If you or a loved one is taking Tysabri for multiple sclerosis or Crohn's disease, you may have heard about the risk of progressive multifocal leukoencephalopathy (PML). Decades of pharmacovigilance have established that PML is a rare but serious brain infection associated with this medication. This page reviews the current evidence on who may be at risk and what the FDA safety communications say.
Tysabri (natalizumab) is a monoclonal antibody used to treat multiple sclerosis and Crohn disease. Its prescribing information carries a boxed warning stating that Tysabri increases the risk of progressive multifocal leukoencephalopathy (PML), an opportunistic viral infection of the brain caused by the JC virus (JCV) that usually leads to death or severe disability (https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c5fdde91-1989-4dd2-9129-4f3323ea2962). PML is a severe demyelinating disease that typically occurs only in patients who are immunocompromised (https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c5fdde91-1989-4dd2-9129-4f3323ea2962). The boxed warning identifies three risk factors for developing PML in Tysabri-treated patients: the presence of anti-JCV antibodies, longer treatment duration (especially beyond two years), and prior use of immunosuppressants (https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c5fdde91-1989-4dd2-9129-4f3323ea2962). These factors should be considered in the context of expected benefit when initiating and continuing treatment (https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c5fdde91-1989-4dd2-9129-4f3323ea2962). The clinical presentation of PML can be subtle and variable. A retrospective national cohort study of 456 Italian PML patients observed between 1987 and 2024 described the demographic, clinical, radiological, and laboratory characteristics of the disease (https://pubmed.ncbi.nlm.nih.gov/40922664/). In that cohort, 376 cases (82.4%) had a definite diagnosis and 80 (17.6%) had a clinico-radiological diagnosis (https://pubmed.ncbi.nlm.nih.gov/40922664/). The study noted that PML characteristics and survival have changed over time and vary according to underlying condition (https://pubmed.ncbi.nlm.nih.gov/40922664/). For Tysabri-treated patients, the prescribing information instructs healthcare professionals to monitor patients for any new sign or symptom that may be suggestive of PML and to withhold Tysabri dosing immediately at the first sign or symptom suggestive of PML (https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c5fdde91-1989-4dd2-9129-4f3323ea2962).
The mechanistic pathway linking Tysabri to PML involves the drug's effect on immune surveillance. Tysabri is an alpha-4 integrin antagonist that inhibits the migration of lymphocytes into the central nervous system. This action reduces inflammation in multiple sclerosis but also impairs the normal immune response to JC virus, allowing the virus to reactivate and cause PML in susceptible individuals. The risk is highest in patients who are anti-JCV antibody positive, have been on therapy for more than two years, or have previously used immunosuppressant medications (https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c5fdde91-1989-4dd2-9129-4f3323ea2962). Adverse event reports submitted to the FDA Adverse Event Reporting System (FAERS) for Tysabri include a wide range of symptoms that may overlap with PML presentation. The most frequently reported events include fatigue (19,150 reports), multiple sclerosis relapse (16,691 reports), headache (9,626 reports), gait disturbance (9,422 reports), memory impairment (7,895 reports), asthenia (7,852 reports), balance disorder (5,621 reports), hypoesthesia (5,343 reports), muscular weakness (4,535 reports), cognitive disorder (3,478 reports), and mobility decreased (3,769 reports) (https://api.fda.gov/drug/event.json?search=patient.drug.medicinalproduct:TYSABRI). These symptoms can be early indicators of PML, and the prescribing information emphasizes that Tysabri should be withheld immediately at the first sign or symptom suggestive of PML (https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c5fdde91-1989-4dd2-9129-4f3323ea2962).
The adequacy of warnings regarding Tysabri and PML is addressed by the boxed warning and the restricted distribution program called the TOUCH Prescribing Program (https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c5fdde91-1989-4dd2-9129-4f3323ea2962). The boxed warning clearly states that Tysabri increases the risk of PML and that the infection usually leads to death or severe disability (https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c5fdde91-1989-4dd2-9129-4f3323ea2962). It also identifies the three known risk factors (https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c5fdde91-1989-4dd2-9129-4f3323ea2962). However, patients who developed PML despite these warnings may have questions about whether the risks were adequately communicated or whether their individual risk factors were properly assessed before and during treatment. For affected patients and their families, attorney-related considerations may include evaluating whether the treating physician followed the recommended monitoring protocols, whether the patient was tested for anti-JCV antibodies, and whether the duration of therapy was appropriate given the patient's risk profile. The timeline between exposure and documented harm is critical: PML risk increases with longer treatment duration, especially beyond two years (https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c5fdde91-1989-4dd2-9129-4f3323ea2962). Patients who developed PML after prolonged Tysabri use may need to examine whether the benefits of continued therapy outweighed the known risks at each point in their treatment course. The prescribing information requires that these factors be considered in the context of expected benefit when initiating and continuing treatment (https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c5fdde91-1989-4dd2-9129-4f3323ea2962). In summary, Tysabri carries a well-documented risk of PML, a devastating brain infection. The drug's labeling provides clear warnings and identifies specific risk factors. Patients who develop PML may benefit from legal consultation to assess whether the warnings were adequate and whether their treatment was managed appropriately given the known risks.
This page is for educational and informational purposes only. It does not provide medical diagnosis, treatment, or legal advice. Consult licensed clinicians and qualified attorneys for case-specific decisions.
Tysabri (natalizumab) is a monoclonal antibody used to treat multiple sclerosis and Crohn disease. It carries a boxed warning stating that it increases the risk of progressive multifocal leukoencephalopathy (PML), a severe brain infection caused by the JC virus that often leads to death or severe disability (https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c5fdde91-1989-4dd2-9129-4f3323ea2962).
The three known risk factors are: presence of anti-JCV antibodies, longer treatment duration (especially beyond two years), and prior use of immunosuppressants (https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c5fdde91-1989-4dd2-9129-4f3323ea2962).
Symptoms can include fatigue, gait disturbance, memory impairment, balance disorder, muscular weakness, cognitive disorder, and others. The prescribing information instructs to withhold Tysabri immediately at the first sign or symptom suggestive of PML (https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c5fdde91-1989-4dd2-9129-4f3323ea2962).
An attorney can evaluate whether the treating physician followed recommended monitoring protocols, whether anti-JCV antibody testing was performed, and whether the duration of therapy was appropriate given the patient's risk profile. Legal consultation may help assess if warnings were adequate and if treatment was managed properly.
No. Submission requests an initial records screening only and does not create an attorney-client relationship.
This page is for educational and informational purposes only and is not medical or legal advice. Consult a licensed professional for case-specific guidance.