The risk of developing a potentially fatal brain infection is higher than previously thought for patients prescribed a genetically engineered multiple sclerosis drug, scientists said Wednesday.

The drug, known as Tysabri, is used to treat patients with relapsing multiple sclerosis, the most common form of the condition. Administered intravenously every 28 days, the drug tamps down the immune system and wards off attacks.

In multiple sclerosis, the immune system assaults the protective coating surrounding nerve fibers, damaging them and causing patients to lose vital functions and suffer overwhelming weakness.

A new medical investigation in the journal Neurology reports that Tysabri’s use by some patients can cause a tenfold risk of developing antibodies to the JC virus.

The virus can lead to a potentially fatal brain disorder known as progressive multifocal leukoencephalopathy, or PML. A German research team, reporting in the journal, said the PML risk is higher than previously thought.

Physicians and advocacy groups Wednesday called the drug a good option despite the findings and emphasized the importance of possibly increasing the amount of routine screening of patients on the drug.

“This sounds like a tempest in teapot to me,” said Dr. Patricia K. Coyle, vice chair of neurology at Stony Brook University Hospital and director of the hospital’s MS Comprehensive Care Center. “Whenever you manipulate the immune system, then theoretically the person may be at risk for PML.”

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She said if any of her patients develop elevated antibody levels, she would advise them about other medication choices. But for those who remain free of JC virus-related antibodies, she will stick with Tysabri for patients who are helped by it.

The JC virus, formally known as the John Cunningham virus, is present in a vast swath of people worldwide, Coyle said Wednesday.

Tysabri, according to the new analysis, aids the virus’ capacity to breach the blood-brain barrier and cause PML. There is no cure.

Many people on Tysabri had not fared well on other therapies for multiple sclerosis, doctors said.

“I don’t want people to say, ‘Well, that’s it. I am not taking the medication,’ because these are the people who need it most,” said Dr. Paul Wright, who chairs the departments of neurology at North Shore University Hospital in Manhasset and Long Island Jewish Medical Center in New Hyde Park. Both hospitals are part of the Northwell Health network.

“We all harbor this virus and we all have the propensity to have the virus,” Wright said. “But when our immune system is compromised, it can come out and be manifested.”

Biogen, maker of Tysabri, has posted a warning on its website about the medication’s propensity to induce PML.

The medication, which costs more than $50,000 a year, is an engineered monoclonal antibody.

The National Multiple Sclerosis Society, which has headquarters in Manhattan, said Wednesday that the drug is one of several medications that help patients who have episodic attacks of multiple sclerosis.

“We are trying to make it as clear as possible for everyone. If you are taking this medication you need to be diligent about following the directions,” said the society’s spokeswoman, Arney Rosenblatt.

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The society posted a bulletin about the new research on its site Wednesday.