Not sure that a pathalogical liar wouldn’t continue to lie even if given truth drugs I know that it is an issue for lie detector tests. Their body doesn’t react when lying like normal people IIRC
Hitlery would take a lie detector test - she laughingly said she no longer believes they are accurate, as her client who raped the 12 year old child passed one. That’s why I wondered if truth serum of sorts would work - there’s a chance it would pass conscience thought and spew out what’s in the brain.