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A high dose of the erectile dysfunction drug sildenafil citrate (Viagra) led to irreversible eye damage in a 31-year-old man, authors of a case study reported.

The patient developed "persistent retinal toxicity" after taking far more than the recommended dose, Richard Rosen, MD, of New York Eye and Ear Infirmary of Mount Sinai in New York City, and colleagues wrote in .

The patient in question presented at an urgent care clinic complaining of red-tinted vision in both eyes. He had begun experiencing the symptoms 2 days earlier after ingesting liquid sildenafil citrate he had purchased over the internet from a non-pharmacy source.

The patient further reported that he drank the sildenafil directly from the bottle and didn't know the exact dosage he ingested, but that it was much more than the recommended 50 mg dose.

Visual disturbances are known to be potential side effects of sildenafil citrate. For example, found that a single oral dose of 100-mg sildenafil citrate given to healthy young men "led to small but statistically significant transient changes of outer and inner retinal function" that resolved in 24 hours.

Here, the patient's symptoms failed to improve in the year after initial diagnosis, despite a wide variety of treatments, which included -- upon presentation -- topical prednisolone acetate four times a day in both eyes, brinzolamide 1% three times a day in both eyes, and brimonidine tartrate twice daily in both eyes. A trial of oral prednisone 60 mg once a day for 3 days also resulted in no improvement.

Rosen's group, using imaging such as optimal coherence tomography and adaptive optics, were able to see microscopic injury to the cones of the retina, the photoreceptor cells that are responsible for color vision.

"To actually see these types of structural changes was unexpected, but it explained the symptoms that the patient suffered from," Rosen said in a release. "While we know colored vision disturbance is a well-described side effect of this medication, we have never been able to visualize the structural effect of the drug on the retina until now."

The authors, noting that had shown that an excessive dose of sildenafil had resulted in retinal toxicity, suggested that "higher doses of sildenafil can lead to persistent retinal damage."

But they also acknowledged that the fact the patient bought the sildenafil over the internet from a non-pharmacy source leads to concerns about the purity of the drug and the accuracy of its concentration -- "factors that might have contributed to the increased risk of toxicity."

"Vision changes as a result of Viagra (sildenafil) administration have been a well-known side effect for many years, discovered during Viagra's preclinical trials before the drug's initial release," Nick Tadros, MD, director of Men's Health and Male Infertility at Southern Illinois University in Carbondale, told MedPage Today.

Tadros, who was not involved in the current study, said it highlights an important potential side effect of unmonitored high-dose sildenafil use: "As the authors point out, it is important to realize that this man procured sildenafil of questionable quality from the internet without a physician's prescription and took several times more than the prescribed amount."

He noted that there are no reported cases of permanent vision damage in patients taking sildenafil at the correct dosage.

"In my practice," Tadros continued, "I counsel patients about this potential risk and instruct them to discontinue the drug and report any vision changes. However, as long as patients are consuming the sildenafil from a provider and taking it as instructed, the chance of serious damage to their vision is exceedingly low."