What the label lists and what reports show
Belzutifan side effects
Belzutifan's FDA label: “Most common (≥25%) adverse reactions, including laboratory abnormalities, were decreased hemoglobin, fatigue, increased creatinine, headache, dizziness, increased glucose, and nausea.” FAERS holds 1,573 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 anaemia (283 reports).
- No generic listed
- Boxed warning
Check it at the source: the belzutifan FDA label on DailyMed (effective June 12, 2026, Merck Sharp & Dohme LLC); 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 WELIREG?WELIREG can cause serious side effects, including: See “What is the most important information I should know about WELIREG?” The most common side effects of WELIREG in adults with VHL disease include: low red blood cell counts tiredness increased blood creatinine (kidney function test) headache dizziness increased blood sugar (glucose) levels nausea The most common side effects of WELIREG when taken with pembrolizumab in adults with ccRCC include: low red blood cell counts increased blood liver enzymes (liver function tests) tiredness decreased white blood cell counts The most common side effects of WELIREG when taken alone in adults with advanced ccRCC include: low red blood cell counts tiredness muscle and joint pain increased blood creatinine (kidney function test) decreased white blood cell counts increased blood liver enzymes (liver function tests) decreased blood salts (sodium) […]
Source: FDA drug label, Merck Sharp & Dohme LLC, effective June 12, 2026 (SPL set 13e15ee0-d679-4fa9-9430-e2e2170474da), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsMost common (≥25%) adverse reactions, including laboratory abnormalities, were decreased hemoglobin, fatigue, increased creatinine, headache, dizziness, increased glucose, and nausea.
Table 9: Adverse Reactions Occurring in ≥10% of Patients with PPGL Who Received WELIREG in LITESPARK-015
| Reaction | Adverse Reaction All Grades Graded per NCI CTCAE v5.0 (%) | WELIREG (n=72) Grade 3-4 (%) |
|---|---|---|
| Anemia | 96 | 22 |
| Fatigue Includes other related terms | 56 | 10 |
| Edema | 24 | 0 |
| Musculoskeletal pain | 56 | 6 |
| Muscle spasms | 13 | 0 |
| Muscle weakness | 13 | 2.8 |
| Dyspnea | 33 | 1.4 |
| Cough | 15 | 0 |
| Hypoxia | 13 | 10 |
| Nasal congestion | 10 | 0 |
| Headache | 29 | 1.4 |
| Dizziness | 26 | 2.8 |
| Peripheral neuropathy | 13 | 0 |
| Nausea | 25 | 1.4 |
| Constipation | 24 | 1.4 |
| Diarrhea | 15 | 0 |
| Abdominal Pain | 13 | 1.4 |
| Vomiting | 10 | 1.4 |
| Hypertension | 21 | 13 |
| Hypotension | 10 | 2.8 |
| Hemorrhage | 10 | 2.8 |
| COVID-19 | 17 | 2.8 |
| Decreased appetite | 14 | 2.8 |
| Weight increased | 13 | 7 |
| Arrhythmia | 11 | 2.8 |
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 clinically significant adverse reactions are discussed elsewhere in the labeling: Anemia [see Warnings and Precautions (5.1) ] Hypoxia [see Warnings and Precautions (5.2) ] VHL disease: Most common (≥25%) adverse reactions, including laboratory abnormalities, were decreased hemoglobin, fatigue, increased creatinine, headache, dizziness, increased glucose, and nausea. ( 6.1 ) ccRCC : Most common (≥25%) adverse reactions for WELIREG, including laboratory abnormalities, in combination with pembrolizumab were decreased hemoglobin, increased alanine aminotransferase (ALT), fatigue, increased aspartate aminotransferase (AST), decreased lymphocytes, and increased alkaline phosphatase. ( 6.1 ) Most common (≥25%) adverse reactions for WELIREG as monotherapy, including laboratory abnormalities were decreased hemoglobin, fatigue, musculoskeletal pain, increased creatinine, decreased lymphocytes, increased ALT, decreased sodium, increased potassium, and increased AST. ( 6.1 ) PPGL : Most common (≥25%) adverse reactions, including laboratory abnormalities were anemia, fatigue, musculoskeletal pain, decreased lymphocytes, increased ALT, increased AST, increased calcium, dyspnea, increased potassium, decreased leukocytes, headache, increased alkaline phosphatase, dizziness, and nausea.
Source: FDA drug label, Merck Sharp & Dohme LLC, effective June 12, 2026 (SPL set 13e15ee0-d679-4fa9-9430-e2e2170474da), via openFDA; the full label on DailyMed
What do FAERS reports show?
1,573 reports list belzutifan among the medicines taken, each a report of something that happened, not proof the medicine caused it; 150 are coded as a death and 678 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 16 reactions below, 9 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?
- Belzutifan: 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