Nucala (mepolizumab) is a biologic medication approved to treat five conditions, all linked by a common thread: overactive eosinophils, a type of white blood cell that drives inflammation when present in excess. It works by blocking a signaling protein called interleukin-5 (IL-5), which is the main chemical messenger responsible for producing, activating, and keeping eosinophils alive. By cutting off that signal, Nucala lowers eosinophil levels in the blood and tissues, reducing the inflammation that fuels each of these conditions.
The Five Approved Uses
Nucala is FDA-approved for the following conditions:
- Severe eosinophilic asthma in adults and children aged 6 and older
- Chronic rhinosinusitis with nasal polyps (CRSwNP) in adults 18 and older
- Chronic obstructive pulmonary disease (COPD) with an eosinophilic component, in adults
- Eosinophilic granulomatosis with polyangiitis (EGPA), a rare blood vessel disease, in adults
- Hypereosinophilic syndrome (HES), a condition of persistently elevated eosinophils lasting six months or more, in adults and children aged 12 and older
In every case, Nucala is either an add-on to existing treatment (not a replacement for inhalers, steroids, or other medications you’re already taking) or a targeted therapy for rare eosinophil-driven diseases. It is not a rescue medication and won’t help during an acute asthma attack or COPD flare.
Severe Eosinophilic Asthma
This is the most common reason Nucala is prescribed. It’s specifically for people whose asthma is driven by high eosinophil levels and isn’t well controlled by standard treatments like inhaled corticosteroids and long-acting bronchodilators. To qualify, you typically need a blood eosinophil count of at least 150 cells per microliter within the past six weeks, or at least 300 cells per microliter within the past year.
The results in clinical practice are meaningful. In a study of U.S. Medicare patients, the average number of asthma flare-ups per person per year dropped by 43% after starting Nucala. The proportion of patients experiencing any flare-up fell by 27%, and hospitalizations for severe attacks dropped by 40%. For people who have been cycling through emergency visits and oral steroid bursts, those numbers translate to substantially fewer disruptions and less reliance on medications that carry long-term side effects.
Nucala is approved for children as young as 6 for this indication, making it one of the few biologics available for younger pediatric patients with severe eosinophilic asthma.
Chronic Rhinosinusitis With Nasal Polyps
Nasal polyps are soft, noncancerous growths in the lining of the sinuses that cause persistent congestion, loss of smell, facial pressure, and frequent sinus infections. When polyps keep coming back despite steroid sprays or surgery, eosinophilic inflammation is often the underlying driver.
In clinical trials, about 53% of patients with both asthma and nasal polyps saw at least a one-point improvement in polyp size scores on Nucala, compared to roughly 30% on placebo. The benefit was even more pronounced in patients with aspirin-exacerbated respiratory disease (a triad of asthma, nasal polyps, and aspirin sensitivity), where 51% improved versus about 21% on placebo. Patients also reported meaningful reductions in nasal obstruction. This indication is approved only for adults 18 and older.
COPD With Eosinophilic Inflammation
Not all COPD is the same. A subset of people with COPD have elevated eosinophil levels that contribute to repeated flare-ups despite standard inhaler therapy. Nucala was approved for this specific subgroup as an add-on maintenance treatment. If your COPD is primarily driven by smoking-related damage without significant eosinophilic inflammation, Nucala is unlikely to be appropriate. Your doctor will check blood eosinophil levels to determine whether you fit the profile.
Eosinophilic Granulomatosis With Polyangiitis
EGPA (formerly called Churg-Strauss syndrome) is a rare autoimmune condition where eosinophils accumulate and inflame blood vessels throughout the body. It often starts with adult-onset asthma, progresses to sinus problems, and can eventually affect the heart, nerves, skin, and kidneys. Treatment has traditionally relied on high-dose oral steroids and immune-suppressing drugs, both of which carry significant long-term risks.
Nucala offers a way to reduce those steroid doses. In clinical data, 76% to 81% of EGPA patients on Nucala achieved a meaningful clinical benefit, defined as reaching remission at any point, cutting their steroid dose by at least half, or avoiding relapses entirely. In the placebo group, only 25% to 39% achieved the same. For a disease that often requires years of steroid use, the ability to taper down is a significant quality-of-life improvement.
Hypereosinophilic Syndrome
HES is a group of disorders defined by persistently high eosinophil counts (typically above 1,500 cells per microliter) for six months or longer, with resulting organ damage or dysfunction. Eosinophils can infiltrate the heart, lungs, skin, and nervous system, causing a wide range of symptoms depending on which organs are affected. Nucala is approved for HES in adults and adolescents aged 12 and older, provided there’s no identifiable blood cancer or other non-eosinophilic cause behind the elevated counts.
The dosing for HES is higher than for asthma. Patients receive 300 mg every four weeks, administered as three separate 100 mg injections, compared to the single 100 mg injection used for asthma.
How Nucala Is Given
Nucala is a subcutaneous injection, meaning it goes just under the skin rather than into a vein. For most indications, it’s given once every four weeks. It’s available as a prefilled syringe and an autoinjector pen, both of which are approved for self-administration at home after proper training. The injection sites are the upper arm, thigh, or abdomen. Many patients start their first dose at a clinic and then transition to giving themselves injections at home.
Because Nucala is a maintenance therapy, it needs to be taken on a regular schedule to keep eosinophil levels suppressed. Stopping the medication typically allows eosinophil counts to rise again over weeks to months.
Common Side Effects
Nucala is generally well tolerated. The most frequently reported side effects in clinical trials include headache, injection site reactions (redness, swelling, or mild pain at the injection spot), back pain, and fatigue. These tend to be mild and often decrease over time.
Allergic reactions are rare but possible, including with the first dose. Signs to watch for include swelling of the face, mouth, or tongue, dizziness, hives, or difficulty breathing. There’s also a small risk of reactivating a herpes zoster (shingles) infection, since eosinophils play a minor role in immune defense against certain infections. Serious side effects are uncommon, which is one reason Nucala has become a preferred biologic option for several eosinophil-driven conditions.