Depression
A real illness — and not a serotonin deficiency
What it is
Major depressive disorder is a persistent low mood or loss of interest and pleasure, lasting at least two weeks and accompanied by changes in sleep, appetite, energy, concentration, and self-worth, to a degree that impairs daily life. It is common, it is treatable, and it is not a character flaw or a failure of gratitude.
Why it matters
Depression is among the leading causes of disability worldwide, and the public understanding of it is built on an explanation that the evidence does not support. For decades people were told depression is a 'chemical imbalance' — a serotonin deficiency corrected by a pill. That model is not supported, and its collapse has produced a second error at least as damaging: the conclusion that antidepressants therefore do not work. They do. Both statements are true at once, and holding them together is the difference between an informed decision and a frightened one.
What BioSignal knows about treating this
What works for Depression
BioSignal’s clinical summary, most important first.
- Psychotherapy — CBT, behavioural activation, interpersonal therapy. First-line, and as effective as medication in mild-to-moderate depression
- Exercise — a real antidepressant effect, not a consolation prize
- Antidepressants (SSRIs/SNRIs) — effective; benefit grows with severity
- Combination of medication and psychotherapy — best evidence in moderate-to-severe depression
- Treating the drivers: insomnia, alcohol, chronic pain, thyroid disease
- For treatment-resistant depression: TMS, esketamine, and ECT (highly effective, and badly stigmatised)
Signal Records relevant to this condition
Interventions and contributing factors — some of these records describe a cause rather than a cure. The rating shown is BioSignal’s confidence in that Signal Record, not a claim about how well it treats this condition. Open any of them for the full evidence.
- Antidepressants (SSRIs & SNRIs)High confidence
They beat placebo — the evidence is strong. The average effect is modest and grows with severity. The chemical-imbalance explanation was wrong; the medication still works. Withdrawal is real: taper slowly, never abruptly, with your prescriber.
- St John's WortModerate confidence
It genuinely works for mild-to-moderate depression. It also strips other medications of their effect — including the contraceptive pill and immunosuppressants — and must never be combined with an SSRI. Do not take it without telling a clinician what else you take.
- Omega-3 Fatty AcidsModerate confidence
Lowers triglycerides, and reduces preterm birth in pregnancy. Routine fish oil does NOT prevent cardiovascular disease, cancer or dementia in the general population. High-dose omega-3 increases atrial fibrillation risk — a real harm most bottles do not mention.
- AlcoholHigh confidence
A Group 1 carcinogen with dose-dependent harms. The heart-protection belief does not survive the methods designed to test it. Cutting back helps, even without quitting.
- Vitamin DHigh confidence
Effective for deficiency and for bone health in at-risk groups. For broad disease prevention in adults who are already replete, the largest trials are null — and confidence in that null is high. Routine testing of healthy adults is not supported.
- AshwagandhaLimited evidence
The best-evidenced hormone supplement — a modest stress effect. Weak testosterone evidence, and a real liver-injury signal that the label won't mention.
New to this? Read these first
How this usually unfolds
- Recognize risk factors
- Get diagnosed
- Track key biomarkers
- Lifestyle first
- Evidence-based treatment
- Long-term monitoring
Who is at risk
- Previous episodes of depression
- Family history
- Chronic illness and chronic pain
- Chronic insomnia (bidirectional — each drives the other)
- Alcohol use
- Social isolation and loneliness
- Adverse life events and trauma
- Postpartum and perimenopausal periods
- Certain medications
How it's diagnosed
There is no blood test for depression. It is a clinical diagnosis based on the pattern, duration, and functional impact of symptoms, commonly supported by a structured questionnaire such as the PHQ-9. Blood tests are used to EXCLUDE conditions that mimic depression — most importantly hypothyroidism, but also anaemia and B12 deficiency — not to confirm it. Anyone told a test has 'confirmed' their depression, or measured their serotonin, has been misinformed.
Key biomarkers
Lifestyle
Explore this condition across BioSignal
Related Foundations
Related Signal Records
Frequently asked questions
Is depression caused by a chemical imbalance?
No — and this deserves a straight answer, because a great many people were told otherwise. A major umbrella review of the serotonin literature found no consistent evidence that depression is caused by low serotonin or reduced serotonin activity. The 'chemical imbalance' phrase was a simplification, and it should be retired. Depression appears to arise from an interaction of genetics, life circumstances, chronic stress, illness, sleep, and social context — which is less tidy, and more true.
If the chemical imbalance theory is wrong, do antidepressants still work?
Yes, and this is the single most important thing on this page. The inference — the theory is wrong, therefore the drugs are useless — feels logical and it is a mistake. A medication can work without the illness being a deficiency of the thing it acts on: aspirin relieves headaches, and headaches are not caused by an aspirin deficiency. Whether antidepressants beat placebo was tested directly in hundreds of randomised trials, and they do. The average effect is modest, and it is larger in more severe depression. Do not let a debunked explanation talk you out of an effective treatment.
Does exercise really treat depression, or is that just something people say?
It really does, and it is one of the few pieces of lifestyle advice that has earned its place in clinical guidelines. Randomised trials show a genuine antidepressant effect for aerobic and resistance exercise in mild-to-moderate depression, comparable in some comparisons to medication — though these trials are harder to blind, which is a real limitation and the reason confidence is moderate rather than high. It is not a substitute for treatment in severe depression. But 'go for a walk' being annoying advice does not make it wrong advice.
Do I have to take medication?
Not necessarily. In mild-to-moderate depression, psychotherapy — particularly CBT and behavioural activation — is about as effective as medication, and exercise has real evidence too. Any of these is a legitimate starting point, and it is entirely reasonable to ask your clinician about them. In moderate-to-severe depression the case for medication is much stronger, and combining it with therapy outperforms either alone. The right answer depends on severity, and that is why the blanket claims on both sides of this argument are unhelpful.
What if nothing has worked?
Treatment-resistant depression is real, and it is not a verdict on you. Options exist that most people have never heard of: transcranial magnetic stimulation, esketamine, and electroconvulsive therapy — which, despite a cultural reputation formed by a film, is among the most effective treatments in psychiatry for severe and resistant depression, and is delivered under anaesthesia. If several treatments have failed, that is a reason to ask about these, not a reason to stop.
I'm struggling right now. What should I do?
Please speak to someone today — your doctor, a crisis line, or someone you trust. Depression is treatable, it distorts your judgement about whether it is treatable, and that distortion is a symptom rather than an insight. This page is here to inform a decision, not to replace one; if you are in crisis, do not work through it alone.
Evidence summary
The serotonin-deficiency model of depression is not supported by the evidence, as concluded by a major umbrella review of the serotonin literature. Independently of that, antidepressants have been shown superior to placebo in acute major depression across hundreds of randomised trials, synthesised in a network meta-analysis of over 500 studies — with a modest average effect that increases with baseline severity. Psychotherapy (CBT, behavioural activation, interpersonal therapy) has strong randomised support and is comparable to medication in mild-to-moderate depression; combination treatment is superior to either alone in moderate-to-severe illness. Exercise shows a genuine antidepressant effect in randomised trials, with the caveat that blinding is difficult. Electroconvulsive therapy remains among the most effective treatments for severe and treatment-resistant depression. Omega-3 (EPA-predominant) shows modest and mixed adjunctive evidence; vitamin D supplementation has not shown benefit for preventing or treating depression in people who are not deficient.
References & sources
- Umbrella review of the serotonin theory of depression (Moncrieff et al.)
- Cipriani et al., network meta-analysis of 21 antidepressants (The Lancet)
- NICE guideline: Depression in adults — treatment and management
- APA Practice Guideline for the Treatment of Patients with Major Depressive Disorder
Educational information — not medical advice
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