What Causes Treatment-Resistant Depression? Biology and Blind Spots

You’ve tried one antidepressant, then another, and you’re still waiting to feel like yourself. It’s easy to conclude that your brain is simply broken, but the research points somewhere more useful: there’s often a specific reason treatment stalled, and some of those reasons can be found and changed.

An antidepressant helps only when a chain of things goes right. You need the right medication, a dose high enough to have a real effect, enough weeks for that effect to build, and a type of depression the drug’s mechanism actually targets. A break anywhere in that chain looks the same from the outside, so finding the cause takes precision.

The standard approach when a first antidepressant falls short is to change course step by step: raise the dose, switch medications, or add a second one. The largest real-world test of that strategy, the STAR*D trial, followed thousands of adults through up to four treatment steps. Stepwise care is still the backbone of depression treatment, and for many people it works.

But a new step only means something if the last one was a fair test, and at Siggy we’d rather understand why a medication didn’t work than reach for the next one by default.

This guide walks through the known causes of treatment-resistant depression, from missed doses and unrecognized bipolar disorder to inflammation and the way your genes process medication. When depression resists treatment, that resistance is often pointing to a specific cause, and finding it is a matter of evidence.

What This Page Covers

• What Counts as Treatment-Resistant Depression?
• How Often Does Antidepressant Treatment Fall Short?
• Is It True Resistance or Pseudo-Resistance?
• Could It Be Bipolar Depression?
• What’s Happening in the Brain and Body?
• Do Your Genes Affect How Antidepressants Work?
• What Else Makes Depression Harder to Treat?
• What Helps When Depression Resists Treatment?
• How Siggy Fits In
• The Bottom Line on Treatment-Resistant Depression
• Start Your Anonymous Intake

What Counts as Treatment-Resistant Depression?

There’s no single agreed-upon definition, which is part of the problem. A 2023 expert review notes that the FDA and the European Medicines Agency use the most common one: an inadequate response to at least two antidepressants, each given at an adequate dose for an adequate time and taken as prescribed. By that definition, at least 30% of people with depression qualify.

The same review makes a point that matters if you’ve been told your depression is resistant. A significant share of people with that label are pseudo-resistant, meaning their treatment trials were never adequate in the first place. Our guide to whether your last two antidepressants really counted goes deeper on the definition.

How Often Does Antidepressant Treatment Fall Short?

In STAR*D, people who didn’t reach remission on the first medication, the SSRI citalopram, could move through up to three more steps that involved switching or adding treatments. Here’s how remission rates changed at each step, as originally reported.

Treatment Step	What It Involved	Reported Remission Rate
Step 1	Citalopram, an SSRI	36.8%
Step 2	Switch or add-on	30.6%
Step 3	Another change in treatment	13.7%
Step 4	Another change in treatment	13.0%

The original STAR*D report described a cumulative remission rate of 67%, and people who needed more steps had higher relapse rates during follow-up. A 2023 reanalysis using the study’s protocol-specified outcome measure put the cumulative remission rate at about 35%, roughly half the published figure.

Either way, the pattern holds: each additional step helps fewer people. That’s why getting the early steps right matters so much.

Is It True Resistance or Pseudo-Resistance?

Before calling depression resistant, clinicians try to rule out reasons a medication never had a fair chance. These are the ones worth checking first:

• The dose was too low to have a real effect
• The trial was too short to judge
• Doses were missed or the medication was stopped early
• Side effects made it hard to stay at a dose that works
• Another condition, like anxiety or substance use, went untreated

If side effects started and you felt on your own until the next appointment, stopping early makes complete sense. Missed doses and early stops say more about how hard it is to stay on treatment alone than about you, and they’re among the most fixable causes on this list.

Could It Be Bipolar Depression?

Bipolar depression can look identical to unipolar depression during a low stretch, and the difference usually lives in your history. Because bipolar disorder is treated differently, a missed bipolar diagnosis can make standard antidepressant treatment look like it’s failing.

This happens often. In a survey of 600 people with bipolar disorder, 69% had initially been misdiagnosed, most often with unipolar depression, and more than a third waited 10 years or more for an accurate diagnosis. The participants came from support groups, so the sample may not represent everyone.

If you’ve ever had periods of unusually high energy, little need for sleep, or racing ideas, tell your prescriber, even if they felt good at the time. Our guide to bipolar depression vs. depression explains what to look for.

What’s Happening in the Brain and Body?

For some people, depression that resists treatment involves biology that standard antidepressants don’t directly target. Two areas have the strongest research behind them.

Inflammation

Higher levels of inflammatory markers in the blood have been linked to weaker responses to antidepressants. In an infliximab trial of 60 adults with treatment-resistant depression, the anti-inflammatory drug infliximab didn’t help the group overall. It did improve symptoms in people whose baseline C-reactive protein (CRP), a common inflammation marker, was above 5 mg/L, while people with lower CRP did slightly better on placebo.

It was a small proof-of-concept study, and anti-inflammatory drugs aren’t a standard depression treatment. Still, it suggests inflammation matters for a subgroup of people, and that anti-inflammatory approaches only make sense when inflammation is actually present.

Glutamate

Most antidepressants act on serotonin, norepinephrine, or dopamine. Glutamate, the brain’s main excitatory chemical messenger, is a different pathway, and it’s where some of the biggest advances for treatment-resistant depression have come from.

In a 2006 randomized trial, a single intravenous dose of ketamine, which blocks a glutamate receptor called the NMDA receptor, produced antidepressant effects within two hours that remained significant for a week. The 2023 review lists IV ketamine and esketamine nasal spray, given alongside an oral antidepressant, as established treatments for treatment-resistant depression.

Do Your Genes Affect How Antidepressants Work?

Your body breaks down many antidepressants using liver enzymes whose activity varies with your genes. Pharmacogenetics guidelines explain that variations in the CYP2D6, CYP2C19, and CYP2B6 genes influence how many SSRIs and SNRIs are metabolized, which can affect dosing, effectiveness, and side effects. The same guidelines say tests of serotonin-related genes like SLC6A4 and HTR2A don’t support use in prescribing.

Testing helps, but modestly. In the PRIME Care trial of 1,944 veterans, testing steered prescribers away from medications with predicted drug-gene interactions, but its effect on remission was small and didn’t hold at 24 weeks. Our guide to genetic testing for psychiatric medications covers what results can and can’t tell you.

What Else Makes Depression Harder to Treat?

Anxiety is one of the most important factors. In a STAR*D analysis, people whose depression came with significant anxiety had poorer outcomes with antidepressant treatment than people without it.

Early life experience matters too. A 2012 meta-analysis of 10 clinical trials found that people with a history of childhood maltreatment had 43% higher odds of not responding or not reaching remission during depression treatment.

Both are signals that a plan may need to address more than one thing at once, such as anxiety alongside depression, or therapy alongside medication.

What Helps When Depression Resists Treatment?

The 2023 review summarizes the options with the strongest evidence. IV ketamine and esketamine are established treatments, and some second-generation antipsychotics, like aripiprazole, are proven add-ons for people with a partial response. Repetitive transcranial magnetic stimulation (TMS) is FDA-approved for treatment-resistant depression, and electroconvulsive therapy (ECT) is effective for both acute and maintenance treatment.

Adding psychotherapy to an antidepressant offers significant relief, while evidence for simply extending a trial, switching, or combining antidepressants is mixed. Our guide to treatment-resistant depression options compares these in more detail.

How Siggy Fits In

Most of the causes in this guide come back to one question: what actually happened during each medication trial? That’s hard to answer when appointments are short and far apart, and you have to retell your story every time you see someone new.

Siggy starts with a free, anonymous intake, with no name or email required. A licensed prescriber reviews your intake, including what you’ve already tried, before anything is prescribed, and you get a summary you can keep and share with any clinician. Once you’re on a plan, Siggy is built to stay with you between appointments, so a missed dose, a side effect, or a stretch of feeling better doesn’t quietly end a medication trial before it’s had a fair chance.

Siggy’s prescribers focus on anxiety, depression, and SSRI-based treatment. If your history points to true treatment resistance or to bipolar disorder, your prescriber can help you understand what kind of specialist care might come next.

This article is for education and isn’t a substitute for care from a licensed clinician.

The Bottom Line on Treatment-Resistant Depression

Treatment-resistant depression is real, and it’s also frequently misread. Before concluding that medication doesn’t work for you, it’s worth asking whether each trial had an adequate dose, enough time, and consistent use, and whether bipolar disorder, anxiety, or other factors were part of the picture. When true resistance is confirmed, ketamine-based treatments, TMS, ECT, and add-on medications offer real options. If you’re at the eight-week mark and unsure what to do next, our antidepressant decision path walks through it step by step.

Start Your Anonymous Intake

If you’re in crisis or thinking about harming yourself, call 911 or call or text 988 first. If you’ve tried antidepressants without the relief you hoped for and want a prescriber to look at your whole history, start your anonymous intake. It’s free, there’s no name or email required, and a licensed prescriber reviews your intake before anything is prescribed. A real doctor reviews everything, and your summary is yours to keep.