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Vagus nerve stimulation (VNS) matters because it offers a longer-term option for depression that has failed multiple other treatments, delivered through a permanently implanted device rather than a course of sessions. A small generator, similar to a pacemaker, is implanted under the skin of the chest and connected to the left vagus nerve (a large nerve running from the brainstem through the neck), delivering regular electrical pulses. For example, imagine a patient who has already tried several antidepressants and other brain-stimulation treatments without lasting benefit: VNS offers a further option because it works through a different route. Its main drawback is that the benefit usually only appears after months, not days or weeks.

VNS stimulates the left vagus nerve (a cranial nerve running from the brainstem to the chest and abdomen), which is thought to influence mood-regulating circuits in the brain over time. It is also an established treatment for certain forms of epilepsy, reflecting its origin as an epilepsy device before its psychiatric use. Say a patient is starting VNS today: they and their clinician should expect the antidepressant benefit to build gradually, becoming clearer only after several months of stimulation. In practice, the main side effects are voice change (hoarseness) and cough, both linked to the vagus nerve's role in supplying the throat, and these often lessen over time.

• Why is the left vagus nerve chosen for stimulation rather than the right?
• Which investigation needs special precautions once a vagus nerve stimulator is implanted?
• How should a patient be counselled about the expected timescale of benefit from vagus nerve stimulation?

• Deep brain stimulation (DBS), stimulates targets inside the brain itself via intracranial electrodes, unlike VNS which stimulates a peripheral nerve in the neck.
• Electroconvulsive therapy (ECT), produces rapid benefit within days, unlike VNS whose antidepressant effect only appears after months.
• Transcranial magnetic stimulation (TMS), fully non-invasive with no implant, whereas VNS requires a surgically implanted generator and lead.
• Neurosurgery for mental disorder (psychosurgery): an irreversible ablative brain procedure, unlike VNS which is reversible and does not damage brain tissue.

• Surgical access is comparable on both sides.
• The vagus carries extensive sensory fibres on both sides.
• Both vagus nerves run through the neck.
• Both left and right vagus nerves are present.
• Plain radiography is safe and will simply show the device.

• Identify the drug, receptor, enzyme or pharmacokinetic change named in the stem.
• Translate it into the immediate mechanism before looking at the options.
• Use timing and the adverse-effect pattern to separate the nearest alternatives.
• Check whether the question asks for mechanism, clinical consequence, monitoring or management.