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In 1970,

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marijuana was classified 
as a schedule 1 drug in the United States:

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the strictest designation possible,

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meaning it was completely illegal 
and had no recognized medical uses.

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For decades, this view persisted

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and set back research 
on the drug's mechanisms and effects.

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Today, marijuana’s therapeutic benefits 
are widely acknowledged,

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and some nations 
have legalized medical use

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or are moving in that direction.

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But a growing recognition 
for marijuana’s medical value

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doesn’t answer the question:

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is recreational marijuana use 
bad for your brain?

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Marijuana acts 
on the body’s cannabinoid system,

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which has receptors 
all over the brain and body.

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Molecules native to the body, 
called endocannabinoids,

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also act on these receptors.

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We don’t totally understand 
the cannabinoid system,

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but it has one feature 
that provides a big clue to its function.

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Most neurotransmitters 
travel from one neuron to the next

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through a synapse to propagate a message.

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But endocannabinoids 
travel in the opposite direction.

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When a message passes 
from the one neuron to the next,

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the receiving neuron 
releases endocannabinoids.

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Those endocannabinoids 
travel backward

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to influence the sending neuron—

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essentially giving it feedback 
from the receiving neuron.

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This leads scientists 
to believe that the endocannabinoid system

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serves primarily 
to modulate other kinds of signals—

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amplifying some and diminishing others.

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Feedback from endocannabinoids 
slows down rates of neural signaling.

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That doesn’t necessarily mean

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it slows down behavior 
or perception, though.

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For example, 
slowing down a signal that inhibits smell

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could actually make smells more intense.

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Marijuana contains 
two main active compounds,

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tetrahydrocannabinol or THC, 
and cannabidiol, or CBD.

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THC is thought to be primarily responsible
for marijuana’s psychoactive effects

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on behavior, cognition, and perception,

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while CBD is responsible 
for the non-psychoactive effects.

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Like endocannabinoids,

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THC slows down signaling 
by binding to cannabinoid receptors.

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But it binds to receptors 
all over this sprawling, diffuse system

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at once,

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whereas endocannabinoids 
are released in a specific place

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in response to a specific stimulus.

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This widespread activity 
coupled with the fact

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that the cannabinoid system
indirectly affects many other systems,

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means that each person’s 
particular brain chemistry, genetics,

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and previous life experience

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largely determine 
how they experience the drug.

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That’s true much more so with marijuana 
than with other drugs

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that produce their effects 
through one or a few specific pathways.

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So the harmful effects, if any, 
vary considerably from person to person.

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And while we don’t know 
how exactly how marijuana

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produces specific harmful effects,

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there are clear risk factors 
that can increase peoples’ likelihood

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of experiencing them.

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The clearest risk factor is age.

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In people younger than 25,

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cannabinoid receptors 
are more concentrated in the white matter

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than in people over 25.

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The white matter 
is involved in communication,

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learning, memory, and emotions.

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Frequent marijuana use

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can disrupt the development 
of white matter tracts,

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and also affect the brain’s ability 
to grow new connections.

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This may damage long-term learning ability
and problem solving.

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For now, it’s unclear 
how severe this damage can be

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or whether it’s reversible.

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And even among young people,

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the risk is higher the younger someone is—

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much higher for a 15 year old 
than a 22 year old, for instance.

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Marijuana can also cause hallucinations 
or paranoid delusions.

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Known as marijuana-induced psychosis,

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these symptoms usually subside 
when a person stops using marijuana.

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But in rare cases, 
psychosis doesn’t subside,

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instead unmasking 
a persistent psychotic disorder.

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A family history of psychotic disorders, 
like schizophrenia,

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is the clearest, though not the only, 
risk factor for this effect.

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Marijuana-induced psychosis 
is also more common among young adults,

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though it’s worth noting 
that psychotic disorders

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usually surface in this age range anyway.

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What’s unclear in these cases 
is whether the psychotic disorder

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would have appeared 
without marijuana use—

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whether marijuana use triggers it early,

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is a catalyst for a tipping point 
that wouldn’t have been crossed otherwise,

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or whether the reaction 
to marijuana is merely an indication

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of an underlying disorder.

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In all likelihood, marijuana’s role 
varies from person to person.

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At any age, as with many other drugs,

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the brain and body

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become less sensitive 
to marijuana after repeated uses,

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meaning it takes more 
to achieve the same effects.

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Fortunately, unlike many other drugs,

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there’s no risk of fatal overdose 
from marijuana,

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and even heavy use 
doesn’t lead to debilitating

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or life-threatening 
withdrawal symptoms if use stops.

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There are more subtle forms 
of marijuana withdrawal, though,

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including sleep disturbances, 
irritability, and depressed mood,

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which pass 
within a few weeks of stopping use.

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So is marijuana bad for your brain?

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It depends who you are.

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But while some risk factors 
are easy to identify,

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others aren’t well understood—

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which means there’s still some possibility
of experiencing negative effects,

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even if you don’t have 
any of the known risk factors.