Phenylephrine (diagnostic)
Dilation & diagnostic · Rx
Trade namesMydfrin 2.5%, Neo-Synephrine Ophthalmic 2.5% / 10%
Drug classAlpha-1 adrenergic mydriatic (no cycloplegia)
Strength2.5%, 10%
AvailabilityRx
Mechanism of actionAlpha-1 agonist; contracts the iris dilator muscle. Produces mydriasis without cycloplegia; also blanches conjunctival vessels.
Indication / reason for useAdjunct to an anticholinergic for fuller dilation; differentiating episcleritis (blanches) from scleritis (does not); breaking recent posterior synechiae (with an anticholinergic); pre-operative dilation.
Usual adult dosing1 drop of 2.5% (combine with tropicamide). Onset ~20 min.
PreservativeBAK
Contact lensesRemove lenses; reinsert after 15 minutes.
Common / important adverse effects10% strength: significant systemic hypertension, reflex bradycardia, arrhythmia, and rare MI/stroke reported, avoid 10% in infants, the elderly, and cardiovascular disease. Rebound miosis and lid retraction. Stinging.
Contraindications & cautions10% in infants, elderly, hypertension, cardiovascular disease, aneurysm, thyrotoxicosis, on MAOIs/tricyclics/atomoxetine. Narrow angles (relative).
Pregnancy / lactationAvoid the 10% strength; 2.5% single diagnostic dose with punctal occlusion is generally acceptable.
Practice notesUse 2.5%, there is almost no diagnostic reason to reach for 10%.
Chair-side reference only. Dosing, contraindications and pregnancy guidance here are not exhaustive. Confirm against the current prescribing information and the patient’s full history before prescribing or advising.