What the label lists and what reports show
Idursulfase side effects
Idursulfase's FDA label: “The most common adverse reactions occurring in at least three patients (≥9%) aged five years and older were headache, pruritus, musculoskeletal pain, urticaria, diarrhea, and cough.” FAERS holds 3,979 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 pyrexia (617 reports).
- No generic listed
- Boxed warning
Check it at the source: the idursulfase FDA label on DailyMed (effective April 15, 2025, Takeda Pharmaceuticals America, Inc.); the FDA FAERS public dashboard.
What does the label list?
From the label: adverse reactionsThe most common adverse reactions occurring in at least three patients (≥9%) aged five years and older were headache, pruritus, musculoskeletal pain, urticaria, diarrhea, and cough.
Table 1. Adverse Reactions that Occurred in the Placebo-Controlled Trial in At Least 9% of Patients in the ELAPRASE 0.5 mg/kg Once Weekly Group and with a Higher Incidence than in the Placebo Group (5 Years and Older)
| System Organ Class Adverse Reaction | ELAPRASE (0.5 mg/kg weekly) N=32 n (%) | Placebo N=32 n (%) |
|---|---|---|
| Diarrhea | 3 (9%) | 1 (3%) |
| Musculoskeletal Pain | 4 (13%) | 1 (3%) |
| Headache | 9 (28%) | 8 (25%) |
| Cough | 3 (9%) | 1 (3%) |
| Pruritus | 8 (25%) | 3 (9%) |
| Urticaria | 5 (16%) | 0 (0%) |
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 most common adverse reactions occurring in at least three patients (≥9%) aged five years and older were headache, pruritus, musculoskeletal pain, urticaria, diarrhea, and cough. The most common adverse reactions occurring in at least three patients (≥10%) aged seven years and younger were pyrexia, rash, vomiting, and urticaria. In all clinical trials, the most common adverse reactions requiring medical intervention were hypersensitivity reactions, and included rash, urticaria, pruritus, flushing, pyrexia, and headache ( 6.1 ). To report SUSPECTED ADVERSE REACTIONS, contact Takeda Pharmaceuticals U.S.A., Inc. at 1-877-TAKEDA-7 (1-877-825-3327) or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch. 6.1 Clinical Trials Experience Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in the clinical trials of another drug and may not reflect the rates observed in practice. The following serious adverse reactions are described below and elsewhere in the labeling: Hypersensitivity Reactions Including Anaphylaxis [see Warnings and Precautions (5.1) ] In clinical trials, the most common adverse reactions (>10%) following ELAPRASE treatment were hypersensitivity reactions, and included rash, urticaria, pruritus, flushing, pyrexia, and headache.
Source: FDA drug label, BLA125151 (Shire), as published by Takeda Pharmaceuticals America, Inc., effective April 15, 2025 (SPL set 60cba843-5aab-4dd7-96dc-66648d413be3), via openFDA; the full label on DailyMed
What do FAERS reports show?
3,979 reports list idursulfase among the medicines taken, each a report of something that happened, not proof the medicine caused it; 684 are coded as a death and 2,174 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 21 reactions below, 8 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?
- Idursulfase: 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