Brimonidine-d4 is intended for use as an internal standard for the quantification of brimonidine by GC- or LC-MS. Brimonidine is an agonist of α2-adrenergic receptors (α2-ARs; Kis = 2.7, 52, and 44 nM for α2A, α2B, and α2C-ARs, respectively, in CHO cells). It is selective for α2-ARs over α1-ARs (Ki = 1,800 nM in human brain). Brimonidine lowers intraocular pressure in DBA/2J mice, a model of glaucoma, to control levels when applied topically to the eye as a 0.1% solution. It also inhibits glutamate release, prevents upregulation of NMDA receptors containing NR1 and NR2A subunits, and protects rat retinal ganglion cells against glutamate excitotoxicity in a rat model of retinal ischemia when administered at a dose of 1 mg/kg per day. Formulations containing brimonidine have been used in the treatment of open-angle glaucoma and ocular hypertension.