The regulator has also granted paediatric exclusivity for the prescription medication.
It was previously approved by the FDA and EU Commission to treat Dravet syndrome-related seizures in children aged two years and above.
The drug is currently under regulatory review with the PMDA in Japan for the same indication.
UCB stated that the Fintepla for LGS is available in the US through a restricted distribution programme, called the Risk Evaluation and Mitigation Strategy (REMS) Program.
UCB global epilepsy head Mike Davis said: “The approval of fenfluramine for Lennox-Gastaut syndrome highlights our continued commitment to bringing differentiated medicines to patients who may not be well controlled on current therapies, and their caregivers.
“We are proud to add fenfluramine as a treatment for Dravet syndrome, and now Lennox-Gastaut syndrome, to our portfolio of epilepsy medicines to help reduce the impact and burden of seizures, including severe epilepsy syndromes that have high paediatric morbidity and mortality rates.”
The regulatory approval was based on the data obtained from a randomised, placebo-controlled, international Phase III clinical trial, which was conducted in 263 patients with LGS.
In the study, Fintepla showed significant improvements on the Clinical Global Impression scale (CG-I) in LGS patients.
The company stated that the drug at a 0.7/mg/kg/day dose has significantly reduced the frequency of drop seizures compared to placebo.
The common adverse reactions in patients treated with fenfluramine included decreased appetite, vomiting, somnolence, diarrhoea, and fatigue.