NAION is a very serious and irreversible condition that can and often does destroy your sight. The condition is a neuropathy in the optic nerve, which of course is what conducts visual signals out of the retina and into the brain. It is sometimes called an "eye stroke" in that it usually presents as sudden and irreversible vision loss.
The study is associative which cannot prove causation but this is extremely troubling in that the risk was more than four times as high in people using it and accelerated with time, being seven times as high at three years approaching seven percent of the people using the drug over that period of time.