Tysabri is a prescription infusion therapy used to treat relapsing forms of multiple sclerosis (MS) and moderately to severely active Crohn’s disease. The generic name is natalizumab. It’s given through an IV, typically once every four weeks, at an authorized infusion center. Because of a rare but serious brain infection risk, Tysabri is only available through a restricted program, and every infusion involves a safety screening beforehand.
How Tysabri Works
In MS and Crohn’s disease, immune cells cross into tissues where they cause damaging inflammation: the brain and spinal cord in MS, the gut lining in Crohn’s. Tysabri is a lab-made antibody that blocks a specific protein on the surface of immune cells called alpha-4 integrin. This protein normally acts like a key, letting immune cells latch onto blood vessel walls and squeeze through into surrounding tissue. By blocking that key, Tysabri keeps inflammatory immune cells circulating in the bloodstream instead of migrating into the brain or intestines where they’d do harm.
What Tysabri Treats
For MS, Tysabri is used in people with relapsing forms of the disease. It’s considered a high-efficacy therapy, meaning it tends to reduce relapses more dramatically than many first-line treatments. In a large observational study, the average annual relapse rate dropped from about 2.0 relapses per year before starting Tysabri to 0.31 on therapy, and that reduction held steady over five years. The cumulative probability of confirmed disability worsening at five years was around 16%.
For Crohn’s disease, Tysabri is reserved for adults with moderate to severe symptoms who haven’t responded adequately to conventional treatments, including steroids and TNF-blocking biologics. It’s used both to bring on remission and to maintain it. If there’s no meaningful improvement after 12 weeks of treatment, the medication is typically discontinued.
What Happens During an Infusion
Tysabri infusions take place at certified infusion centers, not at home. Each session follows a predictable routine. Before the IV is started, you’ll go through a short screening checklist. A staff member will ask whether you’ve had any new or worsening neurological symptoms over the past month (changes in thinking, vision, balance, or strength), whether you have any condition that weakens your immune system, and whether you’ve recently taken immunosuppressant medications. If you answer yes to any of these, the infusion is paused until your prescribing doctor reviews and clears you.
You’re also required to read a patient medication guide before every single infusion, not just the first one. Once cleared, the medication is delivered through an IV line over approximately one hour. After the infusion finishes, you’ll stay at the center for an additional observation period so staff can watch for allergic reactions. The whole visit, including check-in, infusion, and observation, typically takes around two to three hours. You then return roughly every four weeks for the next dose.
The TOUCH Safety Program
Tysabri can only be prescribed, dispensed, and infused through a restricted program called TOUCH (Tysabri Outreach: Unified Commitment to Health). This exists because of the risk of a rare brain infection called progressive multifocal leukoencephalopathy, or PML. The program requires your doctor to be registered, the infusion center to be authorized, and your patient status to be verified as “authorized” in the TOUCH system before every infusion. If your authorization can’t be confirmed, the center will not proceed.
This can feel like a lot of bureaucratic overhead, but the system is designed to ensure that everyone involved, from prescriber to pharmacist to infusion nurse, is actively monitoring for PML warning signs at each visit.
PML Risk and Monitoring
PML is the most serious risk associated with Tysabri. It’s caused by the JC virus, which many people carry without ever knowing it. In most healthy immune systems, JC virus stays dormant and harmless. But because Tysabri reduces immune surveillance in the brain, the virus can reactivate in some patients and cause a progressive, potentially fatal brain infection.
Three factors influence your PML risk: whether you carry antibodies to JC virus (meaning you’ve been exposed to it), how long you’ve been on Tysabri, and whether you’ve previously used immunosuppressant medications. Patients who test negative for JC virus antibodies have the lowest risk. Those who test positive are monitored more closely, and the level of antibodies (measured by a JC virus antibody index) helps estimate how high the risk is. Your doctor will order this blood test periodically throughout treatment.
Tysabri is completely contraindicated in anyone who has ever had PML. It’s also contraindicated if you’ve had a serious allergic reaction to the drug in the past, which can range from hives to full anaphylaxis.
Common Side Effects
The most frequently reported side effects in MS clinical trials were headache (38% of patients vs. 33% on placebo), fatigue (27% vs. 21%), joint pain (19% vs. 14%), urinary tract infections (21% vs. 17%), and depression (19% vs. 16%). Other common effects included lower respiratory infections, stomach discomfort, diarrhea, limb pain, and rash, all occurring in roughly 10 to 17% of patients.
For Crohn’s disease patients, the profile is similar but slightly different in emphasis. Headache (32 to 37%), upper respiratory infections (22%), and nausea (17%) were the most common. During longer-term maintenance therapy, flu-like illness and sinusitis became more noticeable compared to placebo.
Many of these side effects overlap with symptoms that also occurred in the placebo group, meaning not every headache or bout of fatigue is necessarily caused by the drug itself. Still, the rates are consistently a few percentage points higher in patients receiving Tysabri, so the medication does contribute to them. Most side effects are mild to moderate and tend to be manageable. Infusion-related reactions like itching, dizziness, or flushing can also occur, which is one reason for the post-infusion observation period.
Who Is Not a Candidate
Beyond the contraindications for PML history and prior allergic reactions, Tysabri is not used alongside other immunosuppressant therapies. Combining it with drugs that also suppress the immune system raises the risk of serious infections, including PML. For Crohn’s disease specifically, it cannot be given together with TNF-blocking biologics. If you’re on chronic oral steroids when starting Tysabri for Crohn’s, your doctor will begin tapering the steroids once you respond to treatment. If you can’t get off steroids within six months, Tysabri is generally discontinued.
Patients are also routinely monitored for low platelet counts and signs of liver injury during treatment. If either of these emerges, the medication is stopped.