Causes of loss of peripheral vision?

Many many: Most common would be glaucoma and stroke.

Retina-optic nerve: Peripheral vision loss may be a retinal detachment, retinitis pigmentosa, rare drug toxicities, immune retinopathy, retinal infection, and glaucomatous optic neuropathy. In the brain migraine, and certain types of stroke can cause a loss of peripheral vision. Transient obscurations of vision may be low blood pressure. An eye examination with dilation should identify the cause.

Many possibilities: Many things can cause loss of peripheral (side) vision: glaucoma is the most likely. Ocular pressure rise causes vascular and optic nerve damage. Another is a tumor or aneurysm near the pituitary gland, compressing the nerves from the eye as they first join and then re-divide on the way to the back of the brain. Oculo-vascular occlusions can do the same although most often this is only in one eye.