Tysabri is a prescription infusion therapy used to treat two conditions: relapsing forms of multiple sclerosis (MS) in adults and moderate to severe Crohn’s disease. It belongs to a class of high-efficacy treatments that work by blocking immune cells from reaching areas where they cause damage. For MS, it’s one of the most effective options available. For Crohn’s disease, it’s reserved for people who haven’t responded to other treatments.
How Tysabri Treats Multiple Sclerosis
Tysabri is approved for relapsing forms of MS, which includes three subtypes: clinically isolated syndrome (a single episode of neurological symptoms), relapsing-remitting MS, and active secondary progressive MS. In all three, the immune system periodically attacks the protective coating around nerve fibers in the brain and spinal cord, causing flare-ups of symptoms like numbness, vision problems, or difficulty walking.
The drug works by preventing immune cells from crossing from the bloodstream into the brain and spinal cord. Normally, immune cells latch onto the walls of blood vessels using a surface protein called an integrin, which acts like a key fitting into a lock on the vessel wall. Tysabri blocks that key, so inflammatory cells stay in the bloodstream instead of entering the central nervous system where they would cause damage.
The results are striking. In a UK cohort study, patients on Tysabri saw their annual relapse rate drop from 2.21 to 0.29, an 87% reduction. That benefit held up over time, remaining at roughly the same level even after three and a half years of treatment. Relapses severe enough to require hospitalization dropped by 93%. Even patients who had previously tried other MS therapies like interferons saw relapse reductions above 83%.
Current clinical guidelines from the Department of Veterans Affairs classify Tysabri as a high-efficacy disease-modifying therapy, meaning it’s among the strongest options for controlling MS. It’s typically favored for people with highly active relapsing MS, where preventing relapses and disability progression is the top priority.
How Tysabri Is Used for Crohn’s Disease
Tysabri is also approved for adults with moderately to severely active Crohn’s disease who have signs of ongoing inflammation. It’s not a first-line treatment. You’d typically try it only after standard Crohn’s therapies and TNF-alpha inhibitors (another class of biologics commonly used in Crohn’s) have failed or caused intolerable side effects.
The same mechanism that keeps immune cells out of the brain also works in the gut. By blocking the integrin on immune cells, Tysabri reduces the migration of inflammatory cells into intestinal tissue, which helps calm the inflammation driving Crohn’s symptoms. If there’s no meaningful improvement after 12 weeks of treatment, the drug is generally discontinued. For patients who do respond but are also on corticosteroids, the goal is to taper off steroids within six months of starting Tysabri.
What the Infusion Looks Like
Tysabri is given as an intravenous infusion, not a pill or injection you can do at home. The standard dose is 300 mg delivered over about one hour, once every four weeks. After the infusion finishes, you’ll be monitored for an additional hour for any signs of an allergic reaction. This means each visit takes roughly two hours.
Tysabri can only be prescribed and administered through a restricted program called TOUCH. Before every infusion, you’ll complete a checklist that screens for new neurological symptoms, immune-compromising conditions, and recent use of other immune-suppressing medications. If any of those answers raise a red flag, the infusion is paused until your prescribing doctor reviews your situation. This extra layer of oversight exists because of one serious risk in particular.
The PML Risk and How It’s Managed
The most significant concern with Tysabri is progressive multifocal leukoencephalopathy, or PML, a rare but potentially life-threatening brain infection. PML is caused by a common virus called the JC virus, which most people carry harmlessly. Because Tysabri reduces immune surveillance in the brain, the virus can sometimes reactivate and cause damage in people who would otherwise never have a problem with it.
Not everyone on Tysabri faces the same level of risk. Doctors use a blood test that measures JC virus antibody levels, expressed as an “index value.” If your index value is below 1.5 and you haven’t taken immune-suppressing drugs before, the risk of PML is very low, around 0.1% or less, even after up to six years of treatment. If your index is above 1.5, the risk climbs to roughly 1% with long-term use, about ten times higher. Regular screening for JC virus antibodies is standard practice while you’re on the drug.
Tysabri is contraindicated if you’ve already had PML, if you’ve had a previous allergic reaction to the drug, or if your immune system is significantly weakened by conditions like HIV or by organ transplantation. It also cannot be combined with other immunosuppressants or TNF-alpha inhibitors, since layering immune-suppressing drugs increases infection risk.
Common Side Effects
Most side effects of Tysabri are mild to moderate. In MS clinical trials, the most frequently reported issues were headache (38% of patients, compared to 33% on placebo), fatigue (27%), joint pain (19%), urinary tract infections (21%), and depression (19%). Lower respiratory infections, stomach discomfort, diarrhea, and rash each affected roughly 10 to 17% of patients.
For Crohn’s disease patients, the side effect profile looks similar but with some differences in emphasis. Headache (32%), upper respiratory infections (22%), nausea (17%), and fatigue (10%) were the most common during the initial treatment phase. During longer-term maintenance, back pain (12%), flu-like illness (11%), and influenza (12%) became more noticeable.
Many of these side effects overlap with what placebo groups experienced, so the difference attributable to Tysabri itself is often only a few percentage points. Still, because the drug dampens immune cell movement, infections of various kinds, from urinary tract and respiratory infections to vaginal infections, tend to occur somewhat more frequently than they would otherwise.
Stopping or Switching Tysabri
One important feature of Tysabri is that MS disease activity can rebound after stopping it. Unlike some treatments where you can simply discontinue and monitor, stopping Tysabri without a plan increases the chance of a significant flare. Current guidelines recommend switching to another disease-modifying therapy within 8 to 12 weeks of discontinuation to reduce this rebound risk.
For patients who are stable and want to reduce their risk exposure over time, doctors may extend the dosing interval from every four weeks to every eight weeks rather than stopping altogether. This de-escalation approach can lower PML risk while maintaining some level of disease control, and it’s increasingly considered a practical middle ground for long-term management.