In 1970, marijuana was classified  as a schedule 1 drug in the United States: the strictest designation possible, meaning it was completely illegal  and had no recognized medical uses. For decades, this view persisted and set back research  on the drug's mechanisms and effects. Today, marijuana’s therapeutic benefits  are widely acknowledged, and some nations  have legalized medical use or are moving in that direction. But a growing recognition  for marijuana’s medical value doesn’t answer the question: is recreational marijuana use  bad for your brain? Marijuana acts  on the body’s cannabinoid system, which has receptors  all over the brain and body. Molecules native to the body,  called endocannabinoids, also act on these receptors. We don’t totally understand  the cannabinoid system, but it has one feature  that provides a big clue to its function. Most neurotransmitters  travel from one neuron to the next through a synapse to propagate a message. But endocannabinoids  travel in the opposite direction. When a message passes  from the one neuron to the next, the receiving neuron  releases endocannabinoids. Those endocannabinoids  travel backward to influence the sending neuron— essentially giving it feedback  from the receiving neuron. This leads scientists  to believe that the endocannabinoid system serves primarily  to modulate other kinds of signals— amplifying some and diminishing others. Feedback from endocannabinoids  slows down rates of neural signaling. That doesn’t necessarily mean it slows down behavior  or perception, though. For example,  slowing down a signal that inhibits smell could actually make smells more intense. Marijuana contains  two main active compounds, tetrahydrocannabinol or THC,  and cannabidiol, or CBD. THC is thought to be primarily responsible for marijuana’s psychoactive effects on behavior, cognition, and perception, while CBD is responsible  for the non-psychoactive effects. Like endocannabinoids, THC slows down signaling  by binding to cannabinoid receptors. But it binds to receptors  all over this sprawling, diffuse system at once, whereas endocannabinoids  are released in a specific place in response to a specific stimulus. This widespread activity  coupled with the fact that the cannabinoid system indirectly affects many other systems, means that each person’s  particular brain chemistry, genetics, and previous life experience largely determine  how they experience the drug. That’s true much more so with marijuana  than with other drugs that produce their effects  through one or a few specific pathways. So the harmful effects, if any,  vary considerably from person to person. And while we don’t know  how exactly how marijuana produces specific harmful effects, there are clear risk factors  that can increase peoples’ likelihood of experiencing them. The clearest risk factor is age. In people younger than 25, cannabinoid receptors  are more concentrated in the white matter than in people over 25. The white matter  is involved in communication, learning, memory, and emotions. Frequent marijuana use can disrupt the development  of white matter tracts, and also affect the brain’s ability  to grow new connections. This may damage long-term learning ability and problem solving. For now, it’s unclear  how severe this damage can be or whether it’s reversible. And even among young people, the risk is higher the younger someone is— much higher for a 15 year old  than a 22 year old, for instance. Marijuana can also cause hallucinations  or paranoid delusions. Known as marijuana-induced psychosis, these symptoms usually subside  when a person stops using marijuana. But in rare cases,  psychosis doesn’t subside, instead unmasking  a persistent psychotic disorder. A family history of psychotic disorders,  like schizophrenia, is the clearest, though not the only,  risk factor for this effect. Marijuana-induced psychosis  is also more common among young adults, though it’s worth noting  that psychotic disorders usually surface in this age range anyway. What’s unclear in these cases  is whether the psychotic disorder would have appeared  without marijuana use— whether marijuana use triggers it early, is a catalyst for a tipping point  that wouldn’t have been crossed otherwise, or whether the reaction  to marijuana is merely an indication of an underlying disorder. In all likelihood, marijuana’s role  varies from person to person. At any age, as with many other drugs, the brain and body become less sensitive  to marijuana after repeated uses, meaning it takes more  to achieve the same effects. Fortunately, unlike many other drugs, there’s no risk of fatal overdose  from marijuana, and even heavy use  doesn’t lead to debilitating or life-threatening  withdrawal symptoms if use stops. There are more subtle forms  of marijuana withdrawal, though, including sleep disturbances,  irritability, and depressed mood, which pass  within a few weeks of stopping use. So is marijuana bad for your brain? It depends who you are. But while some risk factors  are easy to identify, others aren’t well understood— which means there’s still some possibility of experiencing negative effects, even if you don’t have  any of the known risk factors.