Abnormal, harmful, or undesirable effect on an organism that causes anatomical or functional damage, irreversible physical changes, or increases the susceptibility to other biological, chemical, or environmental stresses.
Optometrists can prescribe glasses, contact lenses, eye exercises, vision aids and therapy, and (in some states) oral and topical medications to treat certain eye diseases. Ophthalmologists are doctors of the eye. They have graduated from medical school and are licensed to practice medicine, so their credentials are either doctor of medicine (MD) or doctor of osteopathy (DO).
Optometrists prescribe, fit and glasses and contact lenses. An optometrist cannot perform surgery, but sometimes works with an ophthalmologist on post-surgical care. Opticians make lenses, frames, and contacts lenses. They analyze and interpret prescriptions and determine the lens best suited for the patient.