Alphagan eye drops contain the active ingredient brimonidine, which is a type of medicine called an alpha agonist. It works by stimulating alpha receptors found on blood vessels in the eye. 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 stimulates alpha receptors found on the blood vessels that supply the ciliary body. This causes the blood vessels to constrict, and reduces the amount of watery fluid that filters out of the blood vessels to form aqueous humour. Brimonidine therefore works by reducing the inflow of aqueous humour into the eyeball, which decreases the pressure within the eye. It is used to treat conditions where there is raised pressure in the eye, such as glaucoma. Brimonidine eye drops are used to treat glaucoma in people who cannot use beta-blocker eye drops. They may also be used in conjunction with other types of eye drop, for example beta-blockers or prostaglandin analogues, to treat glaucoma in people whose eye pressure has not been lowered sufficiently with one medicine.