Most Kesimpta side effects are mild and resolve within a day or two, though the timeline varies depending on the type of side effect. Injection site reactions, the most common complaint, typically clear up the same day or by the following day. Other effects tied to how the drug works on your immune system can persist for weeks or months, and some require ongoing monitoring for as long as you stay on treatment.
Injection Site Reactions
Redness, pain, swelling, or itching at the injection site is the single most reported side effect. These reactions are generally mild to moderate and clear up the same day or by the next day. They tend to be most noticeable with the first few injections and often become less bothersome over time as your body adjusts to the medication.
Systemic Injection-Related Reactions
Some people experience flu-like symptoms after their first injection or during the initial loading doses. This can include fever, headache, muscle aches, chills, and fatigue. These systemic reactions are your body responding to the rapid depletion of a specific type of immune cell (B cells). Kesimpta drives B cell counts down fast, with levels dropping significantly within about 11 days of the first dose.
These flu-like symptoms are most common during the first injection and become rare with subsequent doses. When they do occur, they typically last one to two days. The loading phase (weekly injections for the first three weeks) is when you’re most likely to notice them.
Increased Infection Risk
Because Kesimpta suppresses part of your immune system, infections are the most common ongoing side effect. In a five-year study tracking patients on the drug, the most frequently reported issues were respiratory infections (nasopharyngitis in 19% of patients, upper respiratory infections in about 13%) and urinary tract infections (nearly 13%). These are ordinary infections, not unique to Kesimpta, but they may happen more often or take slightly longer to resolve than they would otherwise.
The rate of serious infections remained low and stable across all five years of follow-up, which is reassuring. Excluding COVID-related infections, serious infection rates actually trended slightly downward over time rather than accumulating with prolonged use. This suggests the immune suppression doesn’t worsen progressively the longer you take the drug.
That said, the infection risk lasts for as long as your B cells remain depleted, which means it persists throughout treatment and for a period after stopping. If you discontinue Kesimpta, B cell counts typically return to the lower end of the normal range in about 23 weeks (roughly five to six months). Until that recovery happens, you remain somewhat more vulnerable to infections.
Low Immunoglobulin Levels
Over time, Kesimpta can lower your levels of immunoglobulins, the antibodies your body uses to fight infections. This is a gradual effect that develops over months to years rather than appearing immediately. There are no formal consensus guidelines on how often to check these levels, but periodic blood tests (roughly every six months) are a reasonable approach for people on long-term treatment.
Low immunoglobulin levels don’t always cause symptoms. Many people with reduced levels never notice a difference. But if your levels drop significantly, it can translate into more frequent or harder-to-shake infections. This is one of the reasons your care team will likely monitor blood work regularly while you’re on Kesimpta.
Rare but Serious Risks
Two uncommon but important risks are linked to Kesimpta’s immune-suppressing mechanism. Hepatitis B reactivation can occur in people who carry the virus, even if it was previously inactive. This is why screening for hepatitis B is required before starting treatment. If you’ve been exposed in the past, your doctor will need to consult a liver specialist before prescribing the drug.
Progressive multifocal leukoencephalopathy (PML) is an extremely rare brain infection caused by a dormant virus that can reactivate when the immune system is suppressed. PML has been associated with other B cell-depleting therapies used in MS. When it occurs, the effects are serious and long-lasting, but the absolute risk with Kesimpta appears very low based on available data.
What to Expect Over Time
The side effect timeline with Kesimpta follows a general pattern. The first few weeks are when you’re most likely to feel something: injection reactions, flu-like symptoms, and the adjustment period as B cells drop. For most people, these early side effects fade quickly and don’t return with ongoing monthly injections.
The longer-term picture is dominated by a modestly increased susceptibility to common infections. Five-year safety data shows that the overall rate of adverse events remained consistent without new safety signals emerging over time. The rate of malignancies was 0.32 per 100 patient-years, which is low and consistent with background rates in the general population.
If you stop Kesimpta, most side effects resolve as your B cells repopulate over the following five to six months. During that recovery window, some residual immune suppression continues, so the infection risk doesn’t disappear the moment you stop injecting.