Treatment-Resistant Depression: When Antidepressants Don't Work

Defining Treatment-Resistant Depression

Treatment-resistant depression (TRD) is one of the most significant unmet needs in psychiatry. The standard definition — depression that has not responded adequately to at least two antidepressant medications at adequate doses for adequate duration — applies to approximately 30–40% of people with major depressive disorder. In absolute numbers, that is tens of millions of people worldwide who have tried the standard treatments and found them insufficient.

The consequences of TRD extend beyond persistent symptoms. People with TRD have significantly higher rates of hospitalisation, disability, suicide attempts, and all-cause mortality than people with treatment-responsive depression. The economic burden is estimated at over $44 billion annually in the United States alone.

Why Antidepressants Don't Work for Everyone

The honest answer is that we don't fully know. The monoamine hypothesis of depression — which underpins all current antidepressants — proposes that depression is caused by deficiency of serotonin, norepinephrine, or dopamine. SSRIs increase serotonin availability; SNRIs increase both serotonin and norepinephrine; MAOIs prevent breakdown of all three.

But the monoamine hypothesis is almost certainly incomplete. A 2022 umbrella review in Molecular Psychiatry — the largest systematic review of the serotonin-depression hypothesis ever conducted — found no consistent evidence that depression is caused by low serotonin levels. This does not mean SSRIs don't work (they clearly do for many people), but it suggests they work through mechanisms we don't fully understand, and that those mechanisms are not present in everyone.

Genetic factors play a role: variants in the serotonin transporter gene (SLC6A4), the CYP450 enzyme system (which metabolises antidepressants), and multiple other genes influence both antidepressant response and side effect profiles. Inflammatory markers, gut microbiome composition, and early life stress all predict antidepressant response. TRD is not a single condition — it is a heterogeneous group of conditions that share the phenotype of not responding to monoamine-targeting drugs.

The Psilocybin Evidence for TRD

Psilocybin is mechanistically distinct from all current antidepressants. Rather than modulating monoamine availability, it acts as a direct agonist at serotonin 2A receptors — producing acute changes in brain network connectivity that persist after the drug has cleared the system.

The most important trial for TRD specifically is the 2021 Imperial College London open-label study of psilocybin therapy in 20 patients with treatment-resistant depression (defined as failure of at least two antidepressants). After two sessions of psilocybin therapy, 67% of patients showed a clinical response (≥50% reduction in depression scores) and 42% achieved remission. These are extraordinary numbers for a population that had failed multiple prior treatments.

A 2022 phase 2 trial published in NEJM Evidence randomised 233 patients with TRD to receive 25mg, 10mg, or 1mg psilocybin in a single session. The 25mg group showed significantly greater reductions in depression scores at three weeks compared to the 1mg control group, with a response rate of 37% vs 18%. Crucially, the 25mg group also showed significantly greater improvements in quality of life and functional ability.

How Psilocybin Works Differently

The 2022 Nature Medicine neuroimaging study by Daws et al. provides the clearest mechanistic explanation. Using fMRI, the researchers showed that psilocybin therapy produced lasting increases in brain network flexibility — the brain's ability to transition between different functional states. This flexibility is specifically what depression reduces: the depressed brain becomes stuck in rigid patterns of default mode network activity, unable to shift into more adaptive states.

Conventional antidepressants do not appear to produce this network flexibility effect. They modulate the neurochemical environment, which helps some people, but they do not produce the structural network change that psilocybin therapy generates. This is why psilocybin can work for people who have failed multiple antidepressants — it is addressing a different aspect of the underlying pathology.

Practical Considerations

For people with TRD who cannot access clinical psilocybin therapy, microdosing represents a lower-intensity approach that engages the same receptor system. The evidence base is observational rather than controlled, but the 2021 Scientific Reports study found that microdosers with depression reported significant improvements in mood and cognitive function — including a subgroup who had previously tried conventional antidepressants without adequate response.

The standard microdosing protocol for depression is 100–150mg psilocybin on a 4-on, 3-off schedule. This is sub-threshold — no psychedelic effects — but provides consistent serotonin 2A receptor stimulation that may produce the cognitive flexibility effects seen in full-dose therapy, at lower magnitude.