Floaters are basically due to vitreous gel pathologies and flashes are due to retinal pathologies.since your ophthalmologist has checked dilated fundus this possiblity is ruled out.so it could be visual hallucinations as a side effect of substance abuse. But it should have been bilateral or random and not in one eye specifically.so my advice to you is: A) Stop the drug for 1 week (If possible without withdrawal symptoms or under mefical supervision)and observe for floaters and flashes;if they didappear ;they were due to visuall hallucinations as a side effect of drug. B) If they persist then hv periodic retina examination If floaters and flashes continue or their number and size increases you should immediately rush to retina specialist C) More over this particular drug is also known to cause 1.redness in eyes(as it dilates blood vessels of eyes) 2. causes increase in eye pressures leading to glaucoma. so you are at risk of developing glaucoma if you are taking this drug for long and in large quantity.so i advice you to have a peroidic eye check up to measure eye pressures and it would be benificial if you are able to discontinue the drug under medical supervision