What the label lists and what reports show
Alemtuzumab side effects
Alemtuzumab's FDA label: “Most common adverse reactions (incidence ≥10% and > interferon beta-1a): rash, headache, pyrexia, nasopharyngitis, nausea, urinary tract infection, fatigue, insomnia, upper respiratory tract infection, herpes viral infection, urticaria, pruritus, thyroid …” FAERS holds 19,229 reports naming it through July 30, 2026; the reaction reported most often, leaving out the conditions it is used for, product-use terms and outcomes, is headache (1,897 reports).
- No generic listed
- Boxed warning
Check it at the source: the alemtuzumab FDA label on DailyMed (effective November 3, 2025, Genzyme Corporation); the FDA FAERS public dashboard.
What does the patient leaflet say about side effects?
From the patient leaflet: What are the possible side effects of LEMTRADA?LEMTRADA may cause serious side effects including: see " What is the most important information I should know about LEMTRADA? " thyroid problems. Some people who receive LEMTRADA may get thyroid problems including an overactive thyroid (hyperthyroidism) or an underactive thyroid (hypothyroidism). Your healthcare provider will do blood tests to check how your thyroid is working. Call your healthcare provider if you have any of the symptoms of thyroid problems. Symptoms of hyperthyroidism may include: – excessive sweating – unexplained weight loss – fast heartbeat – eye swelling – nervousness Symptoms of hypothyroidism may include: – unexplained weight gain – feeling cold – worsening tiredness – constipation low blood counts (cytopenias). LEMTRADA may cause a decrease in some types of blood cells. Some people with these low blood counts have increased infections. […]
Source: FDA drug label, Genzyme Corporation, effective November 3, 2025 (SPL set 6236b0bc-82e9-4447-9a78-f57d94770269), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsMost common adverse reactions (incidence ≥10% and > interferon beta-1a): rash, headache, pyrexia, nasopharyngitis, nausea, urinary tract infection, fatigue, insomnia, upper respiratory tract infection, herpes viral infection, urticaria, pruritus, thyroid gland disorders, fungal infection, arthralgia, pain in extremity, back pain, diarrhea, sinusitis, oropharyngeal pain, paresthesia, dizziness, abdominal pain, flushing, and vomiting.
Table 1: Adverse Reactions in the Pooled 2-Year Active-Controlled Studies in Patients with Relapsing-Remitting Multiple Sclerosis
| Reaction | LEMTRADA (N=811) % | interferon beta-1a 44 mcg (N=389) % |
|---|---|---|
| Rash | 53 | 6 |
| Headache | 52 | 23 |
| Pyrexia | 29 | 9 |
| Nasopharyngitis | 25 | 19 |
| Nausea | 21 | 9 |
| Urinary tract infection | 19 | 8 |
| Fatigue | 18 | 13 |
| Insomnia | 16 | 15 |
| Upper respiratory tract infection | 16 | 13 |
| Herpes viral infection | 16 | 3 |
| Urticaria | 16 | 2 |
| Pruritus | 14 | 2 |
| Thyroid gland disorders | 13 | 3 |
| Fungal infection | 13 | 4 |
| Arthralgia | 12 | 9 |
| Pain in extremity | 12 | 9 |
| Back pain | 12 | 8 |
| Diarrhea | 12 | 6 |
| Sinusitis | 11 | 8 |
| Oropharyngeal pain | 11 | 5 |
| Paresthesia | 10 | 8 |
| Dizziness | 10 | 5 |
| Abdominal pain | 10 | 5 |
| Flushing | 10 | 4 |
| Vomiting | 10 | 3 |
Rows and numbers as the label prints them, under its own column headings (the first rows of a longer table where it runs on). A reaction in a trial table is not necessarily caused by the medicine, and trials differ from everyday use.
From the label: adverse reactions, continuedThe following serious adverse reactions are described below and elsewhere in the labeling: Autoimmunity [see Boxed Warning and Warnings and Precautions (5.1) ] Infusion Reactions [see Boxed Warning and Warnings and Precautions (5.2) ] Stroke and Cervicocephalic Arterial Dissection [see Warnings and Precautions (5.3) ] Malignancies [see Warnings and Precautions (5.4) ] Immune Thrombocytopenia [see Warnings and Precautions (5.6) ] Glomerular Nephropathies Including Anti-glomerular Basement Membrane Disease [see Warnings and Precautions (5.7) ] Thyroid Disorders [see Warnings and Precautions (5.8) ] Other Autoimmune Cytopenias [see Warnings and Precautions (5.9) ] Autoimmune Hepatitis [see Warnings and Precautions (5.10) ] Hemophagocytic Lymphohistiocytosis [see Warnings and Precautions (5.11) ] Adult Onset Still's Disease [see Warnings and Precautions (5.12) ] Thrombotic Thrombocytopenic Purpura (TTP) [see Warnings and Precautions (5.13) ] Autoimmune Encephalitis (AIE) [see Warnings and Precautions (5.14) ] Acquired Hemophilia A [see Warnings and Precautions (5.15) ] Immune-Mediated Colitis [see Warnings and Precautions (5.16) ] Infections [see Warnings and Precautions (5.17) ] Progressive Multifocal Leukoencephalopathy (PML) [see Warnings and Precautions (5.18) ] Acute Acalculous Cholecystitis [see Warnings and Precautions (5.19) ] Pneumonitis [see Warnings and Precautions …
Source: FDA drug label, Genzyme Corporation, effective November 3, 2025 (SPL set 6236b0bc-82e9-4447-9a78-f57d94770269), via openFDA; the full label on DailyMed
What do FAERS reports show?
19,229 reports list alemtuzumab among the medicines taken, each a report of something that happened, not proof the medicine caused it; 2,921 are coded as a death and 6,100 as a hospital stay. Counts follow how widely a medicine is used and watched, and they are not a measure of how risky it is. Of the 22 reactions below, 13 are named by the same term (in US spelling) in the label's adverse reactions, warnings or patient leaflet; a reaction the label does not name may still be there under another word, and a report does not show that the medicine caused it.
Reports received each year
The 25 most reported reactions
Source: FDA Adverse Event Reporting System (FAERS) via openFDA, reports through July 30, 2026; reactions as MedDRA preferred terms
What next?
- Alemtuzumab: the full record
- label, generics, price, shortages, recalls
- Report a side effect to the FDA
- MedWatch
- Side effects hub
- the most reported medicines and reactions