Understand your risk of PML with Natalizumab

This page will give you an overview of the benefits and risks of Natalizumab and allow you to calculate your risk.

What is Natalizumab?

Natalizumab (Tysabri) is a drug that has shown a reduction in relapses for Multiple Sclerosis.

Detailed information about the drug can be found on the European Medicines Agency: Tysabri product information datasheet.

What are the benefits?

There are 3 primary benefits. The extent of these depends on how active your MS is:

  1. Reduction in relapses:


    1+ relapse in the last 12 months


    2+ relapses AND active MRI lesion

  2. Reduction in disability progression:


    1+ relapse in the last 12 months


    2+ relapses AND active MRI lesion

  3. More likely to be free of disease activity*:


    1+ relapse in the last 12 months


    2+ relapses AND active MRI lesion

*Free from disease activity means relapse free, no progression and no new lesions on MRI.

What are the risks?

As with most drugs, there are risks linked to the long term use of Natalizumab. The main risk is a potentially fatal brain infection, called progressive multifocal leukoencephalopathy (PML).

PML is caused by the JC virus, a common virus completely unrelated to MS. The virus has no symptoms so we don't know we've got it. You must have been infected with the JC virus to be at risk of PML.

Almost half of the general UK population are infected with JC virus in childhood and adolescence. It is unknown how you catch the virus, but you excrete it in your urine and we think it is found in the tonsils. A JC virus test will detect if you have anti-JC antibodies, meaning you have been infected with the virus at some point.

What's your risk of developing PML?

It's impossible to predict if you'll get PML, but there are a number of factors that can increase your risk:

  • You must have the JC virus.

  • Your risk increases if you have previously taken an immunosuppressant. These are drugs that reduce the activity of your bodies immune system.

  • Your risk increases the longer you have been on Natalizumab, especially over 2 years.

  • Your risk increases with a higher titre level. This is the level of antibodies in your blood.

  • Your risk increases with 4 week vs 6 week interval dosing.

Calculate your risk of developing PML

Set the options to calculate your risk:

I have
the JCV virus.
I've been on Natalizumab for
and take every

Your estimated risk is:.

1 in 10,000

How can you reduce this risk?

You can switch to a 6 week vs 4 week interval dosing.

The numbers of MS patients developing PML are now falling due to careful monitoring by their clinical team.

If you are currently JC virus negative, you will be retested every 6 months to see if you are still negative.

If you become JC virus positive and are uncomfortable with the increased risk, there are alternative treatment options available. These should be discussed with your clinical team.

Your clinical team can monitor your titre levels and you can be removed from the drug when the risks become too high.

If you are identified as being at higher risk, your clinician may do regular 3 monthly MRI scans to identify PML at a very early stage.

JCV positive patients receiving Natalizumab every 5-6 weeks, rather than every 4 weeks, reduces the risk of PML by more than:


What if you develop PML?

The severity depends on when PML is detected. If it’s detected before it causes physical symptoms the effects are less severe.

PML can be detected early, before it causes symptoms, with regular 3-4 monthly MRI scans.

The symptoms of PML depend on where the infection is in the brain but the most common are personality changes, cognitive problems, similar symptoms to a MS relapse and epileptic attacks.

There is no effective treatment of PML.

With early detection there is low risk of physical disability with a

1 in 30

chance of death.

Late detection is when PML is allowed to develop and presents with neurological symptoms.

With late detection there is high risk of physical disability with a

1 in 3

chance of death.

What if you need to come off Natalizumab?

You may be able to transfer to another treatment.