Firdapse (amifampridine)
An approved treatment for Lambert-Eaton Myasthenic Syndrome.
Drug facts
The same compound appears under different names depending on the context. Here is how to identify Amifampridine wherever you encounter it, plus the key facts at a glance.
- Generic name
- Amifampridine
- Brand name
- Firdapse
- Development code
- 3,4-DAP
- Drug class
- Potassium channel blocker (aminopyridine)
- Manufacturer
- Catalyst Pharmaceuticals
- How it's taken
- Taken by mouth as 10mg scored tablets, split into 3-5 doses a day, with or without food.
A tablet taken by mouth several times a day and the only FDA-approved treatment for Lambert-Eaton myasthenic syndrome (LEMS). It was approved for adults on November 28, 2018 and for children 6 and older on September 29, 2022.
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Where Amifampridine fits
Firdapse is the only FDA-approved drug for LEMS. Because LEMS is often linked to small cell lung cancer, cancer screening and treatment are part of care, and some people also need medicines that calm the immune system.
How Amifampridine works
In LEMS, the immune system attacks calcium channels on nerve endings, so the nerves release too little acetylcholine, the chemical that tells muscles to contract. The result is weakness, especially in the hips and thighs. Amifampridine is thought to work by blocking potassium channels on those nerve endings. That keeps each nerve signal going slightly longer, more calcium gets in, and more acetylcholine is released. It eases symptoms but does not treat the immune attack or any underlying cancer.
Side effects and safety
- Seizures. Seizures happened in about 2% of people, including people who had never had a seizure. Firdapse is not used by anyone with a history of seizures.
- Allergic reactions. None were seen in the trials, but anaphylaxis has been reported with a related drug. Stop and get help for swelling, trouble breathing or a severe rash.
- Lower starting doses for kidney or liver impairment, and for people whose genes make them clear the drug slowly
- Weight in children on 60mg a day or more
Firdapse has no boxed warning, but it must not be used by people with a history of seizures or an allergy to amifampridine or a related drug. Seizures happened in about 2% of people taking it, including people who had never had a seizure. The most common side effect was tingling around the mouth and in the hands and feet. Others included colds, stomach pain, nausea, diarrhea, headache and raised liver enzymes.
This is not a complete list of side effects. Talk to your doctor or pharmacist about what to expect and when to seek medical attention.
Taking Amifampridine
Taken by mouth as 10mg scored tablets, split into 3-5 doses a day, with or without food. Adults and children weighing 45kg or more start at 15-30mg a day and raise it by 5mg every 3-4 days, up to 20mg per dose and 100mg a day. Children under 45kg start at 5-15mg a day, up to 10mg per dose and 50mg a day. For small dose steps, trouble swallowing or a feeding tube, the tablets can be mixed with water at home to make a liquid. It must be refrigerated and thrown away after 24 hours.
Availability and cost
Only available as the brand-name product.
Help paying for Firdapse
Pick your insurance to see which help fits. Drugmaker copay cards can't be used with Medicare, Medicaid or TRICARE; charity funds are the usual route there.
- Copay help
Catalyst says patients with commercial insurance pay $0 a month for Firdapse.
For: private insurance · source - Free medicine program
Patient Assistance Program provides free Firdapse to patients who qualify.
For: no insurance, underinsured · source - Bridge or quick-start supply
Catalyst Bridge supplies free Firdapse to qualifying patients while insurance coverage is being reviewed.
The official page does not say who qualifies. Ask the program. · source
Good to know: Call 1-833-4-CATALYST (1-833-422-8259). The page does not state Medicare or Medicaid rules for the copay program; ask Catalyst Pathways.
- From a charity · NORD RareCareLEMS Premium Copay Assistance fundOpen
Pays for: Insurance premiums and copays.
The foundation says: “Accepting new applications and re-enrollments for current year” - From a charity · NORD RareCareLEMS Medical Assistance fundOpen
Pays for: Medical and medication costs.
The foundation says: “Accepting new applications and re-enrollments for current year” - From a charity · The Assistance FundLambert-Eaton Myasthenic Syndrome (LEMS) fundOpen
Pays for: Copays, coinsurance, deductibles and other health-related expenses.
Every program and charity fund for Lambert-Eaton Myasthenic Syndrome →
Clinical trial results
Approval rested on 2 short trials in adults with LEMS who were on a steady dose of amifampridine. Each was randomly switched to stay on it or to placebo. In LMS-002 (NCT01377922), 38 people were tested after 14 days, and muscle strength scores worsened more on placebo. Those people also rated their own condition worse, and the gap had about a 1 in 300 chance of being a fluke (p=0.003).
In LMS-003 (NCT02970162), 26 people were tested after 4 days. Strength scores stayed steady on Firdapse and worsened by 6.5 points on placebo, with less than a 1 in 1,000 chance of being a fluke.
Development history
Before approval, amifampridine was available for years through an expanded access program. Catalyst Pharmaceuticals won FDA approval for adults on November 28, 2018. A competing version, Ruzurgi, was approved for children in 2019, but after a court ruling for Catalyst the FDA converted it to a tentative approval on February 1, 2022. On September 29, 2022 the FDA approved Firdapse for children 6 and older, and on May 30, 2024 it raised the maximum daily dose to 100mg for adults.
Explore Lambert-Eaton Myasthenic Syndrome trials
Common questions about Amifampridine
▸What is Amifampridine (Firdapse)?
A tablet taken by mouth several times a day and the only FDA-approved treatment for Lambert-Eaton myasthenic syndrome (LEMS). It was approved for adults on November 28, 2018 and for children 6 and older on September 29, 2022.
▸How does Amifampridine work?
In LEMS, the immune system attacks calcium channels on nerve endings, so the nerves release too little acetylcholine, the chemical that tells muscles to contract. The result is weakness, especially in the hips and thighs. Amifampridine is thought to work by blocking potassium channels on those nerve endings. That keeps each nerve signal going slightly longer, more calcium gets in, and more acetylcholine is released. It eases symptoms but does not treat the immune attack or any underlying cancer.
▸What are the side effects of Amifampridine?
Firdapse has no boxed warning, but it must not be used by people with a history of seizures or an allergy to amifampridine or a related drug. Seizures happened in about 2% of people taking it, including people who had never had a seizure. The most common side effect was tingling around the mouth and in the hands and feet. Others included colds, stomach pain, nausea, diarrhea, headache and raised liver enzymes.
▸How is Amifampridine taken?
Taken by mouth as 10mg scored tablets, split into 3-5 doses a day, with or without food. Adults and children weighing 45kg or more start at 15-30mg a day and raise it by 5mg every 3-4 days, up to 20mg per dose and 100mg a day. Children under 45kg start at 5-15mg a day, up to 10mg per dose and 50mg a day. For small dose steps, trouble swallowing or a feeding tube, the tablets can be mixed with water at home to make a liquid. It must be refrigerated and thrown away after 24 hours.
▸Is Amifampridine FDA approved?
Yes, Amifampridine (Firdapse) is FDA approved (2018) for the treatment of Lambert-Eaton Myasthenic Syndrome.
▸Can children with LEMS take Firdapse?
Yes, since September 29, 2022 it is approved for children 6 and older. Children under 45kg start at 5-15mg a day and can go up to 50mg a day.
▸Why is Firdapse taken several times a day?
Its effect wears off within a few hours, so the daily amount is split into 3-5 doses to keep strength more even. The doctor raises the dose slowly every 3-4 days to find the lowest amount that works.
▸Is the tingling from Firdapse dangerous?
Tingling around the mouth and in the fingers and toes was the most common side effect, reported by 62% of people. Tell your doctor if it is bothersome, and get help right away for a seizure.
Sources and references
Every factual claim on this page is drawn from the public sources listed below. Click any reference to open the original document.
- U.S. National Library of Medicine, DailyMed · 2026-04-16. FIRDAPSE (amifampridine) tablets: prescribing information. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=f015fe60-3128-4a43-8c31-19fc6b5def3f
- U.S. Food and Drug Administration · 2018-11-28. NDA 208078 approval letter. http://www.accessdata.fda.gov/drugsatfda_docs/appletter/2018/208078Orig1s000ltr.pdf
- U.S. Food and Drug Administration · 2022-09-29. NDA 208078 supplement S-008 approval letter (ages 6 and older). https://www.accessdata.fda.gov/drugsatfda_docs/appletter/2022/208078Orig1s008ltr.pdf
- U.S. Food and Drug Administration · 2024-05-30. NDA 208078 supplement S-012 approval letter (higher maximum dose). https://www.accessdata.fda.gov/drugsatfda_docs/appletter/2024/208078Orig1s012ltr.pdf
- ClinicalTrials.gov. A Phase 3 Study of Amifampridine Phosphate in Patients With Lambert Eaton Myasthenic Syndrome (LEMS). https://clinicaltrials.gov/study/NCT01377922
- ClinicalTrials.gov. Phase 3 Study to Evaluate Efficacy of Amifampridine Phosphate in Lambert-Eaton Myasthenic Syndrome (LEMS). https://clinicaltrials.gov/study/NCT02970162
- Catalyst Pharmaceuticals. FIRDAPSE access and affordability. https://firdapse.com/taking-firdapse/access-affordability/