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.