What the label lists and what reports show
Amifampridine side effects
Amifampridine's FDA label: “The most common (> 10%) adverse reactions are: paresthesia, upper respiratory tract infection, abdominal pain, nausea, diarrhea, headache, elevated liver enzymes, back pain, hypertension, and muscle spasms.” FAERS holds 2,868 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 asthenia (356 reports).
- No generic listed
Check it at the source: the amifampridine FDA label on DailyMed (effective April 16, 2026, Catalyst Pharmaceuticals, 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 FIRDAPSE?FIRDAPSE may cause serious side effects, including: Seizures. See “ What is the most important information I should know about FIRDAPSE? ” Serious allergic reactions, such as anaphylaxis. FIRDAPSE can cause serious allergic reactions. Stop taking FIRDAPSE and call your doctor right away or get emergency medical help if you have: shortness of breath or trouble breathing swelling of your throat or tongue hives The most common side effects of FIRDAPSE include: tingling around the mouth, tongue, face, fingers, toes, and other body parts upper respiratory infection stomach pain nausea diarrhea headache increased liver enzymes back pain high blood pressure muscle spasms Tell your doctor if you have any side effect that bothers you or that does not go away. These are not all the possible side effects of FIRDAPSE. Call your doctor for medical advice about side effects. […]
Source: FDA drug label, Catalyst Pharmaceuticals, Inc., effective April 16, 2026 (SPL set f015fe60-3128-4a43-8c31-19fc6b5def3f), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsThe most common (> 10%) adverse reactions are: paresthesia, upper respiratory tract infection, abdominal pain, nausea, diarrhea, headache, elevated liver enzymes, back pain, hypertension, and muscle spasms.
Table 2. Adverse Reactions in ≥5% of LEMS Patients Newly Treated with FIRDAPSE in Study 1
| Adverse Reaction | *Includes paresthesia, oral paresthesia, oral hypoesthesia **Includes elevated alanine aminotransferase (ALT), aspartate aminotransferase (AST),… FIRDAPSE N=42 % |
|---|---|
| Paresthesia | 62 |
| Upper respiratory tract infection | 33 |
| Abdominal pain | 14 |
| Nausea | 14 |
| Diarrhea | 14 |
| Headache | 14 |
| Elevated liver enzymes | 14 |
| Back pain | 14 |
| Hypertension | 12 |
| Muscle spasms | 12 |
| Dizziness | 10 |
| Asthenia | 10 |
| Muscular weakness | 10 |
| Pain in extremity | 10 |
| Cataract | 10 |
| Constipation | 7 |
| Bronchitis | 7 |
| Fall | 7 |
| Lymphadenopathy | 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, continuedThe following serious adverse reactions are described elsewhere in the labeling: Seizures [see Warnings and Precautions ( 5.1 )] Hypersensitivity [see Warnings and Precautions ( 5.2 )] The most common (> 10%) adverse reactions are: paresthesia, upper respiratory tract infection, abdominal pain, nausea, diarrhea, headache, elevated liver enzymes, back pain, hypertension, and muscle spasms. ( 6 ) To report SUSPECTED ADVERSE REACTIONS, contact Catalyst Pharmaceuticals at 1-844-347-3277 (1-844-FIRDAPSE) or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch. 6.1 Clinical Trials Experience Adults 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. In controlled and uncontrolled clinical trials (Study 1 and 2) in patients with LEMS [see Clinical Studies ( 14 )], 63 patients were treated with FIRDAPSE, including 40 patients treated for more than 6 months, and 39 patients treated for more than 12 months. In an expanded access program, 139 patients with LEMS were treated with FIRDAPSE, including 102 patients treated for more than 6 months, 77 patients treated for more than 12 months, and 53 patients treated for more than 18 months. …
Source: FDA drug label, Catalyst Pharmaceuticals, Inc., effective April 16, 2026 (SPL set f015fe60-3128-4a43-8c31-19fc6b5def3f), via openFDA; the full label on DailyMed
What do FAERS reports show?
2,868 reports list amifampridine among the medicines taken, each a report of something that happened, not proof the medicine caused it; 207 are coded as a death and 759 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, 12 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?
- Amifampridine: 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