“I keep getting double vision that comes and goes — it’s probably just tiredness, right?”

Diplopia in a young adult is approached structurally. First split MONOCULAR (persists with one eye open, resolves only when that eye is covered — usually an ocular/refractive/lens problem, less sinister) from BINOCULAR diplopia (present with both eyes open, resolves when EITHER eye is covered — implying ocular misalignment from a neurological or neuromuscular cause), the latter being the concerning type that demands a neuro work-up. In a young woman, consider MULTIPLE SCLEROSIS — her recent painful, blurred monocular visual episode is a classic description of OPTIC NEURITIS (pain on eye movement, reduced acuity/colour vision, often resolving), and MS produces episodes separated in time and space (e.g. internuclear ophthalmoplegia) — warranting urgent neurology/ophthalmology and MRI, not "tiredness". Consider MYASTHENIA GRAVIS, which causes FATIGABLE diplopia and ptosis worse with effort and toward the end of the day, with possible bulbar (swallow/speech) and limb fatigue and a risk of bulbar/respiratory compromise (myasthenic crisis). Know the cranial-nerve RED FLAGS: a painful THIRD nerve palsy with a FIXED DILATED PUPIL is a posterior communicating artery ANEURYSM until proven otherwise — a neurosurgical emergency — and a SIXTH nerve palsy can be a false-localising sign of RAISED INTRACRANIAL PRESSURE (screen headache, papilloedema, vomiting). Examine acuity, pupils (RAPD), eye movements, ptosis and fatigability, and fundi IN PERSON (on video, arrange a same-day face-to-face examination), and set urgency by findings — emergency for a pupil-involving III palsy, thunderclap headache (NICE NG228), raised ICP with papilloedema or bulbar/breathing symptoms; same-day on-call ophthalmology or neurology discussion for new persistent binocular diplopia or current visual loss; urgent neurology ± ophthalmology and MRI for suspected MS/optic neuritis (NICE NG220) or ocular myasthenia (ABN guidelines). Advise that a driver with diplopia must stop driving and tell the DVLA. The hidden agenda is the wish to be reassured it is just tiredness/screens while minimising recurrent neurological symptoms; take them seriously, work the serious differential and red flags, and refer with appropriate urgency honestly but without catastrophising — never colluding with "it’s just eye strain". NICE NG12 (Suspected cancer: recognition and referral, last updated April 2026) offers a route rather than a cancer label here: consider an urgent direct-access MRI brain (within 2 weeks) for progressive, sub-acute loss of central neurological function.