What the label lists and what reports show
Imetelstat side effects
Imetelstat's FDA label: “Most common adverse reactions (incidence ≥10% with a difference between arms of >5% compared to placebo), including laboratory abnormalities are decreased platelets, decreased white blood cells, decreased neutrophils, increased AST, increased alkaline …” FAERS holds 176 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 platelet count decreased (27 reports).
- No generic listed
Check it at the source: the imetelstat FDA label on DailyMed (effective April 22, 2026, Geron 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 RYTELO?RYTELO may cause serious side effects, including: See " What is the most important information I should know about RYTELO? " Infusion-related reactions. RYTELO can cause infusion-related reactions during or after your infusion that can be severe, including a severe sudden increase in blood pressure called hypertensive crisis. Your healthcare provider will give you medicines before each RYTELO infusion to help prevent or lessen infusion-related reactions and will watch you for at least 1 hour after your infusion. If you develop infusion-related reactions, your healthcare provider may infuse RYTELO more slowly, temporarily stop, or permanently stop your treatment. […]
Source: FDA drug label, Geron Corporation, effective April 22, 2026 (SPL set b0fab7ca-e578-43c5-9df6-bdaff4182257), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsMost common adverse reactions (incidence ≥10% with a difference between arms of >5% compared to placebo), including laboratory abnormalities are decreased platelets, decreased white blood cells, decreased neutrophils, increased AST, increased alkaline phosphatase, increased ALT, fatigue, prolonged partial thromboplastin time, arthralgia/myalgia, COVID-19 infections, and headache.
Table 5: Adverse Reactions (≥5%) in Patients with MDS Who Received RYTELO with a Difference Between Arms of >2% Compared to Placebo in IMerge
| Reaction | All Grades % | Grades 3 or 4 % | All Grades % | Grades 3 or 4 % |
|---|---|---|---|---|
| Fatigue Fatigue: asthenia, fatigue, and malaise. | 29 | 0 | 20 | 3.4 |
| Arthralgia/myalgia Arthralgia/myalgia: arthralgia, back pain, bone pain, musculoskeletal… | 25 | 2.5 | 19 | 5 |
| COVID-19 COVID-19: asymptomatic COVID-19, COVID-19, COVID-19 pneumonia, and SARS-CoV-2… | 19 | 1.7 | 14 | 5 |
| Urinary tract infection Urinary tract infection: cystitis, Escherichia urinary tract… | 9 | 2.5 | 7 | 0 |
| Headache | 13 | 0.8 | 5 | 0 |
| Syncope Syncope: fall, pre-syncope, and syncope. | 7 | 1.7 | 1.7 | 0 |
| Infusion-related reactions Infusion-related reactions: abdominal pain, arthralgia,… | 8 | 1.7 | 3.4 | 0 |
| Epistaxis | 7 | 0 | 0 | 0 |
| Hematoma | 6 | 0 | 0 | 0 |
| Pruritus | 6 | 0 | 1.7 | 0 |
| Atrial arrhythmia Atrial arrhythmia: atrial fibrillation and atrial flutter. | 6 | 1.7 | 3.4 | 1.7 |
| Fractures Fractures: femur fracture, hand fracture, hip fracture, humerus fracture,… | 5 | 3.4 | 1.7 | 1.7 |
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, continuedADVERSE REACTIONS The following clinically significant adverse reactions are described elsewhere in the labeling: Thrombocytopenia [see Warnings and Precautions (5.1) ] Neutropenia [see Warnings and Precautions (5.2) ] Infusion-Related Reactions [see Warnings and Precautions (5.3) ] Most common adverse reactions (incidence ≥10% with a difference between arms of >5% compared to placebo), including laboratory abnormalities are decreased platelets, decreased white blood cells, decreased neutrophils, increased AST, increased alkaline phosphatase, increased ALT, fatigue, prolonged partial thromboplastin time, arthralgia/myalgia, COVID-19 infections, and headache. ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Geron Corporation at 1-855-437-6664 (1-855-GERONMI) 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. …
Source: FDA drug label, Geron Corporation, effective April 22, 2026 (SPL set b0fab7ca-e578-43c5-9df6-bdaff4182257), via openFDA; the full label on DailyMed
What do FAERS reports show?
176 reports list imetelstat among the medicines taken, each a report of something that happened, not proof the medicine caused it; 11 are coded as a death and 35 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, 11 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.
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?
- Imetelstat: 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