There has also been a third suspected but unconfirmed case.

The infection has also been reported in patients taking Biogen Idec's multiple sclerosis drug Tysabri.

A milder warning was included in the original Adcetris label based on a single case reported in a patient who had received four chemotherapy regimens prior to receiving the drug.

Following the occurrence of a second case of the infection, the company began working with U.S. regulators to add a stronger, boxed warning.

The infection, known as progressive multifocal leukoencephalopathy (PML), is a rare disease of the central nervous system that often leads to death or severe disability and is believed to be caused by the JC virus.

"Although PML in lymphoma patients can be caused by factors such as underlying disease and prior therapies that affect the immune system, a contributory role of Adcetris cannot be excluded," Chief Medical Officer Thomas C. Reynolds said in a statement.

Adcetris, known chemically as brentuximab vedotin, received accelerated approval by the U.S. Food and Drug Administration in August 2011 for two types of relatively rare blood cancers -- Hodgkin's lymphoma and anaplastic large cell lymphoma.

The drug links a tumor-targeting antibody to a cancer-killing chemotherapy drug with the goal of limiting side effects.

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