As the most widely used illegal drug in both the UK and the US (yes, okay, it's legal in some US states now, but even if it weren't it would still be the top dog), so it makes sense to start here. Cannabis basically works because it contains substances that are very similar to ones that naturally occur in our brains, cannabinoids, although in much smaller quantities. Usually, when the neurons in your brain fire, they are programmed to take a short break afterwards to allow your brain to keep ticking over in a nice, calm, suitable-behaviour-for-the-workplace kinda way. However, when the cannabinoids get hold of your neurons, they force them the keep firing, meaning that you thought process starts running away with itself. This is largely why people on marijuana tend to talk such gibberish in the classic stream-of-consciousness fashion. It also makes it very difficult to hold anything in your short term memory because new impulses are constantly being fed through, meaning that your brain doesn't have time to stop and take stock. It has also been found, that people who smoke a lot of pot tend to have a smaller orbitofrontal cortex, the part of your brain that deals with emotions and decisions, but stronger, thicker connections within the orbitofrontal cortex to compensate. It is not properly understood yet whether weed is shrinking the brains of smokers, or whether people with a naturally smaller orbitofrontal cortex are more likely to take up the habit in the first place, as a separate study has found that this might be a good predictor. At the moment, more study needs to go into the effects of marijuana, particularly as it is being legalised. The only main side effect that we can be really sure is caused by smoking a lot of pot, is never shutting up about smoking pot.
