If you can just drug people to tell the truth why doesn't the FBI just drug the Guantanamo detainees or in fact why don't we just drug every suspected criminal arrested on probable cause?
Because testifying under the influence of psycho altering substances won't hold up in court of probably any modern nation.
Because those drugs don't really work. It's like getting someone drunk and then asking them questions. It makes it harder to lie effectively because you have trouble thinking and people may trust people the shouldn't nothing we know about can force you to be truthfull.
tl;dr difficulty lying != forced to tell the truth