's chemical signals for "I'm hungry."
Plus, we know that cannabis has an indirect effect on the reward circuit. That suggests that our enjoyment of an already-enjoyable thing is heightened when we're high.
Taking all of this together, it seems like marijuana may make us hungrier by working on a few different levels: It increases our appetite, it makes already-good food even more appetizing, and it makes food taste better when we get it. So, basically, just remember to order the pizza before you light up.