Tysabri was approved for use in Canada in 2006. (CBC)

There are reports of a rare brain infection among people taking the multiple sclerosis drug Tysabri, Health Canada warns.

The brain infection, called progressive multifocal leukoencephalopathy, or PML, usually happens in people with weakened immune systems.

Since Tysabri first came on markets several years ago, five people around the world who were taking the drug developed PML and one case resulted in death, Health Canada said Wednesday.

The drug has been approved for use in Canada since 2006 for the treatment of the relapsing-remitting form of MS, a progressive neurodegenerative disease.

In this type of MS, patients alternate between periods when they experience symptoms and when they don't.

The symptoms of PML are similar to those of MS, and include:

• Progressive weakness on one side of the body.
• Clumsy limbs.
• Disturbed vision.
• Changes in thinking, memory and orientation.
• Confusion.
• Personality changes.

People with MS who are taking Tysabri who experience these or other unusual symptoms, or who find their MS is getting worse, should contact their doctors immediately, Health Canada advised.

Tysabri (natalizumab) is generally recommended in MS patients who have not responded to or cannot tolerate other treatments for MS.

The drug is taken intravenously once a month. It should not be used in combination with other medicines that affect the immune system.

It's designed to work by preventing the body's affected immune cells from migrating from the bloodstream into the brain, where they can cause inflammation and damage nerve fibres and their myelin coating.

Prescribing information for Tysabri has been updated to include more information about PML, the drug's manufacturer, Biogen Idec, said Thursday.

As of the end of December 2008, approximately 37,600 patients were receiving Tysabri worldwide, according to the company.