You might think that antidepressants  are only for your brain.

After all, people take them for brain stuff. Depression, anxiety, OCD, and so on.

But there’s research to  suggest that antidepressants may be able to help with stuff all over the body. Stuff like cancer. Yes, really.

We do not clickbait around here. Here’s how this would work and how it could start helping people soon. [♪INTRO] There are several different  types of antidepressants, and they work by adjusting the balance of neurotransmitters in the brain. Each of the medication types  might have different side effects, and people might respond better to  one antidepressant over another, even within the same class.

For today, we’re just going  to talk about two main types: MAOIs and SSRIs. MAOIs were the first  antidepressants on the market. The acronym is short for  monoamine oxidase inhibitors, and as the name suggests, it works by blocking an enzyme called monoamine oxidase.

The usual function of monoamine  oxidase is to break down neurotransmitters in your brain,  like serotonin and dopamine. While every neurotransmitter is complicated, these two very generally are  involved in helping you feel good, so you might think “hang on, I need those.” But breaking down of it is an  important part of maintaining balance and making sure you don’t  get too much of a good thing. But sometimes, you don’t have  enough of the good thing, and monoamine oxidase is slashing too many of your feel-good chemicals.

That’s where the inhibitor in MAOI comes in. The MAOIs basically say, “Good  job, enzyme, but that’s enough for now. We’re going to keep some of  these neurotransmitters around.” MAOIs aren’t super specific about  which neurotransmitters they affect, so this can sometimes cause unwanted side effects.

Over the years, scientists have  developed new antidepressants with fewer side effects, but plenty of  people still take MAOIs to treat their depression, especially if those  other meds haven’t worked out. One of those newer types of  antidepressants is the SSRIs, short for selective serotonin reuptake inhibitors. And you might already be familiar  with some of these, but don’t worry, I am getting to the good stuff,  just gotta explain real quick.

SSRIs work by blocking the  protein that’s usually responsible for vacuuming up all the serotonin in your brain. By preventing it from being whisked away, all that serotonin is now free to keep binding with your neurons without interruption. Same idea as MAOIs – it’s about adjusting the balance when it needs adjusting.

Since SSRIs work pretty specifically on serotonin, they don’t have as many side effects, which makes them a good first  choice for treating depression. But neurotransmitters, including serotonin, can have functions outside of the brain, including important roles in cancer cells. Told you I was getting there.

For example, scientists have found  that serotonin is an important regulator of the anti-tumor  response in your immune system. In 2021, a group of researchers  published two papers that found that MAOIs could help suppress tumors. Now, fasten your seatbelts, because there’s going to be a lot of back and forth here.

But it’s going to make sense by the end. One form of monoamine oxidase, called MAO-A, is expressed more in the immune cells that are supposed to fight cancer. Imagine your immune system  is made up of a bunch of cars that carry different  immune cells all over your body.

Some cars will drive to  fight an infection over here, some will drive over to any  cuts or scrapes you have, and many are just cruising around, keeping an eye out for anything that might pop up. MAO-A is part of the immune system, but it also interacts with neurotransmitters, crashing into molecules like serotonin and dopamine, and destroying them. And under normal circumstances,  in the immune system, MAO-As stop your body from  attacking your own cells.

But your immune system isn't  the only one on the road. Tumors also have their own immune cells that can find ways to bypass your patrol. Some of these tumor immune cells are called tumor-associated macrophages, or TAMs.

These TAMs work for the cancer to promote tumor growth and inhibit any  of your natural defenses. When your immune cells recognize a tumor, the tumor takes this opportunity to turn your MAO-A to the dark side. Now, instead of just preventing the immune system from attacking your healthy cells,  the MAO-A stops your body from attacking the tumor as well, and  boosts the pro-tumor action of TAMs.

Hence, the MAOIs. Remember,  the I is short for inhibitor. The inhibitors kick the MAO-A out of the driver’s seat so it can’t do the tumor’s work any more.

TAMs can’t stop MAOIs, so MAOIs bring everything back into balance and make  the tumor vulnerable again. To sum up: Normal immune cells bad for tumor. Tumor turn immune cells into good for tumor.

MAOI turn off bad immune cells.  Bad for tumor. Good for us. To verify the role of MAO-A,  the researchers made mice that didn’t produce MAO-A at all  and found that those mice were really good at keeping tumors under control.

In that paper, they also showed how tumor-induced MAO-As found in those hijacked immune cells prevent your immune system from fighting tumors. In the process of MAO-A’s  usual role of breaking up different neurotransmitters, it produces highly reactive oxygen molecules as byproducts. This is usually fine in small  amounts, but when the tumor cells are increasing how many MAO-As  are expressed in your cells, it creates too reactive an  environment for your cancer-fighting cells to handle.

The TAMs,  however, thrive in the chaos It’s literally creating an  environment that’s so toxic, your immune cells don’t want to work there. Which, fair. I wouldn’t want to either.

But the researchers didn’t stop there. In a 2025 paper, they went on to  apply the same concept to SSRIs. This time, they used SSRIs  to more specifically target serotonin transporters in the nervous system.

And the SSRIs were able to wake up the anti-tumor immune cells to go do their job. And just like in treating depression, the more specific function of SSRIs  meant that the researchers found all of the great anti-tumor results  of MAOIs, but with fewer side effects! Antidepressants aren’t really being proposed as a stand-alone cancer treatment.

Rather, it seems to be a good option to explore as a boost for other treatments. One type of cancer therapy,  called PD-1 inhibitors, seems to be a promising candidate for this boost. PD-1 is a protein that works  with your immune system.

When it binds to the surface of a cell, that’s basically a signal  to your immune system like, “Everything’s good! This guy’s  with me. Nothing to see here.” .

But some cancer cells are  really good at binding to PD-1, which makes them basically  invisible to your immune system. So PD-1 inhibitors can stop PD-1  from binding to other cells, revealing the imposters to be targeted. When PD-1 inhibitors work, they work great.

But the treatment only seems to be  effective in 20 to 25% of patients. And while more research will be needed, the initial studies have found  that both MAOIs and SSRIs seem to boost the effectiveness  of PD-1 inhibitors in mice. Now for the weirdly good and  practical part of all this.

About 20% of cancer patients already  take antidepressants like SSRIs. As I can attest to, having  cancer can make you pretty sad. Well, it seems like it could  be helping those patients’ outcomes in a way we didn’t even know about.

And having so many cancer patients  already on antidepressants provides a golden opportunity,  even if it doesn’t feel so hot. The study population already  exists to compare the combination of SSRIs and PD-1 inhibitors  to other treatment plans. Researchers could start that  clinical trial, like, yesterday.

The people are already out there! Not only that, but since we  already know the safety profile of MAOIs and SSRIs, we can  more safely consider whether they would fit as part of  more people’s treatment plan. A safe drug you already have is  better than one you have to invent.

Even though we still need to do more research, we have SSRIs and MAOIs  already. We know how they work. That removes a lot of barriers  to using them to treat patients.

It’s a way to start helping people really soon. And that’s the farthest thing from  depressing I’ve heard all day. [♪OUTRO]