What the label lists and what reports show
Fenfluramine side effects
Fenfluramine's FDA label: “The most common adverse reactions (incidence at least 10% and greater than placebo) in patients with Dravet syndrome were decreased appetite; somnolence, sedation, lethargy; diarrhea; constipation; abnormal echocardiogram; fatigue, malaise, asthenia; …” FAERS holds 4,666 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 tricuspid valve incompetence (580 reports).
- No generic listed
- Boxed warning
Check it at the source: the fenfluramine FDA label on DailyMed (effective October 27, 2025, UCB, 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 FINTEPLA?FINTEPLA may cause serious side effects, including: See " What is the most important information I should know about FINTEPLA? " serotonin syndrome. Serotonin syndrome is a life-threatening problem that can happen in people taking FINTEPLA, especially if FINTEPLA is taken with certain other medicines to include: anti-depressant medicines called SSRIs, SNRIs, TCAs, and MAOIs tryptophan lithium antipsychotics St. John's Wort dextromethorphan tramadol Call your healthcare provider right away if you have any of the following symptoms of serotonin syndrome. mental status changes such as seeing things that are not there (hallucinations), agitation, or coma changes in blood pressure tight muscles fast heartbeat nausea, vomiting, diarrhea high body temperature trouble walking high blood pressure (hypertension). Hypertension is both a serious and common side effect. […]
Source: FDA drug label, NDA212102 (Ucb Inc), as published by UCB, Inc., effective October 27, 2025 (SPL set e88f360e-33ad-4cd6-b2de-5ef885857c5d), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsThe most common adverse reactions (incidence at least 10% and greater than placebo) in patients with Dravet syndrome were decreased appetite; somnolence, sedation, lethargy; diarrhea; constipation; abnormal echocardiogram; fatigue, malaise, asthenia; ataxia, balance disorder, gait disturbance; blood pressure increased; drooling, salivary hypersecretion; pyrexia; upper respiratory tract infection; vomiting; decreased weight; fall; status epilepticus.
Table 4: Adverse Reactions in 5% or More of Patients Treated with FINTEPLA and Greater Than Placebo in Placebo-Controlled Trials for Dravet Syndrome (Study 1 and 2)
| Reaction | N=39 % | N=40 % | N=43 % | N=84 % |
|---|---|---|---|---|
| Decreased appetite | 23 | 38 | 49 | 8 |
| Somnolence, sedation, lethargy | 26 | 25 | 23 | 11 |
| Abnormal echocardiogram Consisted of trace and mild mitral regurgitation, and trace… | 18 | 23 | 9 | 6 |
| Diarrhea | 31 | 15 | 23 | 6 |
| Constipation | 3 | 10 | 7 | 0 |
| Fatigue, malaise, asthenia | 15 | 10 | 30 | 5 |
| Ataxia, balance disorder, gait disturbance | 10 | 10 | 7 | 1 |
| Abnormal behavior | 0 | 8 | 9 | 0 |
| Blood pressure increased | 13 | 8 | 0 | 5 |
| Drooling, salivary hypersecretion | 13 | 8 | 2 | 0 |
| Hypotonia | 0 | 8 | 0 | 0 |
| Rash | 8 | 8 | 5 | 4 |
| Blood prolactin increased | 0 | 5 | 0 | 0 |
| Chills | 0 | 5 | 2 | 0 |
| Decreased activity | 0 | 5 | 0 | 1 |
| Dehydration | 0 | 5 | 0 | 0 |
| Insomnia | 0 | 5 | 5 | 2 |
| Pyrexia | 15 | 5 | 21 | 14 |
| Stereotypy | 0 | 5 | 0 | 0 |
| Upper respiratory tract infection | 21 | 5 | 7 | 10 |
| Vomiting | 10 | 5 | 5 | 8 |
| Weight decreased | 13 | 5 | 7 | 1 |
| Croup | 5 | 3 | 0 | 1 |
| Ear infection | 8 | 3 | 9 | 5 |
| Gastroenteritis | 8 | 3 | 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 following clinically significant adverse reactions are described elsewhere in labeling: Valvular Heart Disease and Pulmonary Arterial Hypertension [see Warnings and Precautions (5.1) ] Decreased Appetite and Decreased Weight [see Warnings and Precautions (5.3) ] Somnolence, Sedation, and Lethargy [see Warnings and Precautions (5.4) ] Suicidal Behavior and Ideation [see Warnings and Precautions (5.5) ] Withdrawal of Antiepileptic Drugs [see Warnings and Precautions (5.6) ] Serotonin Syndrome [see Warnings and Precautions (5.7) ] Increase in Blood Pressure [see Warnings and Precautions (5.8) ] Glaucoma [see Warnings and Precautions (5.9) ] The most common adverse reactions (incidence at least 10% and greater than placebo) in patients with Dravet syndrome were decreased appetite; somnolence, sedation, lethargy; diarrhea; constipation; abnormal echocardiogram; fatigue, malaise, asthenia; ataxia, balance disorder, gait disturbance; blood pressure increased; drooling, salivary hypersecretion; pyrexia; upper respiratory tract infection; vomiting; decreased weight; fall; status epilepticus. ( 6.1 ) The most common adverse reactions (incidence at least 10% and greater than placebo) in patients with Lennox-Gastaut syndrome were diarrhea; decreased appetite; fatigue; somnolence; vomiting.
Source: FDA drug label, NDA212102 (Ucb Inc), as published by UCB, Inc., effective October 27, 2025 (SPL set e88f360e-33ad-4cd6-b2de-5ef885857c5d), via openFDA; the full label on DailyMed
What do FAERS reports show?
4,666 reports list fenfluramine among the medicines taken, each a report of something that happened, not proof the medicine caused it; 114 are coded as a death and 1,399 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. Reports rose sharply in 2025 (1,307, against a median of 44 a year): FAERS counts follow reporting, which news, litigation and label changes can drive, not how often a reaction happens. Of the 20 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?
- Fenfluramine: 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