The Canadian regulator is investigating two possible cases in Canada among men who were using Viagra, but has not confirmed the drugs were related to the medical problems.
In May, the U.S Food and Drug Administration issued a similar warning for users of Viagra, Cialis and Levitra, which are used to treat impotence and erectile dysfunction.
The vision loss, called nonarteritic anterior ischemic optic neuropathy (NAION), occurs when blood flow to the optic nerve is blocked.
The condition can cause sudden and painless loss of some or all vision in one or both eyes.
There have been no confirmed reports of excess hair growth on users' hands, however.