Augmentation Strategies
Augmentation means adding a second agent to a partially effective primary antidepressant to convert partial response into remission. The rationale for any augmentation strategy should be pharmacological: what mechanism is the primary antidepressant not providing, and how does the augmenting agent supply it? Augmentation is distinct from switching (abandoning the primary agent entirely) and from combining (using two antidepressants of different classes).
Atypical antipsychotics have the strongest evidence base of any augmentation class, with Food and Drug Administration-approved indications for adjunctive use in major depressive disorder for aripiprazole, quetiapine extended-release, brexpiprazole, and cariprazine when added to antidepressants in patients with inadequate response. The mechanistic rationale differs by agent. Aripiprazole is a dopamine D2 partial agonist and serotonin-1A partial agonist; its augmenting effect is attributed to partial restoration of dopaminergic tone in the prefrontal cortex — particularly relevant for residual anhedonia, low motivation, and cognitive slowing. Quetiapine at low doses (25 to 200 mg) augments primarily through serotonin-2A antagonism and histamine H1 blockade, improving sleep and reducing anxiety rather than through dopamine blockade. Brexpiprazole combines dopamine D2 partial agonism with serotonin-1A partial agonism and serotonin-2A antagonism, with more favorable rates of akathisia than aripiprazole.
The shared adverse effect burden of atypical antipsychotic augmentation includes weight gain (most prominent with quetiapine, less with aripiprazole and brexpiprazole), metabolic effects, and long-term risks of tardive dyskinesia requiring informed consent and periodic neurological monitoring.
Lithium augmentation has the longest historical evidence base in augmentation pharmacology, with controlled trials supporting its use when added to tricyclic antidepressants and selective serotonin reuptake inhibitors. The mechanism is incompletely understood but likely involves enhancement of serotonergic neurotransmission — lithium increases serotonin synthesis and release and may sensitize postsynaptic serotonin-1A receptors. Effective augmenting plasma concentrations (0.6 to 1.0 milliequivalents per liter) require the same monitoring infrastructure as lithium for any indication: baseline renal and thyroid function, regular serum concentration monitoring, and hydration counseling. Lithium's narrow therapeutic index and the monitoring burden have made it less commonly used in contemporary practice relative to atypical antipsychotics, despite its extensive evidence base.
Bupropion augmentation of selective serotonin reuptake inhibitors was included as a Sequenced Treatment Alternatives to Relieve Depression Step 2 option and is widely used in primary care. The pharmacological rationale is straightforward: bupropion blocks the norepinephrine and dopamine reuptake transporters without serotonergic activity, adding noradrenergic and dopaminergic enhancement to the primary serotonin transporter blockade of the selective serotonin reuptake inhibitor. Residual symptoms of fatigue, hypersomnia, anhedonia, cognitive slowing, and low motivation after selective serotonin reuptake inhibitor treatment are more closely linked to insufficient noradrenergic and dopaminergic tone than to insufficient serotonergic tone — bupropion directly addresses this gap. Bupropion augmentation also mitigates serotonin transporter inhibitor-induced sexual dysfunction through dopaminergic activity. The key pharmacokinetic caution: bupropion is a potent cytochrome P450 2D6 inhibitor, which raises plasma concentrations of selective serotonin reuptake inhibitors metabolized by this isoform.
Buspirone, a serotonin-1A partial agonist, was the other augmentation option at Sequenced Treatment Alternatives to Relieve Depression Step 2. Its proposed augmenting mechanism involves direct engagement of serotonin-1A autoreceptors — accelerating or enhancing the autoreceptor desensitization that is required before full antidepressant response develops. Buspirone augmentation is most rationally applied when residual anxiety is a prominent feature of the partial response. It produces no dependence, no withdrawal, and no respiratory depression — advantages over benzodiazepines in anxious patients.
Triiodothyronine (T3) augmentation was a Sequenced Treatment Alternatives to Relieve Depression Step 3 option with a signal of efficacy in small trials. The mechanism likely involves thyroid hormone's role in regulating central noradrenergic and serotonergic receptor sensitivity — even subclinical hypothyroidism within the normal reference range can blunt antidepressant response. Triiodothyronine is preferred over thyroxine (T4) for augmentation because it does not require peripheral conversion to the active form.
Fatigue, anhedonia, cognitive slowing, low motivation: bupropion (norepinephrine and dopamine) or aripiprazole/brexpiprazole (dopamine partial agonism). Residual anxiety and insomnia: quetiapine low dose (serotonin-2A antagonism, sedation) or buspirone (serotonin-1A agonism). Inadequate overall response with no clear residual pattern: lithium, aripiprazole, or brexpiprazole. Sexual dysfunction from selective serotonin reuptake inhibitor: bupropion. The augmenting pharmacology should address a mechanism the primary antidepressant is not covering.
