What the label lists and what reports show
Temozolomide side effects
Temozolomide's FDA label: “The most common adverse reactions (≥20%) are: alopecia, fatigue, nausea, vomiting, headache, constipation, anorexia, and convulsions.” FAERS holds 20,949 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 thrombocytopenia (1,571 reports).
- Generic listed
Check it at the source: the temozolomide FDA label on DailyMed (effective February 20, 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 TEMODAR?TEMODAR can cause serious side effects, including: Decreased blood cell counts. TEMODAR can affect your bone marrow and cause you to have decreased blood cell counts. Decreased white blood cell count, red blood cell count and platelet count are common with TEMODAR but it can also be severe and lead to death. Some people need to be hospitalized or need to receive transfusions to treat their decreased blood cell counts. Your healthcare provider will do blood tests regularly to check your blood cell counts before you start and during treatment with TEMODAR. Your healthcare provider may need to change the dose of TEMODAR or when you get it depending on your blood cell counts. People who are age 70 or older and women have a higher risk for developing decreased blood cell counts during treatment with TEMODAR. Liver problems. […]
Source: FDA drug label, Merck Sharp & Dohme LLC, effective February 20, 2026 (SPL set 046a9011-3911-4d3f-a15f-fbb56d5aad56), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsThe most common adverse reactions (≥20%) are: alopecia, fatigue, nausea, vomiting, headache, constipation, anorexia, and convulsions.
TABLE 4: Adverse Reactions (≥10%) in Patients with Refractory Anaplastic Astrocytoma
| Reaction | Adverse Reactions All Reactions (%) | TEMODAR N=158 Grades 3-4 (%) |
|---|---|---|
| Nausea | 53 | 10 |
| Vomiting | 42 | 6 |
| Constipation | 33 | 1 |
| Diarrhea | 16 | 2 |
| Headache | 41 | 6 |
| Fatigue | 34 | 4 |
| Asthenia | 13 | 6 |
| Fever | 13 | 2 |
| Convulsions | 23 | 5 |
| Hemiparesis | 18 | 6 |
| Dizziness | 12 | 1 |
| Coordination abnormal | 11 | 1 |
| Amnesia | 10 | 4 |
| Insomnia | 10 | 0 |
| Edema peripheral | 11 | 1 |
| Infection viral | 11 | 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 following clinically significant adverse reactions are described elsewhere in the labeling: Myelosuppression [see Warnings and Precautions (5.1) ] Hepatotoxicity [see Warnings and Precautions (5.2) ] Pneumocystis Pneumonia [see Warnings and Precautions (5.3) ] Secondary Malignancies [see Warnings and Precautions (5.4) ] The most common adverse reactions (≥20%) are: alopecia, fatigue, nausea, vomiting, headache, constipation, anorexia, and convulsions. ( 6.1 ) The most common Grade 3 to 4 hematologic laboratory abnormalities (≥10%) in patients with anaplastic astrocytoma are: decreased lymphocytes, decreased platelets, decreased neutrophils, and decreased leukocytes. ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Merck Sharp & Dohme LLC at 1-877-888-4231 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. Newly Diagnosed Glioblastoma The safety of TEMODAR was evaluated in study MK-7365-051 [see Clinical Studies (14.1) ]. Severe or life-threatening adverse reactions occurred in 49% of patients treated with TEMODAR; …
Source: FDA drug label, Merck Sharp & Dohme LLC, effective February 20, 2026 (SPL set 046a9011-3911-4d3f-a15f-fbb56d5aad56), via openFDA; the full label on DailyMed
What do FAERS reports show?
20,949 reports list temozolomide among the medicines taken, each a report of something that happened, not proof the medicine caused it; 4,436 are coded as a death and 6,699 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 20 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?
- Temozolomide: 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