Cannabidiol (CBD) for use in the treatment of epilepsy, wherein the epilepsy is treatment-resistant epilepsy (TRE), and wherein the epilepsy to be treated is characterised by atonic seizures, is provided. Preferably the atonic seizures to be treated are in patients diagnosed with Lennox-Gastaut syndrome, tuberous sclerosis complex, Dravet syndrome, Doose syndrome, CDKL5 and Dup15q. The CBD is preferably for use in combination with one or more concomitant anti-epileptic drugs (AED), in particular clobazam, levetiracetam, topiramate, stiripentol, phenobarbital, lacsamide, valproic acid, zonisamide, perampanel and fosphenytoin, wherein the dose of the anti-epileptic drug/s that is/are used in combination with CBD is reduced. Preferably the CBD is present as a highly purified extract of cannabis which comprises at least 98% (w/w) CBD. Preferably the extract further comprises up to 1% CBDV (cannabidivarin) and less than 0.15% THC (tetrahydrocannabinol). Preferably the dose of CBD is from 5mg/kg/day to 25mg/kg/day.
