A prominent consumer advocacy group is calling for the government to add the most serious of warnings to the labels of Viagra (search) and other impotence drugs, noting that some users have gone blind.

Public Citizen (search) petitioned the Food and Drug Administration Thursday to add black-box warnings to the labels of Viagra, Cialis (search) and Levitra (search), as well as the hypertension drug Revatio (search), which is similar to Viagra.

A small number of people taking the impotence drugs have developed NAION, or non-arteritic ischemic optic neuropathy (search), a loss of vision that is frequently irreversible.

NAION is considered one of the most common causes of sudden vision loss in older people, with anywhere from 1,000 to 6,000 cases a year. Risk factors include diabetes and heart disease, which are also two of the leading causes of impotence.

In July, the FDA ordered less prominent warnings be added to the labels of the three impotence drugs. The warnings noted that some users have suffered vision loss but cautioned it was impossible to know if the pills are to blame.

Public Citizen, however, used FDA data to compare the rates of people who developed NAION while on the impotence drugs with those who were taking Lipitor, the cholesterol drug that is used by people with the same risk factors.

The group found that number of reports of the condition per million prescriptions was 18 times higher for patients taking Viagra and 25 times higher for Cialis compared to Lipitor. The group contends this data more closely associates the risk of blindness with the impotence drugs than the FDA has acknowledged.

The actual number of cases are few: between 1998 and 2004, there were 48 reports to FDA's database of people taking Viagra developing the condition. Public Citizen found an additional 50 cases of other visual problems among Viagra patients, the group said in its petition. That's out of 89 million prescriptions written during that period.

Vision problems reported among those taking Cialis and Levitra number 12 — including only two NAION cases, both in Cialis users, according to Public Citizen. Roughly 5.2 million prescriptions for the two drugs were written over those six years.

"It is the Food and Drug Administration's job to immediately and coherently tell consumers about the risks associated with the drugs they put into their bodies," Dr. Sidney Wolfe, director of Public Citizen's Health Research Group, said in a statement. "The agency has once again failed in this responsibility. These drugs need much stronger warnings, especially a black box warning such as the one we have proposed."

In July, FDA advised patients to stop taking the pills and call a doctor if they experience sudden or decreased vision loss in one or both eyes — and to tell their doctor if they have ever suffered an episode of sudden vision loss, because such patients are at increased risk of a second episode.