Tysabri – Safety & Side Effects


The most common side effects or adverse effects associated with Tysabri in clinical trials included headache, fatigue, arthralgia (joint pain), urinary tract infection, lower respiratory tract infection, gastroenteritis, vaginitis, depression, pain in extremity, abdominal discomfort, diarrhea, and rash. Percentages of Tysabri and placebo patients affected by adverse effects in clinical trials are shown below.

The most serious adverse effects associated with Tysabri include progressive multifocal leukoencephalopathy (PML, a serious brain infection), hypersensitivity, and immunosuppression and infections.

Adverse effects in a study of Tysabri in MS

Adverse effect

Placebo (n= 312)

Tysabri (n=627)

Headache 33% 38%
Fatigue 21% 27%
Arthralgia 14% 19%
Urinary tract infection 17% 21%
Lower respiratory tract infection 16% 17%
Gastroenteritis 9% 11%
Vaginitis 6% 10%
Depression 16% 19%
Pain in extremity 14% 16%
Abdominal discomfort 10% 11%
Diarrhea NOS 9% 10%
Rash 9% 12%
Urinary urgency/frequency 7% 9%


Precautions and warnings

There are several warnings and precautions to be aware of before starting treatment with Tysabri.

Progressive multifocal leukoencephalopathy (PML). PML is an infection of the brain, caused by the John Cunningham (JC) Virus, that can occur in people with weakened immune systems. The JC Virus is a harmless virus in most people (exposure to JC Virus usually happens during childhood), but in people with a weakened immune system, reactivation of JC Virus can cause PML. PML can lead to death or severe disability.

Since Tysabri can weaken the immune system by inhibiting normal functioning of leukocytes (a type of white blood cell that is an important immune system cell), use of Tysabri can increase your risk of developing PML. In clinical trials of Tysabri as treatment for MS or Crohn’s disease, 3 cases of PML occurred among 1,869 people who received Tysabri, many of whom received the drug for over 2 years.

If you have been exposed to the JC Virus, your risk for PML increases the longer you take Tysabri (especially after 2 years) and with prior and/or current use of other medications that weaken the immune system. Known as immunosuppressants, these medications include common treatments for cancer and rheumatoid arthritis (such as methotrexate, imuran, cytoxan, cellcept, novantrone), but do not include some common treatments for MS (such as short-term steroids or interferons).  Before starting Tysabri, your doctor may suggest you take the JC Virus test to see if you have antibodies to the JC virus.

Hypersensitivity and anaphylaxis. Hypersensitivity reactions, such as anaphylaxis, a severe systemic reaction can occur in a small number of people who receive Tysabri and will usually happen within 2 hours of starting an infusion of Tysabri. If such a reaction occurs with Tysabri, the infusion should be discontinued and appropriate treatment started.

Immunosuppression and infections. Since Tysabri affects the immune system, it can increase your risk for infections. If you take antineoplastic, immunosuppressant or immunomodulating medications at the same time as Tysabri, your risk for infections (including PML) may increase.

Impaired liver function. Tysabri can cause impaired liver function and significant liver damage in some people. You should discontinue Tysabri if there is evidence of liver damage, including elevated liver enzymes or total bilirubin, which can occur as early as 6 days after starting therapy, or jaundice.

Pregnancy and nursing. Tysabri is a pregnancy category C drug and may cause harm to your unborn child. If you are pregnant or planning to become pregnant, talk to your doctor before taking Tysabri. If you are pregnant, you should only take Tysabri if the potential benefits outweigh the risk to your fetus. If you are nursing an infant, talk to your doctor before taking Tysabri as it may pass into your breast milk. However, the risk to your nursing infant is not known.


Drug interactions

Because of the risk for infections (including PML), if you are receiving chronic immunosuppressant or immunomodulatory treatment, you should not take Tysabri. Examples of immunosuppressants, commonly used to treat cancer and rheumatoid arthritis, include methotrexate, Novantrone, Imuran, Azasan, Rheumatrex, Trexall, Cytoxan, Cellcept, and Myfortic.