What the label lists and what reports show
Natalizumab side effects
Natalizumab's FDA label: “Most common adverse reactions (incidence ≥ 10%): MS - headache, fatigue, arthralgia, urinary tract infection, lower respiratory tract infection, gastroenteritis, vaginitis, depression, pain in extremity, abdominal discomfort, diarrhea NOS, and rash ( 6.1 ) CD …” FAERS holds 180,844 reports naming it through July 30, 2026; the reaction reported most often, leaving out the conditions it is used for, product-use terms and outcomes, is fatigue (19,617 reports).
- No generic listed
- Boxed warning
Check it at the source: the natalizumab FDA label on DailyMed (effective March 21, 2025, Biogen Inc.); the FDA FAERS public dashboard.
What does the patient leaflet say about side effects?
From the patient leaflet: What are the possible side effects of TYSABRI?TYSABRI may cause serious side effects, including: See “ What is the most important information I should know about TYSABRI? ” Herpes Infections. TYSABRI may increase your risk of getting an infection of the brain or the covering of your brain and spinal cord (encephalitis or meningitis) caused by herpes viruses that may lead to death. Call your doctor right away if you have sudden fever, severe headache, or if you feel confused after receiving TYSABRI. Herpes infections of the eye, causing blindness in some patients, have also occurred. Call your doctor right away if you have changes in vision, eye redness, or eye pain. Liver damage. Symptoms of liver damage can include: yellowing of the skin and eyes (jaundice) unusual darkening of the urine nausea feeling tired or weak vomiting Call your doctor right away if you have symptoms of liver damage. […]
Source: FDA drug label, Biogen Inc., effective March 21, 2025 (SPL set c5fdde91-1989-4dd2-9129-4f3323ea2962), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsMost common adverse reactions (incidence ≥ 10%): MS - headache, fatigue, arthralgia, urinary tract infection, lower respiratory tract infection, gastroenteritis, vaginitis, depression, pain in extremity, abdominal discomfort, diarrhea NOS, and rash ( 6.1 ) CD - headache, upper respiratory tract infections, nausea, and fatigue ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Biogen at 1-800-456-2255 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch.
Table 2: Adverse Reactions in Study MS1 (Monotherapy Study)
| Adverse Reactions (Preferred Term) | TYSABRI n=627 % | Placebo n=312 % |
|---|---|---|
| Headache | 38 | 33 |
| Fatigue | 27 | 21 |
| Arthralgia | 19 | 14 |
| Chest discomfort | 5 | 3 |
| Other hypersensitivity reactions | 5 | 2 |
| Acute hypersensitivity reactions | 4 | <1 |
| Seasonal allergy | 3 | 2 |
| Rigors | 3 | <1 |
| Weight increased | 2 | <1 |
| Weight decreased | 2 | <1 |
| Urinary tract infection | 21 | 17 |
| Lower respiratory tract infection | 17 | 16 |
| Gastroenteritis | 11 | 9 |
| Vaginitis | 10 | 6 |
| Tooth infections | 9 | 7 |
| Herpes | 8 | 7 |
| Tonsillitis | 7 | 5 |
| Depression | 19 | 16 |
Rows and numbers as the label prints them, under its own column headings (the first rows of a longer table where it runs on). A reaction in a trial table is not necessarily caused by the medicine, and trials differ from everyday use.
From the label: adverse reactions, continuedThe following serious adverse reactions are described below and elsewhere in the labeling: Progressive Multifocal Leukoencephalopathy (PML) [see Warnings and Precautions ( 5.1 )] Herpes Infections [see Warnings and Precautions ( 5.3 )] Hepatotoxicity [see Warnings and Precautions ( 5.4 )] Hypersensitivity/Antibody Formation [see Warnings and Precautions ( 5.5 )] Immunosuppression/Infections [see Warnings and Precautions ( 5.6 )] Hematological Abnormalities [see Warnings and Precautions ( 5.8 )] Most common adverse reactions (incidence ≥ 10%): MS - headache, fatigue, arthralgia, urinary tract infection, lower respiratory tract infection, gastroenteritis, vaginitis, depression, pain in extremity, abdominal discomfort, diarrhea NOS, and rash ( 6.1 ) CD - headache, upper respiratory tract infections, nausea, and fatigue ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Biogen at 1-800-456-2255 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch. 6.1 Clinical Trials Experience Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in the clinical trials of another drug and may not reflect the rates observed in practice. The most common adverse reactions (incidence ≥ 10%) were headache and fatigue in both the multiple sclerosis (MS) and Crohn's disease (CD) studies.
Source: FDA drug label, Biogen Inc., effective March 21, 2025 (SPL set c5fdde91-1989-4dd2-9129-4f3323ea2962), via openFDA; the full label on DailyMed
What do FAERS reports show?
180,844 reports list natalizumab among the medicines taken, each a report of something that happened, not proof the medicine caused it; 5,103 are coded as a death and 34,611 as a hospital stay. Counts follow how widely a medicine is used and watched, and they are not a measure of how risky it is. Of the 23 reactions below, 8 are named by the same term (in US spelling) in the label's adverse reactions, warnings or patient leaflet; a reaction the label does not name may still be there under another word, and a report does not show that the medicine caused it.
Reports received each year
The 25 most reported reactions
Source: FDA Adverse Event Reporting System (FAERS) via openFDA, reports through July 30, 2026; reactions as MedDRA preferred terms
What next?
- Natalizumab: the full record
- label, generics, price, shortages, recalls
- Report a side effect to the FDA
- MedWatch
- Side effects hub
- the most reported medicines and reactions