What You Should Know About Kesimpta Side Effects

Every medication comes with potential side effects, and Kesimpta (Ofatumumab) is no exception. As a biologic that works by depleting certain immune cells, Kesimpta has a specific side effect profile that's important to understand before — and during — treatment.

This guide breaks down the common and serious side effects reported in clinical trials and real-world use, who may be more at risk, and when you should pick up the phone and call your doctor.

What Is Kesimpta? (Quick Overview)

Kesimpta is a fully human anti-CD20 monoclonal antibody approved for relapsing forms of multiple sclerosis (MS) in adults. It works by targeting and depleting B cells — a type of white blood cell involved in the immune attacks that damage nerve tissue in MS. It's self-administered as a subcutaneous injection once monthly (after initial loading doses). For a deeper dive, read our complete guide to Kesimpta.

Common Side Effects

These are the side effects reported most frequently in clinical trials. Most are mild to moderate and tend to improve over time:

• Upper respiratory tract infections — The most commonly reported side effect. Symptoms include runny nose, sore throat, and sinus congestion.
• Headache — Reported by a notable percentage of patients, especially in the early weeks of treatment.
• Injection-related reactions (systemic) — Fever, chills, muscle aches, and fatigue that can occur within 24 hours of your injection. These are most common after the first dose and tend to lessen with subsequent injections.
• Injection-site reactions (local) — Redness, pain, itching, or swelling at the injection site. Usually mild and short-lived.
• Urinary tract infections (UTIs) — More frequent in some patients, likely related to immune system changes.
• Back pain — Reported in clinical trials, though the exact mechanism isn't fully understood.
• Decreased immunoglobulin M (IgM) — Kesimpta can lower IgM antibody levels over time. Your doctor will monitor this with periodic blood work.

Serious Side Effects

These are less common but require immediate medical attention:

Infections

Because Kesimpta suppresses part of your immune system, it increases your risk of serious infections. This includes bacterial, viral, and fungal infections that your body might normally fight off. Signs to watch for:

• Fever above 100.4°F (38°C)
• Chills or persistent sweating
• Cough that won't go away
• Painful or frequent urination
• Skin sores or wounds that aren't healing

Hepatitis B Virus (HBV) Reactivation

If you've ever had hepatitis B — even if you recovered — Kesimpta can cause the virus to reactivate. This can lead to serious liver damage and even liver failure. Your doctor should screen you for HBV before starting treatment. If you have a history of HBV, you'll need close monitoring.

Progressive Multifocal Leukoencephalopathy (PML)

PML is a rare but potentially fatal brain infection caused by the JC virus. While PML has not been reported in Kesimpta clinical trials for MS, it has occurred with other anti-CD20 therapies and is listed as a warning. Symptoms include:

• Progressive weakness on one side of the body
• Vision changes
• Confusion or personality changes
• Problems with thinking or memory

Contact your doctor immediately if you notice any of these symptoms.

Severe Injection-Related Reactions

In rare cases, injection-related reactions can be severe, including difficulty breathing, throat swelling, or a drop in blood pressure. This is why your first injection should be supervised by a healthcare professional.

Low Immunoglobulins

Over time, Kesimpta can reduce your body's levels of immunoglobulins (antibodies), particularly IgM and sometimes IgG. Low immunoglobulins can increase your vulnerability to infections. Your doctor will check these levels periodically.

Side Effects in Specific Populations

Pregnancy and Breastfeeding

Kesimpta has not been adequately studied in pregnant women. Because it depletes B cells, there's a theoretical risk to the developing immune system of a fetus. Novartis recommends that women of childbearing potential use effective contraception during treatment and for 6 months after the last dose. It's unknown whether Ofatumumab passes into breast milk.

Older Adults

Clinical trial data in patients over 65 is limited. Older adults may have a higher baseline risk of infection, so the immunosuppressive effects of Kesimpta warrant extra caution.

People With a History of Hepatitis B

As noted above, HBV screening is mandatory before starting Kesimpta. Patients with active HBV infection should not take Kesimpta.

How to Manage Side Effects

Many common side effects can be managed at home with straightforward strategies:

• Injection-related reactions: Your doctor may recommend premedication with acetaminophen, antihistamines, or corticosteroids — especially for early doses.
• Injection-site reactions: Rotate injection sites (abdomen, thigh, outer upper arm). Apply a cold compress after injecting.
• Upper respiratory infections: Stay hydrated, rest, and treat symptoms with OTC cold remedies. Call your doctor if symptoms worsen or last more than 10 days.
• Headaches: Over-the-counter pain relievers like acetaminophen or ibuprofen are usually fine, but check with your doctor.
• Staying healthy overall: Get your vaccinations up to date before starting Kesimpta, wash your hands frequently, and avoid close contact with people who are sick.

When to Call Your Doctor

Call your doctor or seek emergency care if you experience:

• Fever, chills, or signs of infection
• Difficulty breathing or throat tightness after an injection
• Yellowing of the skin or eyes (possible liver issue)
• Progressive weakness, vision changes, or confusion (possible PML)
• Unusual bruising or bleeding
• Symptoms that are new, persistent, or worsening

When in doubt, call. Your neurology team would rather hear from you than have you wait it out.

Final Thoughts

Kesimpta is a powerful and effective treatment for relapsing MS, and most patients tolerate it well. But understanding the potential side effects — and knowing when to act — helps you stay safe while getting the benefits of treatment.

If cost or access is a concern alongside side effect management, explore our guides on saving money on Kesimpta and finding Kesimpta in stock. And if you're ready to locate a pharmacy, Medfinder can help.