PharmaceuticalSerotonin releaserAnorecticWithdrawn (weight loss)

Fenfluramine

Also known as Pondimin · Ponderax · fen-phen (with phentermine)

Written by enwyReviewed 5 Aug 20261 sources0 comments

A serotonin-releasing anorectic used from the 1970s, most infamous as the 'fen' in the fen-phen combination. It was withdrawn in 1997 after being linked to cardiac valvulopathy and pulmonary hypertension. Its d-enantiomer was later re-approved at low doses as an anticonvulsant for Dravet syndrome under strict monitoring.

01 Overview

Fenfluramine releases serotonin and inhibits its reuptake, suppressing appetite with little psychostimulant effect. Combined with phentermine ('fen-phen') it became an enormously popular off-label weight-loss regimen in the mid-1990s.

Reports of valvular heart disease and primary pulmonary hypertension in fen-phen users led the FDA to request withdrawal of fenfluramine and dexfenfluramine in 1997. The valvulopathy is attributed to serotonin (5-HT2B) stimulation of heart valves. Notably, low-dose fenfluramine (Fintepla) was approved in 2020 for seizures in Dravet syndrome, with mandatory echocardiographic monitoring — a reminder the molecule is not inert, only repurposed under tight controls.

02 Mechanism

Releases serotonin and inhibits its reuptake; appetite suppression is serotonergic, while 5-HT2B agonism on cardiac valves and pulmonary vasculature drives its characteristic toxicity.

03 Dosing

Tier	Dose	Route	Notes
Dravet syndrome (Fintepla)	0.2–0.7 mg/kg/day	Oral	Low-dose anticonvulsant use with echocardiographic monitoring; capped ~26 mg/day.
Former anorectic dose	40–120 mg/day	Oral	Historically 20-40 mg two to three times daily; withdrawn for weight loss.

04 Effects

Effect	Magnitude	Evidence
Appetite suppressionSerotonergic satiety enhancement with low stimulation.	short-term weight loss	Clinical
Seizure reduction (Dravet)At low doses it markedly reduces convulsive seizures in Dravet syndrome.	significant seizure-frequency reduction	Clinical

05 Side effects

Effect	Severity	Frequency	Evidence	Countermeasures
Cardiac valvulopathyFibrotic thickening of heart valves causing regurgitation, the basis for withdrawal.	Severe	Notable in fen-phen users	Clinical
Pulmonary arterial hypertensionIncreased risk of primary pulmonary hypertension with prolonged use.	Life-threatening	Rare but serious	Observational

06 Legal status

United States

Withdrawn for weight loss 1997; re-approved (Fintepla) for Dravet with REMS

Schedule IV; restricted distribution program. · as of Jan 2026

Withdrawn for obesity; approved for Dravet

Monitored anticonvulsant use only. · as of Jan 2026

07 References

Valvular heart disease associated with fenfluramine-phentermineNew England Journal of Medicine, 1997 (Connolly et al.)

08 Discussion0 comments

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This page is reference information, not medical advice. Doses and protocols are documented as they appear in the clinical literature and in practice — describing them is not a recommendation to use them. Countermeasures listed here are not a substitute for a physician. Legal status varies by jurisdiction and changes.