What the label lists and what reports show
Zonisamide side effects
Zonisamide's FDA label: “The most common adverse reactions with zonisamide (an incidence at least 4% greater than placebo) in controlled clinical trials and shown in descending order of frequency were somnolence, anorexia, dizziness, ataxia, agitation/irritability, and difficulty …” FAERS holds 10,560 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 fatigue (496 reports).
- Generic listed
Check it at the source: the zonisamide FDA label on DailyMed (effective June 4, 2026, Sun Pharmaceutical Industries, 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 Zonisamide Capsules?Zonisamide capsules can cause serious side effects. See “What is the most important information I should know about Zonisamide Capsules?” Other serious side effects include: kidney stones: back pain, stomach pain, or blood in your urine may mean you have kidney stones. Drink plenty of fluids while you take zonisamide capsules to lower your chance of getting kidney stones. problems with mood or thinking (new or worse depression; sudden changes in mood, behavior, or loss of contact with reality, sometimes associated with hearing voices or seeing things that are not really there; feeling sleepy or tired; trouble concentrating; speech and language problems). Call your healthcare provider right away if you have any of the symptoms listed above. high blood ammonia levels. High ammonia in the blood can affect your mental activities, slow your alertness, make you feel tired, or cause vomiting. […]
Source: FDA drug label, Sun Pharmaceutical Industries, Inc., effective June 4, 2026 (SPL set 3164d438-90bf-420a-9a56-5498b987f91c), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsThe most common adverse reactions with zonisamide (an incidence at least 4% greater than placebo) in controlled clinical trials and shown in descending order of frequency were somnolence, anorexia, dizziness, ataxia, agitation/irritability, and difficulty with memory and/or concentration.
Table 4. Adverse Reactions in Placebo-Controlled, Add-On Trials (Events that occurred in at least 2% of Zonisamide-treated patients and occurred more frequently in Zonisamide -treated than placebo-treated patients)
| BODY SYSTEM/PREFERRED TERM | ZONISAMIDE (n=269) % | PLACEBO (n=230) % |
|---|---|---|
| Headache | 10 | 8 |
| Abdominal Pain | 6 | 3 |
| Flu Syndrome | 4 | 3 |
| Anorexia | 13 | 6 |
| Nausea | 9 | 6 |
| Diarrhea | 5 | 2 |
| Dyspepsia | 3 | 1 |
| Constipation | 2 | 1 |
| Dry Mouth | 2 | 1 |
| Ecchymosis | 2 | 1 |
| Weight Loss | 3 | 2 |
| Dizziness | 13 | 7 |
| Ataxia | 6 | 1 |
| Nystagmus | 4 | 2 |
| Paresthesia | 4 | 1 |
| Confusion | 6 | 3 |
| Difficulty Concentrating | 6 | 2 |
| Difficulty with Memory | 6 | 2 |
| Mental Slowing | 4 | 2 |
| Agitation/Irritability | 9 | 4 |
| Depression | 6 | 3 |
| Insomnia | 6 | 3 |
| Anxiety | 3 | 2 |
| Nervousness | 2 | 1 |
| Schizophrenic/Schizophreniform Behavior | 2 | 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 most common adverse reactions with zonisamide (an incidence at least 4% greater than placebo) in controlled clinical trials and shown in descending order of frequency were somnolence, anorexia, dizziness, ataxia, agitation/irritability, and difficulty with memory and/or concentration. In controlled clinical trials, 12% of patients receiving zonisamide as adjunctive therapy discontinued due to an adverse reaction compared to 6% receiving placebo. Approximately 21% of the 1,336 patients with epilepsy who received zonisamide in clinical studies discontinued treatment because of an adverse reaction. The most common adverse reactions leading to discontinuation were somnolence, fatigue and/or ataxia (6%), anorexia (3%), difficulty concentrating (2%), difficulty with memory, mental slowing, nausea/vomiting (2%), and weight loss (1%). Many of these adverse reactions were dose-related (see WARNINGS and PRECAUTIONS ). Adverse Reaction Incidence in Controlled Clinical Trials: Table 4 lists adverse reactions that occurred in at least 2% of patients treated with zonisamide in controlled clinical trials that were numerically more common in the zonisamide group. In these studies, either zonisamide or placebo was added to the patient's current AED therapy. Table 4.
Source: FDA drug label, Sun Pharmaceutical Industries, Inc., effective June 4, 2026 (SPL set 3164d438-90bf-420a-9a56-5498b987f91c), via openFDA; the full label on DailyMed
What do FAERS reports show?
10,560 reports list zonisamide among the medicines taken, each a report of something that happened, not proof the medicine caused it; 677 are coded as a death and 3,245 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, 10 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?
- Zonisamide: 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