Kesimpta (ofatumumab) has a half-life of about 16 days at steady state, meaning the drug itself clears from your blood within roughly three to four months after your last injection. But its biological effects last longer than that. Because Kesimpta works by depleting a specific type of immune cell, its impact on your body persists well after the drug itself is gone. The median time for those immune cells to recover is about 24.6 weeks, or roughly six months.
How Quickly the Drug Leaves Your Blood
Kesimpta’s half-life at steady state is approximately 16 days. That means every 16 days after your last dose, the amount of drug circulating in your blood drops by half. Pharmacologists generally consider a drug fully eliminated after about five half-lives, which for Kesimpta works out to roughly 80 days, or a little under three months. By that point, the concentration in your blood is negligible.
Unlike many medications that are processed by your liver or filtered through your kidneys, Kesimpta is a monoclonal antibody. Your body breaks it down the same way it breaks down any protein: enzymes throughout the body gradually chop it into small fragments (peptides and amino acids) that get recycled. Because the liver and kidneys aren’t doing the heavy lifting here, neither liver disease nor kidney problems meaningfully change how fast Kesimpta is cleared.
Why Its Effects Last Longer Than the Drug Itself
Kesimpta targets and depletes CD20-positive B cells, a subset of white blood cells involved in the immune response. Once those cells are gone, your body needs time to produce new ones, regardless of whether the drug is still circulating. In the ASCLEPIOS I and II clinical trials, the median time for B-cell counts to recover to the lower limit of normal after stopping treatment was 24.6 weeks, about six months.
This distinction matters. When people ask “how long does Kesimpta stay in my system,” they often really want to know how long it continues to affect their immune function. The answer is that your immune system remains suppressed for several months beyond the point where the drug itself has been eliminated. During this window, you’re still more vulnerable to infections, and vaccines may be less effective.
Factors That Affect Clearance
One reassuring finding from the FDA’s pharmacokinetic review is that body weight, sex, age, race, and baseline B-cell count do not have a clinically meaningful effect on how Kesimpta is processed. This is somewhat unusual for a monoclonal antibody, since many are dosed by weight. But Kesimpta’s fixed 20 mg monthly dose produces consistent drug levels across a wide range of patients.
There is one nuance worth understanding. Kesimpta is cleared through two pathways: one that depends on the number of B cells available to bind (called target-mediated clearance) and one that doesn’t. Early in treatment, when B-cell counts are still high, the drug is consumed faster because more cells are soaking it up. Once B cells are depleted, clearance slows, and the drug lingers at slightly higher levels. This is why the 16-day half-life figure applies specifically to steady state, after several months of dosing.
Pregnancy Planning and the Six-Month Window
The FDA-approved prescribing information recommends that women use contraception for six months after their last Kesimpta injection before trying to conceive. This six-month window aligns with the median B-cell recovery time and provides a buffer well beyond the drug’s physical elimination from the blood. It gives the immune system time to normalize and reduces potential exposure during early pregnancy.
What Happens If You Miss a Dose
Because Kesimpta’s half-life is 16 days and the dosing schedule is every four weeks, your drug levels are already declining between doses under normal circumstances. If you miss or delay a dose, levels drop further, but B-cell depletion doesn’t reverse quickly. A single missed dose is unlikely to trigger an immediate return of B cells, though prolonged gaps could allow gradual recovery. If you’ve missed a dose, the key step is simply to take your next injection as soon as possible rather than doubling up.
The Two Timelines to Remember
Think of Kesimpta’s presence in your body on two separate clocks. The first is the drug itself: roughly 80 days (about three months) after your final injection, the protein is essentially gone from your bloodstream. The second clock is biological. Your B cells need approximately six months to bounce back to normal levels. Both timelines matter depending on what you’re planning, whether that’s switching medications, scheduling vaccines, or thinking about pregnancy.