WASHINGTON (AP) -- Regulators are exploring whether organ transplant drugs made by Roche and Novartis increase the risk of an often-fatal neurological disease.

The Food and Drug Administration said Thursday it has identified 16 cases of patients developing the rare neurological ailment while taking Roche's drug CellCept, which is used to avoid rejection in organ transplant patients. U.S. regulators approved CellCept in 1995.

The Swiss drug maker Roche alerted regulators to the issue last November. FDA said it is looking into similar risks with Myfortic, a drug made by fellow Swiss drugmaker Novartis that includes the same key ingredient as CellCept.

The agency said it will take about two months to review the reports and approve new labeling for the medications. Until then, regulators advise doctors and patients to watch for symptoms of neurological problems.

The disease, known as progressive multifocal leukoencephalopathy, attacks the brain and central nervous system and is usually fatal. Symptoms include vision problems, loss of coordination and memory loss. Patients who survive are often permanently disabled, according to the FDA.

Roche has independently confirmed 10 cases of the disease in CellCept patients, according to company spokesman Christopher Vancheri. He added that the drug has been used by an estimated 500,000 patients worldwide.

CellCept was Roche's sixth best-selling drug last year with sales of $1.99 million.

Roche submitted new labeling for CellCept last November which the FDA is still reviewing.

European regulators have already added language about the disease to CellCept. In February, Roche sent a letter to European doctors, highlighting the changes. The letter said problems were reported in kidney, heart and lung transplant patients. The disorder was also seen in patients taking the drug for a form of lupus, a use that is not approved by regulators.

Roche said that it is difficult to sort out the role of its drug in the reports, since many patients had other diseases and were taking other drugs. "However, the contributory role of CellCept cannot be excluded," states the letter, which FDA posted to its Web site Thursday.

A spokeswoman for Novartis said the company plans to cooperate with any labeling changes recommended by regulators.

FDA said it has not received any reports of the disease with Myfortic, which was approved in 2004.

Shares of Novartis AG fell $1.89, or 3.8 percent, Thursday to close at $47.28.

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