What the label lists and what reports show
Asfotase alfa side effects
Asfotase alfa's FDA label: “Most common adverse reactions (≥ 10%) are injection site reactions, lipodystrophy, ectopic calcifications and hypersensitivity reactions.” FAERS holds 9,959 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 injection site pain (1,327 reports).
- No generic listed
- Boxed warning
Check it at the source: the asfotase alfa FDA label on DailyMed (effective December 12, 2025, Alexion Pharmaceuticals, Inc.); 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 STRENSIQ?STRENSIQ may cause serious side effects, including: severe allergic (hypersensitivity) reactions. See " What is the most important information I should know about STRENSIQ? " skin thickening or pits at the injection site (lipodystrophy). Lipodystrophy is common and has happened after several months in people treated with STRENSIQ. calcium build-up in your eyes and kidneys. People with HPP are at increased risk for developing calcium build-up in the body. Calcium build-up in the eyes and kidneys has happened and is a common side effect of STRENSIQ. Calcium build-up in the eyes and kidneys may also happen in people with HPP who are not treated with STRENSIQ. Your healthcare provider should check your eyes and kidneys before and during treatment with STRENSIQ. immune-related effects. You may develop antibodies during treatment that may decrease how well STRENSIQ works. […]
Source: FDA drug label, Alexion Pharmaceuticals, Inc., effective December 12, 2025 (SPL set 3387574f-5eaa-4501-a71d-4cbfbd563031), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsMost common adverse reactions (≥ 10%) are injection site reactions, lipodystrophy, ectopic calcifications and hypersensitivity reactions.
Table 4: Adverse Reactions Reported in at Least 10% of Patients with Perinatal/Infantile- or Juvenile-onset HPP Enrolled in STRENSIQ Clinical Trials
| Adverse Reaction Category or Term | STRENSIQ less than or equal to 6 mg/kg per week (N=66) n (%) | STRENSIQ more than 6 mg/kg/week Adverse reactions are from the combined period of 6 mg/kg… | Total (N=79) n (%) | STRENSIQ (N=20) n (%) |
|---|---|---|---|---|
| Injection site reactions | 38 (58) | 6 (46) | 44 (56) | 18 (90) |
| Erythema | 29 (44) | 3 (23) | 32 (41) | 15 (75) |
| Discoloration/ Hypopigmentation | 11 (17) | 1 (8) | 12 (15) | 8 (40) |
| Pain/ Tenderness | 10 (15) | 1 (8) | 11 (14) | 8 (40) |
| Pruritus/ Itching | 10 (15) | 0 (0) | 10 (13) | 7 (35) |
| Swelling | 8 (12) | 0 (0) | 8 (10) | 6 (30) |
| Induration | 9 (14) | 1 (8) | 10 (13) | 3 (15) |
| Macule | 4 (6) | 0 (0) | 4 (5) | 7 (35) |
| Reaction, not otherwise specified | 6 (9) | 1 (8) | 7 (9) | 4 (20) |
| Bruising | 6 (9) | 0 (0) | 6 (8) | 4 (20) |
| Nodule | 2 (3) | 0 (0) | 2 (3) | 2 (10) |
| Other injection site reactions Other injection site reactions include injection site… | 10 (15) | 3 (23) | 13 (17) | 4 (20) |
| Ectopic calcifications | 3 (5) | 0 (0) | 3 (4) | 11 (55) |
| Lipodystrophy | 12 (18) | 2 (15) | 14 (18) | 14 (70) |
| Injection site atrophy | 4 (6) | 2 (15) | 6 (8) | 8 (40) |
| Injection site hypertrophy | 5 (8) | 0 (0) | 5 (6) | 6 (30) |
| Other lipodystrophy Other lipodystrophy includes lipohypertrophy. | 4 (6) | 0 (0) | 4 (5) | 1 (5) |
| Hypersensitivity reactions | 7 (11) | 3 (23) | 10 (13) | 2 (10) |
| Vomiting/emesis | 2 (3) | 2 (15) | 4 (5) | 2 (10) |
| Other hypersensitivity reactions Other hypersensitivity reactions include… | 6 (9) | 2 (15) | 8 (10) | 2 (10) |
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 adverse reactions are described below and elsewhere in the labeling: Hypersensitivity Reactions [see Warnings and Precautions (5.1) ] Lipodystrophy [see Warnings and Precautions (5.2) ] Ectopic Calcifications [see Warnings and Precautions (5.3) ] Possible Immune-Mediated Clinical Effects [see Warnings and Precautions (5.4) ] Most common adverse reactions (≥ 10%) are injection site reactions, lipodystrophy, ectopic calcifications and hypersensitivity reactions. ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Alexion Pharmaceuticals, Inc. at 1-844-259-6783 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 data described below reflect exposure to STRENSIQ in 99 patients with perinatal/infantile- or juvenile onset HPP (age 1 day to 58 years) treated with STRENSIQ, most for more than 2 years (range 1 day to 312 weeks [78 months]): 51 patients received at least 96 weeks (24 months) of treatment and 39 patients received 168 weeks (42 months) or more of treatment [see Clinical Studies (14) ]. …
Source: FDA drug label, Alexion Pharmaceuticals, Inc., effective December 12, 2025 (SPL set 3387574f-5eaa-4501-a71d-4cbfbd563031), via openFDA; the full label on DailyMed
What do FAERS reports show?
9,959 reports list asfotase alfa among the medicines taken, each a report of something that happened, not proof the medicine caused it; 72 are coded as a death and 752 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. Reports rose sharply in 2024 (2,432, against a median of 785 a year): FAERS counts follow reporting, which news, litigation and label changes can drive, not how often a reaction happens. Of the 24 reactions below, 6 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?
- Asfotase alfa: 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