Anxiety and IBS: What's Actually Happening Between Your Brain and Your Gut

This is a situation I see all the time. Someone comes in to talk about anxiety, and somewhere in the second half of the appointment they mention, almost as an aside, that their stomach has been a disaster for years. Cramping before meetings. Urgency on the drive to work. A bathroom map of every route they take. They have usually been worked up at least once, told everything looked fine, and left with the impression that it was stress and therefore not quite real.

Stress is part of it. But "it's stress" is a lazy stopping point, and it leaves out the part that actually helps people, which is how stress does this.

Why these two keep showing up together

Roughly half of people with a disorder of gut-brain interaction, the umbrella term that now includes IBS, also meet criteria for a psychiatric condition. That is not a coincidence of miserable people having miserable guts. The two systems are wired together.

The gut has its own nervous system, and it talks to the brain constantly through the vagus nerve, immune signaling, and hormones. Most of that traffic goes upward, not downward: the brain is getting far more information from the gut than it is sending back. About 90 to 95 percent of the body's serotonin is not in the brain at all. It is made by enterochromaffin cells in the lining of the intestine, where it regulates motility and fires the sensory nerves that report back to the brain.

The piece worth understanding is visceral hypersensitivity - the gut is not more damaged, it is more loud. Normal amounts of gas and normal stretching of the bowel produce pain signals that most people never notice consciously. And anxiety does something specific here: it degrades descending pain modulation, the brain's volume knob on incoming pain signals. Turn that knob down and ordinary gut sensation becomes pain. Imaging studies back this up. In people with IBS, the brain regions that assign threat and emotional weight to a sensation light up harder during rectal distension than they do in controls.

So this is not a metaphor about being tense. It is a measurable change in how a real signal gets amplified.

Which came first

Patients ask me this constantly, usually because someone implied their gut problems were self-inflicted.

The prospective data says it runs both ways. In population studies that followed people for years, some started with anxiety and developed gut symptoms later, and others started with gut symptoms and developed anxiety later. The gut-to-brain direction was at least as common as the reverse. There is also shared genetic vulnerability, which means for a meaningful number of people neither one caused the other - both came from the same underlying wiring.

I bring this up because the direction changes nothing about the treatment, and the question is usually really a question about blame.

Making sure it's actually IBS

I am not the person who makes this diagnosis. I am fairly often the person who notices it was never properly made.

IBS has positive criteria, not just leftover-after-everything-else criteria. Rome IV requires recurrent abdominal pain at least one day per week over the last three months, associated with at least two of: pain related to defecation, a change in stool frequency, or a change in stool form. Subtype matters for treatment - diarrhea-predominant, constipation-predominant, or mixed.

The American College of Gastroenterology recommends making the diagnosis this way rather than through an open-ended hunt for something else. Their guidance is fairly narrow: check celiac serology if there is diarrhea, check fecal calprotectin (or lactoferrin) plus CRP to screen for inflammatory bowel disease, skip routine stool pathogen testing, and skip routine colonoscopy in patients under 45 with no warning signs.

Warning signs that do change the plan: rectal bleeding, unintentional weight loss, iron deficiency anemia, symptoms that wake you from sleep, onset after 50, or a family history of colorectal cancer, IBD, or celiac disease. Any of those and this stops being an IBS conversation until it is sorted out.

Treating both at once

Neuromodulators

Tricyclic antidepressants carry a strong ACG recommendation for global IBS symptoms, which is unusual company for an old drug.

The ATLANTIS trial is the one I find most useful to point to. It randomized 463 primary care patients with IBS to low-dose amitriptyline, self-titrated between 10 and 30 mg, or placebo for six months. Symptom scores improved significantly on the drug. What makes it interesting is what did not change: anxiety and depression scores were no different between groups. The gut benefit was not coming from treating mood. At these doses you are using the medication as a visceral analgesic - a drug acting on gut pain signaling and transit speed - not as an antidepressant.

Subtype drives the choice, and the reason is mechanical. TCAs are anticholinergic, so they slow gut transit, which is useful in diarrhea-predominant IBS and the wrong move in constipation-predominant. SSRIs raise serotonin availability in the gut as well as the brain, which tends to speed transit - sometimes helpful in IBS-C, often unhelpful if diarrhea is the main problem. That is also why the first two weeks of an SSRI so often come with nausea and loose stools, and why that usually settles.

The overall evidence for antidepressants in IBS gives a number needed to treat around four to five, which is respectable. I would not oversell it. Most of these trials are small, short, and heterogeneous, and the SSRI data is weaker than the TCA data. Worth knowing about too: TCAs affect cardiac conduction even at therapeutic doses, and SSRIs and SNRIs carry a real hyponatremia risk (low sodium), particularly in older patients and in the first couple of weeks.

Brain-gut behavioral therapy

This is the part most patients have never been offered, and I think it is undersold.

Cognitive behavioral therapy adapted for IBS has a number needed to treat around four. Gut-directed hypnotherapy, which sounds soft and is not, sits around five, with benefit still present at one year. Both have versions that work with minimal therapist contact, including app-based and self-administered formats, which matters when there is no GI-trained therapist within an hour's drive.

The ACG rates the quality of this evidence as very low, and honestly the trials are hard to blind - you cannot give someone placebo hypnotherapy. I still think it earns its place. It is the only intervention here that treats the gut symptoms and the anxiety at the same time and has no side effects to weigh.

What I'm cautious about

Benzodiazepines. They get requested for this, they will calm someone down in the moment, and they do nothing for visceral pain while adding tolerance and dependence to a chronic problem.

Opioids for abdominal pain, which reliably make things worse over time through narcotic bowel syndrome, where escalating doses produce escalating pain.

And the quiet one: letting "it's anxiety" become the reason nobody checks anything. Anxious people get celiac disease, IBD, and colon cancer at the same rate as everyone else.

A few questions I get

If my anxiety improves, will the IBS go away?

Usually it improves rather than resolves. Treating anxiety turns down amplification; it does not reset gut motility or sensitivity on its own. Expect better, not gone.

Do I have to be depressed to take an antidepressant for this?

No. Low-dose TCAs for IBS are a separate indication at a separate dose, and any prescriber starting one should say that out loud rather than letting you find out from the pharmacy leaflet.

What about the low-FODMAP diet and probiotics?

Low-FODMAP has reasonable short-term evidence and belongs in a conversation with a GI dietitian, not a printout. Probiotic evidence is genuinely a mess - the trials use different strains, doses, and outcomes, and ACG recommends against routine use. I am also careful about restrictive diets in anyone with a history of disordered eating, where they can do more harm than the IBS.

My take

If you have both, treat both, and say both out loud in both offices. Tell your GI provider what your anxiety is doing and tell your prescriber what your gut is doing, because the medication choice changes depending on whether you are constipated or running to the bathroom - and because a prescriber who does not know about your IBS may hand you the drug that makes it worse.