What the label lists and what reports show
Istradefylline side effects
Istradefylline's FDA label: “The most common adverse reactions (at least 5% and more frequent than placebo) were dyskinesia, dizziness, constipation, nausea, hallucination, and insomnia ( 6.1 ).” FAERS holds 5,920 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 hallucination (608 reports).
- No generic listed
Check it at the source: the istradefylline FDA label on DailyMed (effective May 20, 2025, Kyowa Kirin, 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 NOURIANZ?NOURIANZ may cause serious side effects, including: uncontrolled sudden movements (dyskinesia). Uncontrolled sudden movements is one of the most common side effects. NOURIANZ may cause uncontrolled sudden movements or make such movements you already have worse or more frequent. Tell your healthcare provider if this happens. hallucinations and other symptoms of psychosis. […]
Source: FDA drug label, Kyowa Kirin, Inc., effective May 20, 2025 (SPL set a7d008cb-b273-4049-a5d2-9c6902910d58), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsThe most common adverse reactions (at least 5% and more frequent than placebo) were dyskinesia, dizziness, constipation, nausea, hallucination, and insomnia ( 6.1 ).
Table 1: Adverse Reactions with an Incidence of at Least 2% in Patients Treated with NOURIANZ, and Greater than on Placebo, in Pooled Studies 1, 2, 3, and 4
| Adverse Reactions | NOURIANZ 20 mg/day (N=356) % | NOURIANZ 40 mg/day (N=378) % | Placebo N=426 (%) |
|---|---|---|---|
| Dyskinesia | 15 | 17 | 8 |
| Dizziness | 3 | 6 | 4 |
| Constipation | 5 | 6 | 3 |
| Nausea | 4 | 6 | 5 |
| Diarrhea | 1 | 2 | 1 |
| Hallucination Includes hallucinations, hallucinations visual, hallucinations olfactory,… | 2 | 6 | 3 |
| Insomnia | 1 | 6 | 4 |
| Decreased appetite | 1 | 3 | 1 |
| Blood alkaline phosphatase increased | 1 | 2 | 1 |
| Blood glucose increased | 1 | 2 | 0 |
| Blood urea increased | 1 | 2 | 0 |
| Upper Respiratory Tract Inflammation | 1 | 2 | 0 |
| Rash | 1 | 2 | 1 |
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 in greater detail in other sections of the labeling: Dyskinesia [see Warnings and Precautions (5.1) ] Hallucinations / Psychotic Behavior [see Warnings and Precautions (5.2) ] Impulse Control / Compulsive Behaviors [see Warnings and Precautions (5.3) ] The most common adverse reactions (at least 5% and more frequent than placebo) were dyskinesia, dizziness, constipation, nausea, hallucination, and insomnia ( 6.1 ). To report SUSPECTED ADVERSE REACTIONS, contact Kyowa Kirin Inc. at 1-844-768-3544 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 safety of NOURIANZ was evaluated in 734 patients with Parkinson's disease (PD) taking a stable dose of levodopa and a DOPA decarboxylase inhibitor, with or without other PD medications, in four randomized, multicenter, double-blind, placebo-controlled trials 12 weeks in duration (Studies 1, 2, 3 and 4) [see Clinical Studies (14) ]. Of the patient population exposed to NOURIANZ, 50% were male, 32% White, 67% Asian, and the mean age was 65 years (range: 33 to 84 years). …
Source: FDA drug label, Kyowa Kirin, Inc., effective May 20, 2025 (SPL set a7d008cb-b273-4049-a5d2-9c6902910d58), via openFDA; the full label on DailyMed
What do FAERS reports show?
5,920 reports list istradefylline among the medicines taken, each a report of something that happened, not proof the medicine caused it; 928 are coded as a death and 763 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 19 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?
- Istradefylline: 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