These are both medicines that decrease raised pressure within the eye. they work in different ways to produce a combined effect greater than either medicine produces on its own.the pressure within the eyeball is naturally maintained by a continuous flow of liquid called aqueous humour through the eyeball. aqueous humour is produced by a part of the eye called the ciliary body. it drains out of the eyeball through channels called the trabecular meshwork. if the outflow of aqueous humour is blocked, as in glaucoma, the aqueous humour builds up inside the eye, increasing the pressure within the eyeball. this pressure needs to be reduced, as otherwise it can damage the optic nerve and impair vision as a result.brimonidine is a type of medicine called an alpha agonist. it works by stimulating alpha receptors found on the blood vessels that supply the ciliary body. this causes the blood vessels to constrict,