Why aren't criminals hypnotised or given a "truth drug" to get them to reveal the locations of victims bodies?

I realise this is not guaranteed but say, if one of my loved ones was murdered I'd want anything to be tried to try to locate their remains. I think this approach could be used in the Moors murders or the Peter Falconio case in Australia, for example. Unorthodox methods have been used in police work before, like employing psychics.