The 10-day ordeal, 12 young boys and their soccer coach

found alive in a flooded cave in Thailand.

Narrator: Remember when that happened?

It was one of the most difficult

rescue missions of all time,

involving as many as 10,000 people,

and now we know that rescuers gave the boys a drug

which helped the mission succeed.

That drug was ketamine.

Ketamine was first synthesized in the 1960s

as an anesthetic, a sedative.

That's why it was so useful in the rescue mission.

But ketamine actually has a lot of different uses.

In the '80s, it became a popular club drug,

gaining nicknames like Special K and Cat Valium,

and more recently, in early 2019,

a form of ketamine was approved

by the FDA as an antidepressant.

So how does a drug that's so multipurpose actually work?

Ketamine is what scientists call a dirty drug.

That means it doesn't just target

one system in your brain, but dozens.

It has a weak effect on opiate receptors

in the dopamine system,

which drugs like heroine and cocaine target.

But most importantly, ketamine manipulates

a neurotransmitter called glutamate,

earning it the attention of psychiatrists nationwide.

Glutamate is what many of the neurons in your brain

use to communicate with each other,

and without it, well, your brain would essentially shut down

like a city grid without power.

Now, at high doses, ketamine seems to block glutamate.

That's why it's such an effective anesthetic.

But in low doses, like what you might find at a club

or in a spray of the FDA-approved drug esketamine,

it actually ramps up glutamate production,

and that comes with all kinds of side effects.

It can make you hallucinate or feel as though

you're losing touch with reality.

And it might also help build new connections or synapses

between neurons, electrifying new parts of that city grid.

Chadi Abdallah: When people are stressed for a long time

or when they suffer from depression for a while,

they start losing these connections,

and when we give them ketamine 24 hours,

they reverse. Now they look like more a normal brain.

So we believe that perhaps ketamine

is working by regenerating these connections

that are needed for normal brain functioning.

Narrator: And that could make ketamine

one of the best drugs out there for treating depression.

This isn't how other antidepressants work.

Medications like Prozac and Zoloft take weeks if not months

to kick in, and they regulate serotonin,

another chemical in the brain,

which scientists have long tied to depression.

These more traditional drugs may work for some people,

but not for everyone.

In fact, as many as 4 million American adults

have treatment-resistant depression.

And for them, well, ketamine might be

the only drug out there that can provide relief.

That's according to Doctor Andre Atoian.

He's the founder of Ketamine Specialists,

a clinic where he administers ketamine to patients with

mood disorders, pain, and addiction.

Andre Atoian: Ketamine is the agent that works when

most others have failed.

It is something that really allows us to give patients

a sort of new hope 'cause a lot of people that I treat

have basically already tried everything,

and they're in this situation

where nothing's really working,

and they're suffering, they're miserable.

Narrator: So it's hard to deny that ketamine's effect

on depression sounds promising,

and, clearly, it's a useful sedative.

But here's the thing...

Nolan Williams: As far as effect, it's, you know,

there's still a lot we don't know.

Narrator: That's Doctor Nolan Williams,

an assistant professor of psychiatry

at the Stanford University Medical Center.

He says that because ketamine manipulates so many different

receptors in the brain,

it's been hard to nail down all of them.

So it's still unclear how it might affect different patients

both short- and long-term,

and researchers still haven't figured out

how to preserve its benefits as an antidepressant.

Abdallah: The challenge is how to maintain

these new connections.

Is it to give ketamine repeatedly?

We know that we cannot give it every day,

that's like substance of abuse,

because then it will harm the brain.

Narrator: So while FDA approval is considered

a victory for the millions of people

with treatment-resistant depression,

doctors still caution that ketamine should only be used

as a last resort.